Advanced ecology notes 2020,

Advanced ecology notes 2020, CLA BOYS
1 | KING SOYEKWO 2020 ABE TING
ECOLOGY
It is the scientific study of the interactions that determine
the distribution and abundance of organisms within a
particular environment.
OR:
 Is the scientific study of the complex relationships
between organisms and their environment.
 These interactions determine the distribution and
abundance of organism within a particular
environment.
OLD DEFINITION OF ECOLOGY
The study of inter-relations betweenliving organisms and
their natural environment.
TERMS USED IN ECOLOGY
1. Ecosystem
A natural unit composed of living (biotic) and non-living
(abiotic) components whose interactions lead to a self-
sustaining systeme.g. ponds, lakes, forest,desert,stream.
2. Species
Group of organisms showing resemblance among
themselves in appearance, behaviour, chemistry and
genetic makeup.
Organisms that reproduce sexually are classified as
members of the same species if, under natural conditions,
they can (i) actually or potentially breed with one another
and (ii) produce fertile offspring.
3. Population
Total number of members of a species occupying a
specific area at the same time. E.g. tilapia fish in a pond,
mahogany trees in a forest, people in a country
4. Community
All the organisms of different species that interact in a
given, well defined area e.g. all organisms within a pond
5. Synecology
Study of many species within an ecosystem
6. Autecology
Study of single organisms or populations of single
species and their relationship to their environment e.g.
considering a lion in the bush, what does it feed on?, how
does it reproduce?,what are its competitors?, what are its
predators?, etc
7. Habitat
Specific locality where anorganism normally lives within
the environment e.g. the underside of a log for
earthworms, intestines of man for tapeworms, ponds for
frogs, kitchen for cockroaches, etc. Habitat is like the
“address” of an organism.
Microhabitat
Small locality within the habitat with particular
conditions (microclimate) that support specific
organisms e.g. mosses can grow at the upper side of a
fallen log, while the underside supports earthworms.
a) Niche /Ecological niche
The role an organism plays in the habitat, and its
interactions with other organisms. i.e. the sum of all
environmental factors that influence the growth, survival
and reproduction of a species. A niche is like the
“profession” of an organism
b) Fundamental niche
The physical conditions under which a speciesmight live,
in the absence of interactions with other species.
c) Realised niche
The role an organism plays in the habitat, and its
interactions with other organisms in the presence of
competition and other constraining factors i.e. is the set
of conditions under which an organism exists in nature.
8. Native species
Species that normally and shrive in a particular
ecosystem
9. Non-native/alien/exotic species
Species that migrate into the ecosystem or are
deliberately or accidentally introduced into an ecosystem
by humans e.g. crops and game species
10. Indicator species
Species that serve as early warnings of damage to a
community or an ecosystem
11. Keystone species
Species that play more important roles than others in
maintaining the structure and function of ecosystems of
which they are a part i.e. it is a dominant species that
dictates community structure by affecting abundances of
other species.

Examples:
1) Top predator keystone species like lions, crocodiles
and great white sharks exert a stabilizing effect on
their ecosystems by feeding on and helping regulate
the populations of certain species.
2) Bats and birds regenerate deforested areas by
depositing plant seeds in their droppings.
3) Elephants uproot and break trees, creating forest
openings in the savanna grasslands and woodlands,
which promotes growth of grasses for grazers and
also accelerates nutrient recycling.
4) Dung beetles remove,bury and recycle animal wastes
(dung)
Note: all species play some role in their ecosystems and
thus are important, therefore the assertion that some
species are more important than others remains
controversial
THE MAJOR PARTS OF THE EARTH’S LIFE
SUPPORT SYSTEM
a) Atmosphere:
It is a thin envelope or membrane of air around the
planet.
b) Biosphere:
The part of the earth in which living organisms exist
and interact with one another and with their nonliving
environment. It reaches from the deepest ocean floor,
20 kilometers (12 miles) below sea level, to the top
of the highest mountains.
c) Troposphere:
Inner layer of the earthextending about 17 kilometers
(11miles) above sea level, but containing most of the
planet’s air, mostly nitrogen (78%) and oxygen
(21%).
d) Stratosphere:
This is the layer stretching 17 – 48 kilometers (11 –
30 miles) above the earth’s surface. Its lower portion
contains ozone (O3) to filter out most of the sun’s
harmful ultraviolet radiation.
e) Hydrosphere:
It consists of the earth’s (i) liquid water (both surface
and underground), (ii) ice (polar ice, icebergs, and ice
in frozen soil layers) (iii) water vapour in the
atmosphere.
f) Lithosphere:
This is the earth’s crust and upper mantle. The crust
contains nonrenewable fossil fuels and minerals as
well as renewable soil nutrients needed for plant life.
BIOMES
These are large regions of the biosphere characterized by
a distinct climate and specific life-forms (especially
vegetation) adapted to it.
Examples of the major terrestrial biomes of the world:
a) Tropical forests
i) Tropical rainforests: occur at low latitudes where the
rain falls abundantly all year long and temperature is
warm
ii) Tropical seasonal forests: occur where climate is
ratherdrier, and treesmay lose their leaves during the dry
season.
A forest biome is divided into ground zone (consisting
of millipedes & earthworms) and canopy zone/aerial
zone;(consisting of birds & monkeys); with eachof these
zones supporting different animals that are adapted to the
conditions within them.
b) Savannah: warm and dry grasslands with few trees,
typically supporting grazing of animals
c) Deserts: areas of very little rainfall, ranging from
entire barrennessto seasonalrainfall that supports growth
of some vegetation.
Deserts can be: Hot and dry desert regions
(evaporation is high and there is too much heat), cold
deserts (precipitation coming from colder water sources
than rain, such as snow or ice), temperate region (winters
and summers).
d) Chaparral (temperate shrub land): grow where
summers are hot and dry, and the winters are cool and
wet. Vegetation is composed of mainly dense spiny
shrubs with tough evergreen leaves.
e) Temperate grassland: are similar to savannas but
occur in cooler climates e.g. Canadian prairies and
pampas of Argentina.
f) Temperate forests: include deciduous and evergreen
forests but contain far fewer species than a tropical
rainforest.
g) Taiga (coniferous forest/boreal forest): conifer
forests in cold subarctic or subalpine conditions

h) Tundra: treeless plains in the arctic; cold for most of
the year. It is characterized by grasses, lichens, sedges,
and mosses as the most predominant species.
 Organisms live within a relatively narrow sphere
(land, water and air) and the earth’s surface and this
is known as Biosphere/ecosphere.
 The biosphere is divided into two major regions
namely;
i. Aquatic regions; made up of fresh water (lakes
and ponds, rivers and streams, wetlands), marine water
(oceans), and estuaries.
ii. Terrestrial regions covering a few meters deep
in the soil and a few kilometers into the atmosphere.
 On land, there are several bio-geographical areas,
each with specific conditions that support distinct
species of plants and animals. Such areas include the
present day continents.
ECOSYSTEM
It is natural unit of environment composed of living
(biotic) and non-living (abiotic) components whose
interactions lead to a self-sustaining system.
(i) Water (aquatic) ecosystems may be fresh water
bodies (e.g. lakes, ponds, rivers) or marine water bodies
(e.g. sea, ocean).
Organisms in water may be of large size (nektons) e.g.
fish, whales, turtles or very tiny (planktons) e.g.
phytoplankton and zooplanktons.
(ii) Land (terrestrial) ecosystems include forests,
deserts, savanna, etc
THE MAJOR COMPONENTS OF AN
ECOSYSTEM
a) Abiotic / non-living things: these are physical and
chemical factors that influence living organisms on land
(terrestrial) ecosystems and in water (aquatic).
Examples of abiotic components:
Climatic factors, which include; Temperature, Light,
Wind, Humidity, rainfall etc
i. Soil (edaphic) factors e.g. Soil pH, Soil air,
Inorganic particles, Soil water,Organic matter (dead
organic matter and living organisms), Soil
temperature etc
ii. Topography
iii. Other physical factors e.g fire and wave action etc
Question. How do abiotic factors affect the
distribution and abundance of organisms?
(i) Climatic factors
Temperature
1) Affects physiological processes (respiration,
photosynthesis, and growth etc) in organisms which
in turn influence their distribution.
2) Ultimate heating and cooling of rocks cause air to
break and crack into small pieces and finally form
soil.
3) These changes in turn may result into migration of
organisms e.g birds to avoid over heating or freezing.
4) Low temperatures inactivate enzymes while
excessive temperatures denature enzymes.
5) High temperature increase transpiration and sweating
6) Low temperatures break dormancy of some plants.
7) Temperatures stimulate flowering in some plants e.g
cabbage (vernalisation)
8) Exposure to low temperature (stratification)
stimulate germination in some seeds after imbibition.
9) Organisms have evolved to have structural,
physiological and behavioral adaptations to maintain
their temperature in an optimum range.
Adaptations ofanimals for life in hot and dry deserts.
A. Structural adaptations,
(i) Large body extremities e.g ear lobes; to increase
surface area over which heat is lost.
(ii) Small sized; to increase the surface area to volume
ratio, for heat loss
(iii)Some animals like the camel, have long skinny non
fatty legs to increase heat loss during locomotion
(iv)Little or no fur to reduce on insulation, and increase
amount of heat lost
(v) Thin subcutaneous fatlayer under the skin to increase
heat loss from the body
(vi)Have tissues tolerant to extreme temperature
changes, maintaining the body’s main functions
B. Physiological adaptations
 Enzymes work under a high optimum temperature
range to maintain metabolism during day and night.

C. Behavioral adaptations
(i) Most are nocturnal, i.e most active at night, when
temperatures are relatively low
(ii) Aestivation (seasonalresponse by animals to drought
or excessive heatduring which they become dormant,
and the metabolic rate followed by body temperature
fall to the minimum required for maintaining the vital
activities of the body) ; allows them to survive
extremes of hot temperatures e E.g. African lungfish
burrows into mud till the dry season ends,
earthworms , garden snails , desert rats, termites also
aestivate
(iii) Movement with some body parts raised to minimize
direct contact with hot grounds e.g desert snakes
(iv) Salivation of the neck and legs; increasing heat loss
by evaporation e.g in tortoise
Adaptations of animals for life in cold environments
Structural adaptations
1) Thick layer of fat under the skin; to increase on
insulation by avoiding heat loss
2) Small body extremities to reduce the surface area
over which heat is lost
3) Large sized; thus small surface area to volume ratio;
reducing amount of heat lost to the surrounding
4) Thick fur; to increase on insulation
5) Tissues tolerant to extreme changes in temperature;
maintaining their normal functions in the body
 Physiological adaptations
 Enzymes work under a high optimum temperature
range to maintain metabolism during day and night
 Behavioral adaptations
 Hibernation (is seasonal response by animals to cold
temperature during which they become dormant,
body temperature and metabolic rate fall to the
minimum required for maintaining the vital activities
of the body) The animals, said to be in ‘deep sleep’
ably reduce energy needs to survive the winter when
food is scarce allowing them survive extreme cold
conditions eg in polar bears.
 Gathering in groups to warm themselves e.gpenguins
Question
How does temperature influence the distribution of
organisms? (10 marks)
Approach
Small temperature range ; because enzymes work within
narrowoptimumtemperaturerange;mostorganismsare
foundwheretemperatureismoderate;like in tropicsand
temperate regions ; high temperatures cause Enzyme
denaturation ; rapid evaporation of water ; dehydration
; low temperature in-activates enzymes ; makescrystals
in cells so, killing them ; therefore very few inhabits
regionswith extreme high/lowtemperature@ 1 mark =
10 marks
Rain fall;
Amount of rainfall in a given area determines the
abundance, distribution and types of plants in the area.
Vegetation cover is influenced by the amount of
precipitation in an area.
Ecological significances of water
1) Habitat for many aquatic organisms e.g frogs,
fish etc
2) Raw material for photosynthesis; main energy
source for body processes of other organisms
3) High thermal capacities; acting as cooling agent
for terrestrial organisms e.g plants during
transpiration, some animals during sweating.
4) Agent for fruit, seed, spore, larva and gamete
dispersal
5) Condition for germination
6) Highly transparent; therefore allowing light to
reach acquatic organisms, for photosynthesis;
and aquatic predators to locate their prey
7) Important factor in decay and decomposition;
therefore,increasesin recycling of nutrients in an
ecosystem.
Humidity;
Amount of water in the atmosphere affects the rate at
which water evaporates from organisms’ i.e Low
humidity results to increasing evaporation while high
humidity causeslow rate of evaporation; through stomata
of leaves in plants.
Accordingly, organisms within areas of low humidity are
adapted to avoid excessive loss of water by;
1) Having reduced number of sweat glands e.g in
kangaroo rat
2) Presence ofleaf spines in cactusplants; to reduce
surface area over which water is lost through
transpiration.

3) Controls other activities of animals like feeding,
hunting, and movements e.g earth worms
experience a larger ecological niche when the
environment is humid.
4) Controls opening and closure of stomata;
therefore, affecting rate of photosynthesis and
transpiration.
Wind / air currents;
It influences the following,
1) Dispersal or migration of flying mammals, winged
insects; thus reducing the level of competition.
2) Pollination
3) Dispersal of seeds and spores; increasing the spread
of non-motile organisms e.g fungi and some bacteria.
4) Takes part in rain formation
5) Current and wave formation in seasand lakes enables
distribution of mineral salts.
6) Increase transpiration; thus promoting water and
mineral salt uptake from the soil by plant roots
7) Increases evaporation and reduces sweating.
8) Causes physical damage to vegetation and soils e.g
soil erosion.
• Increases dissolution of oxygen in aquatic
bodies; thereby increasing aerobic activities of
organisms.
Light (intensity, quality, and duration)
1) Influences many physiological activities of
organisms ie
2) It is a source of energy for photolysis (breakdown of
water during photosynthesis.).
3) Absence of light causes etiolation (elongation of
shoot inter nodes).
4) Induces flowering in long-day plants e.g. barley, but
inhibits flowering in short day plants.
5) Phototropism, by redistributing auxins on the darker
sides of shoots and roots, with cells on darker side
elongating more than those on illuminated side.
6) Germination; some seeds are positively photoblastic;
germination only in presence of light while other do
not require light to germinate.(are negatively
photoblastic)
7) Stomatal opening and closure; with most plant
species opening their stomata during day (when there
is light) and closing during night (in absence of
light/darkness).
8) Predation; (hunting and killing of prey by predators
require certain levels of illumination and visibility
9) Courtship; with some animals preferring light so as
to carry out courtship while others prefer darkness
10) Light breaks dormancy of seeds.
11) Stimulates synthesis of vitamin D in mammals;
where lipids(sterols) in the dermis are converted to
vitamin D by uv light
12) It enables the mechanisms photoreceptions in eyes
13) Absence of light results in failure of chlorophyll
formation in plants i.e. plant remains yellow, and
leaves fail to expand.
14) Photoperiod affects migratory and reproductive
behaviour in various animals e.g. sunlight polarised
by water acts as a compass for migration of salmon
fish.
15) Necessary for the germination of certain seeds e.g.
lettuce
(ii) Topography.;
1) Refers to the nature of the landscape, which
includes features like mountains, valleys, lakes
etc.
2) High altitude is associatedwith, low atmospheric
pressure; low average temperatures, increased
wind speed; decreased partial pressures of
oxygen, thus few organisms live permanently
here.
3) Slope reduces water logging and there is a lot of
soil erosion preventing proper plant
establishment especially at steep slopes
4) At low altitudes, average temperatures are high,
high atmospheric pressure, partial pressures of
oxygen are high, and in some places there is
water logging.
Assignment. Describe different adaptations of
organisms that live in high altitude.
(iii). Edaphic (soil) factors,
Soil formed by chemical and physical weathering of
rocks, possess both living components(living organisms
like bacteria, fungi, algae and animals like protozoans,
nematodes earthworms, insects, burrowing mammals)
and non living components (particles of different sizes)
o Also present are; mineral salts, water, organic matter,
and grasses.

Soil pH
1) Influences physical properties of soil and
availability of certain minerals to plants, thus
affecting their distribution in soil; i.e tea and
coffee plants thrive well in acidic soils
2) Affects activity of decomposers e.g in acidic
medium, the rate of decomposition is reduced,
subsequently recycling of matterin an ecosystem
reduced.
Water content;
1) Varies markedly in any well-defined soil,
2) Any finely drained soil holding much water as
possible is said to be at full capacity
3) Addition of more water which cannot be drained
away leads to water logging; and anaerobic
conditions, affectingmineral ion uptake by active
transport, subsequently affecting osmotic uptake
of water, due to decreased osmotic potential
gradient, causing plants to dry out.
4) Plants like rice, marshes, and sedges have
developed air spacesamong root tissues allowing
some diffusion of oxygen from aerial parts to
help supply the roots.
Biotic component;
1) Microorganisms like bacteria and fungi carry out
decomposition of dead organic material, therefore
recycling nutrients back to the soil.
2) Burrowing organisms e..g earthworms improve
drainage and aeration by forming air spaces in the
soil.
3) Earthworms also improve soil fertility by mixing of
soil, asthey bring leached minerals from lower layers
within reach of plant roots.
4) They also improve humus content, by pulling leaves
into their burrows
5) Also press soil through their bodies making its
texture fine.
Air content;
• Spaces between soil particles is filled with air
from which plant roots obtain oxygen by diffusion for
aerobic respiration,
• Also essential for aerobic respiration by
microorganisms in the soil that decompose the humus.
Salinity;
 Is the measure of salt concentration in aquatic bodies
and soil water.
1) Determines the osmotic pressure of water; therefore
the organisms have developed structural, behavioral,
and physiological adaptations to osmo regulate in the
respective salt concentration, (read adaptations of
fresh water fish,marine water fish and migratory fish
to their osmo regulatory problems).
2) Mineral salts in water affect the distribution of plant
species,which in turn affects the animals that depend
on plants for food.
3) Plants growing in soils deficient of certain salts, e.g
insectivorous plants in nitrogen deficient soils, obtain
nitrogen feeding on insects.
4) Significances of mineral salts to plants
5) Mineral salts together with other solutes determine
the osmotic pressure of cells and body fluids
6) Determinants in anion and cation balance in cells, e.g
Na+ and Cl-, involved in transmission of nerve
7) Constituents of certain pigments like haemoglobin,
and chlorophyll containing iron and magnesium
respectively.
8) Metabolic activators; some ions activate enzymes,
e.g chloride ions activate salivary amylase,
magnesium activate enzymes in phosphate
metabolism, and phosphorus as phosphate is required
in activation of sugars during Glycolysis in tissue
respiration.
9) Mineral salts like potassium are involved in
formation of cell membrane and opening of stomata;
10) Development of stem and root e.g. calcium pectate in
formation of plant cell wall. Etc
(v) Fire;
Types of fire
a) Natural fires; are set up by natural causes like
lightening, volcanic eruptions etc
b) Artificial fires; are set up by man either intentionally
or carelessly
c) Wild fires; burn in the direction of wind

d) Early fires; set up at beginning of dry season
e) Prescribed fires; under ecological management
where prevention measures are taken when stting up
the fire.
Properties of fire
a) Fire intensity;
Is the heat content of the fire,
Depends on environmental factors such as wind,
temperature as wellas the amount and type of vegetation.
b) Fire duration;
Is the time taken by the fire to destroy a given area.
c) Fire severity; is measured in terms of major
vegetation destroyed by the fire.
Ecological effects of fire
Positive effects
1) Removes old leaves and stimulates trees and
grasses to produce new buds.
2) Breaks dormancy (seed dormancy), incase seed
coats are hard and impermeable.
3) Causes release of mineral nutrients in form ash;
on burning organic matter, releasing nitrate and
phosphate compounds into soil, and
subsequently improving on soil fertility.
4) Improves on visibility of organisms such as
predators,prey, mates allowing them easily carry
out their activities.
5) Improves on food productivity in terms of
quality, quantity and productivity, because after
burning new species with high protein content
grows.
6) Destroys pests
7) Controls undesirable plant species and weeds
Negative effects
1) Increase soil erosion; leading soil infertility
2) Kills slow moving animals e.g snails, earthworms
3) Destruction of habitat for most of the animal species
may leading migration or extinction.
4) Increases fire resistant species.
5) Reduction in population density and biodiversity.
6) Destroys food for animals like herbivores which may
lead to starvation and eventually death.
7) Air pollution by products such as carbon monoxide
and carbon dioxide, increasing on global warmimg.
8) It disrupts the hydrological cycle (water cycle),since
it destroys vegetation which would contribute to rain
formation.
9) It also disrupts the nitrogen cycle by killing nitrogen
fixing bacteria.
10) When fire kills decomposers, organic pollutants
accumulate and recycling of matter is hindered.
Adaptations of plants to fire
 Thick succulent shoot system to reduce the effects of
heat.
 Grasses grow in tussocks to protect the young
growing buds.
 Some tree stems are succulent i.e. store water in
parenchyma cells to reduce on the effectsoffire heat.
 Many plants are annuals to avoid fire severity in form
of seeds, which may be underground. Some trees
have heat resistant tissues.
(b). Biotic / living components: these are the plants,
animals and decomposers.
Adaptation of Desert-Dwelling Flora to Challenges in
their Habitat
1. Structural adaptations
2. Possession of extremely deep roots so as to
obtain water from deep down below the water
table e.g. acacia and Oleander.
3. Shallow root system for absorbing moisture even
after slight showering e.g. cactus
4. Possession of fleshy succulent stems and leaves
that store water in large parenchyma cells e.g.
Bryophylum and cactus.
5. Reduction in stomata number to reduce on
transpiration.
6. Possession of stomata sunken with a hairy leaf
surface to trap air and reduce on transpiration.
7. Rolling / curling / folding of leaves to reduce
Transpiration e.g. Marram grass (Ammophila)

8. Hairy epidermis for reflecting solar radiation and
trapping humid air next to leaf surface andreduce
transpiration.
9. Possession of thick cuticle, which is
impermeable to water e.g. prickly pear
(Opuntia).
10. Reduction of surface area over which
transpiration has to occur by having small leaves.
Physiological adaptations
a) Reversal of the normal stomatal rhythm in some
plants e.g. opening stomata at night and closing
during day time so as to reduce on waterevaporation.
b) Increased levels of abscisic acid, which induces
stomatal closure so as to reduce water loss.
c) Possession of tissues tolerant to desiccation e.g. low
solute potential of cytoplasm and production of
resistant enzymes.
d) Leaf fall in deciduous trees so as to cut down
transpiration
e) Survival of drought as seeds or spores that are highly
dehydrated and protected within a hard coat
THE MAJOR BIOTIC / LIVING COMPONENTS
OF ECOSYSTEMS
1. Producer:
Are autotrophs capable of synthesizing complex organic
food materials from simple inorganic food raw materials
e.g carbon dioxide and water. Examples include; large
greenterrestrialplants e.g trees,shrubs, grass.For aquatic
ecosystem, the producers are microscopic algae, blue
green bacteria.Othersare flagellates like euglena, volvox,
chlamydomonas etc. They are collectively called
Phytoplanktons (microscopic marine producers)
NB; Some producers use chemical energy derived from
breakdown of chemical compounds like sulphur to
convert carbon dioxide and water into high energy
compounds like carbohydrates e.g sulphur bacteria i.e
they are chemosynthetic.
2. Consumer:
Are organisms that get energy and nutrients by feeding
on other organisms or their remains.
Are classified as;
a) Primary consumers (Herbivore):
A consumer that eats plants. E.g. insects, birds, most
mammals (grazers),
In aquatic ecosystem, they include; water fleas, fish,
crabs, mollusks, and protozoans, collectively known as
zooplanktons (microscopic marine consumers).
b) Secondary consumers (Carnivore):
Aconsumer that eatsother animals. E.g. birds of prey like
eagle, kites, kingfishers; and lions, cheetahs, tigers,
hyenas, snakes, big fish
c) Tertiary consumers:
These feed on both primary and secondary consumers
Can be predators that hunt and kill others for food or
scavengers (animals that feed on dead organisms but do
not kill them. E.g. vultures, hyenas, marabou stocks etc
d) Omnivore: A consumer that eats both plants and
animals.e.g. man, pigs, etc
3. Decomposer:
An organism that feeds on dead organic matter.
Classified into;
a. Detrivore/ macro decomposers;
An animal that eats detritus. (dead and waste matter not
eaten by consumers)
E.g earth worms, rag worms, mites, maggots, wood lice,
termites etc.
b. Saprophyte:
A microbe (bacterium or fungus) that lives on detritus.
Importance of decomposition:
(1) It enables dead bodies to be disposed off which,
if left would accumulate everywhere.
(2) Recycles nutrients to be used by other organisms
e.g. Mineral salts are released from dead bodies into soil
for plant growth.
(3) Unlocks trapped energy in the body of dead
organisms.
ENERGY FLOW THROUGH AN ECOSYSTEM
The sun is the primary source of energy in the ecosystem.
Light energy is trapped by photosynthetic organisms
(green plants, algae, and some bacteria); converted
into chemical energy by during photosynthesis.

It is then transferred from one feeding level to another
through feeding relationships like food chains or food
webs.
Most of the energy from sun getting the earth’s surface is
reflected by vegetation, soil, and water or absorbed and
radiated to atmosphere; leaving only between 5%-10%
for the producers to make use of.
• Along the food chain, only a small proportion of
the available energy is transferred from one feeding level
to another; much energy is lost as heat during sweating
and evaporation, excretion, respiration, egestion, and
some remains locked up in indigestible parts of the plant
like cellulose, or bones, hooves, hair, skin etc of animals.
The number of organisms decrease at each successive
feeding level because of the great energy losses, so the
energy left in organisms is little to support large numbers
of top consumers; limiting the length of food chain ( not
exceeding five trophic levels( feeding level in a food
chain containing given amount of energy).
Energy flow through an ecosystem and the relative
efficiency with which it occurs.
The primary source of energy is the sun, whose energy
(less than 0.1%) is fixed by photosynthetic plants as
chemical energy;
As primary consumers (herbivores) feed on producers,
they obtain about 5 – 10% of the energy (a loss of 90 –
95% occurs) because of egestion, excretion and
indigestibility of materials like lignin, cellulose
As secondary consumers (lower order carnivores) feed
on herbivores, they obtain only about 10-20% of energy
(loss of 80 – 90% occurs) because:
(i) Animal tissues e.g. bones, hooves, hides not readily
digestible
(ii) Feeding is not 100% efficient – much digestible
material e.g. blood and food fragments may be lost to
the environment.
However, energy transfer is more efficient than
producer to herbivore because:
(i) Animal tissue is more digestible than plant tissue
(ii) Animal tissue has a higher energy value
(iii) Carnivores may be extremely specialized for prey
consumption.
Feeding levels are thus limited to 4 or rarely 5 because
of the cumulative energy losses along successive trophic
levels.
TROPHIC EFFICIENCY/ ECOLOGICAL
EFFICIENCY
 Is the percentage of energy at one trophic level that is
converted into organic substances at the next trophic
level.
Productivity in ecosystem
 Is the amount of organic material manufactured by
organisms.
Can be measured using several methods i.e
 Harvest crop
 Through oxygen production of the given area of
the ecosystem.
 Amount of carbon dioxide consumed during
photosynthesis.
 .Rate of consumption or use of raw materials
ENERGY BUDGETS
An energy budget shows the percentage allocation of
energy consumed by an individual organism to the
various processes in the body such as respiration, growth
and reproduction.
TERMSASSOCIATEDWITH ENERGYBUDGETS
a) Gross primary productivity (GPP)
It is the rate at which producers convert solar energy into
chemical energy stored in organic substances.
It is the total amount of energy fixed by producers per
unit area of photosynthetic surface per unit time.
Productivity may be expressed as units of energy (e.g.
kJm-2yr-1 or kCal m-2yr-1), or units of mass (e.g. kg m-
2yr-1)
GPP is greatest:
(i) In shallow waters near continents
(ii) Along coral reefs where abundant light, heat
and nutrients stimulate the growth of algae.
(iii) Where upwelling currents bring nitrogen
and phosphorus from the oceanbottom to the
surface.

GPP is lowest in:
(i) deserts due to low precipitation and intense
heat
(ii) the open ocean due to lack of nutrients and
sunlight except near the surface.
Gross productivity; is the total amount of energy and
organic matterstored in an organism over a period of time
b) Net primary productivity (NPP)
It is the rate at which energy for use by heterotrophs or
consumers is stored in new organic substances.
NPP is the energy that remains to be used by consumers
after producers have used part of GPP for their own
respiration.
NPP = GPP – (respiration + metabolism)
NPP most productive ecosystems are:
(i) Estuaries
(ii) Swamps and marshes
(iii) Tropical rainforests
NPP least productive ecosystems are:
(i) Open ocean
(ii) Tundra – arctic and alpine grasslands
(iii) Desert.
Despite its low net productivity, the open ocean produces
more of the earth’s NPP per year than any other
ecosystem because of its large size.
Net productivity; is the amount of energy and organic
matter stored in an organism and passed onto the next
trophic level.
c) Primary productivity; Is the amount of energy
and organic material stored in primary producers.
Measured in mass per unit area per unit time
(kilogram per unit area per year, Kg/M /yr.)
d) Secondary productivity; Is the amount of
energy incorporated into the body of consumers.
Also known as Gross secondary productivity.
Net secondary productivity; is the amount of energy
that cansuccessfully be transferredfrom one consumer to
another.
Carnivores have a higher secondary productivity than
herbivores because;
1. Diet of carnivores is rich in proteins; easily
digestible and therefore absorbed efficiently,
allowing little energy to be lost. Herbivores their
diet mainly consists of plant materials which are
not easily digested.
2. Carnivores do not have symbiotic microbes to
consume part of the energy of their diet in their
digestive tracts,
3. Their faeces contain much less undigested
matter.
Netsecondary productivity is higher in exotherms than in
endotherms, because;
 Energy from absorbed food, is usedin replace the
lost heat to their surroundings, in order to
maintain a constant body temperature, unlike
exotherms that depend mostly on behavioral
means to maintain their body temperature.
Biomass
It is the dry weight of all organic matter contained in
organisms per unit area of ground or water
Biomass is expressed as g/m2
Standing biomass (Standing crop biomass)
It is the dry weight of all organic matter contained in
organisms per unit area of ground or water at a given
moment in time
Trophic efficiency (Ecological efficiency)
It is the percentage of energy at one trophic level that
is converted into organic substances at the next
trophic level
Trophic efficiencies range from less than 1% (e.g.
herbivores eating plant material) to over 40% (e.g.
zooplanktons feeding on phytoplanktons).
FOOD CHAIN AND FOOD WEB.
FOOD CHAIN
A linear sequence of energy flow from producers
through a series of organisms in which there is
repeated eating and being eaten.

Two types exist i.e
(i) Grazing food chain (ii) Detritus food chain
i Grazer food chain,
Starts with autotrophs (producers)/ green plants
which convert carbon dioxide & water into
chemical compounds.
These are grazed upon by herbivores.
Energy is further transferred to carnivores. It can
be in grass land or water body (aquatic). E.g.
Grass millipedes toads snakes hawks
Green algae haplochromics tilapia kingfisher
ii Detritus food chain
 Is the one where the consumers obtain
energy from fragments of dead decaying
organic matter.
 Exists in both aquatic and terrestrial
habiats.
 1st
trophic level is occupied by a
decomposing organic matter
E.g Tree log wood lice - toad python
Dead animal maggot birds python
FOOD WEB
Is a complex nutritional interrelationship that
illustrates alternative food sources and predator
for each organism.
This is a complex nutritional relationship
showing alternative sources of food for each
organism in a food chain i.e. a complex network
of food chains linked to one another.
In a food web, there are several food chains.
Examples of food webs in a grassland
NB. Techniques used in constructing food webs and food
chains
1. Direct observation of organisms as it feeds so as to
establish the organisms prey.
2. Examination of stomach content through dissecting
the animals’ stomach
3. Faecal method; observation of faecal materials
egested by an animal.
4. Use of radioactive tracers to label the environment
from which organisms obtain their food and then
trace them in the organisms gut.
Typical examination question:
The figure below shows energy flow process in a food
chain;
a) Assuming that 10% of the energy received by the
herbivores is lost, calculated the energy retained
by the herbivores
b) Explain why;
i) Energy transfer from herbivores to
carnivores is more efficient than
from producers to herbivores

ii) The efficiency of energy transfer
from herbivores to carnivores is less
than 100%
c) State the factors which limit the number of
trophic levels in a food chain
Approach
a) Energy received = 800kJ
Energy lost = 10%
=
𝟏𝟎 𝐗 𝟖𝟎𝟎
𝟏𝟎𝟎
= 80 kJ
Energy retained = energy recievd – energy lost
= 800kJ - 80 kJ = 720kJ
b) (i)
 Producers contain a high proportion of cellulose and
sometimes wood which are relatively indigestible
and therefore unavailable as a source of energy for
most herbivores.
 The herbivores transfer animal tissue to the
carnivores, which is digestible and can therefore be
utilized by the carnivores. As a result, a large
proportion of energy is transferredfrom the herbivores
to carnivores than from producers to herbivores.
ii) - Some energy is lost in respiration and cannot be
transferred to other living organisms
Energy is also lost in form of excreta and egesta and is
transferred and is transferred to detritivores and
decomposers and never reaches the carnivores.
c) - Amount of energy received by producers
- Proportion of received energy that is converted
into net primary productivity (NPP)
- Extent of energy loss at each trophic level.
ECOLOGICAL PYRAMIDS
These are histograms that provide information about
trophic levels in ecosystems.
Pyramid of numbers
Pyramid of biomass
Pyramid of energy flow
1. Pyramid of numbers
It is a histogramatic representation of the numbers of
different organisms at each trophic level in an ecosystem
at any one time.
The number of organisms at any trophic level is
represented by the length (or area) of a rectangle
NB.
 As a pyramid is ascended, the number of organisms
decreases but the size of each individual increases.
 In some cases,the consumers may be more than the
producers e.g in a parasitic food chain, inverted
pyramids B & C are obtained, because parasites
progressively become smaller and many along a food
chain.
Disadvantages:
1. Drawing the pyramid accurately to scale may be very
difficult where the range of numbers is large e.g. a
million grass plants may only support a single top
carnivore.
2. Pyramids may be inverted; particularly if the
producer is very large e.g. an oak tree or parasites
feed on the consumers e.g. fleas on a dog.
3. The trophic level of an organism may be difficult to
ascertain.
4. The young forms of a species may have a different
diet from adults, yet they are considered together.
Pyramid of biomass
Is a histogram showing the total dry mass of organisms
present at each feeding level.
It is a histogramatic representation of the biomass
(number of individuals x mass of each individual) at each
trophic level in an ecosystem at any one time.

Advantages
 Reduces the possibility of forming inverted
pyramids because its construction depends on
biomass of organisms
NB. Inverted pyramid of biomass is typical of an aquatic
ecosystem, because diatoms (phytoplankton) have a
lower biomass but with higher productive rate (caused by
so rapid turnover rate),therefore capable of supporting a
larger biomass of zooplanktons.
Disadvantages/limitations of pyramid of biomass
1. Does not allow for changes in biomass at
different times of the year e,g deciduous trees have larger
biomass in summer than in winter when they shed off
leaves.
2. Does not take into account rate at which biomass
accumulates e.g a mature tree has a large biomass which
increases over many years.
3. Impossible to measure exactly biomass of the
organisms in an ecosystem,because the sample used may
not true representation of the whole population.
4. Results may not be accurate, e.g where killing is
not allowed, the results are obtained by estimating the
fresh mass.
Pyramid of energy flow
it is a histogram showing the total amount of energy
present at each feeding level.
It is a histogramatic representation of the flow of energy
through each level of an ecosystem during a fixed time
period (usually one year, to account for seasonaleffects).
Energy values may be expressed variously e.g. kJm-2yr-
1 or kCal m-2yr-1
Note:
(i) Because such pyramids represent energy flows, not
energy storage,they should not be called pyramids of
energy (a common error in some books)
(ii) Energy flow pyramids explain why the earth can
support more people if they eatatlower trophic levels
by consuming grains, vegetables and fruits directly
rather than passing such crops through another
trophic level and eating grain eaters.
Advantage:
1. It compares productivity because a time factor is
incorporated.
2. Biomass may not be equivalent to energy value, e.g.
1g of fat has many more kJ than 1g of cellulose or
lignin.
3. No inverted pyramids are obtained because of the
automatic degradation of energy quality.
4. The solar input of energy may be included as an extra
rectangle at the base.
5. Explains why the earth can support more people if
they eat at lower trophic level (by consuming grains,
vegetables and fruits directly ratherthan passing such
crops through another trophic level and eating grain
eaters
Disadvantage:
Obtaining the necessary data required in constructing
pyramids of energy flow is difficult.

WHAT IS BIODIVERSITY (BIOLOGICAL
DIVERSITY)?
The different life forms and life-sustaining processes that
can best survive the variety of conditions currently on
earth.
KINDS OF BIODIVERSITY:
Genetic diversity– variety in the genetic make up among
individuals within a species.
Species diversity (species richness) – number of species
present in a habitat.
Ecological diversity – the different biological
communities e.g. forests, deserts, lakes etc.
Functional diversity – biological and chemical
processes or functions such as energy flow and matter
cycling needed for the survival of species and biological
communities.
Distinguish between Species diversity (species richness)
and species abundance.
Species abundance: the number of individuals of each
species
Rare species
Species with small populations either restricted
geographically with localized habitats or with widely
scattered individuals.
Endangered species
Species with low population numbers that are in
considerable danger of becoming extinct.
Extinct species
Species, which cannot be found in areas they previously
inhabited nor in other likely habitats.
FACTORS THAT AFFECT SPECIES DIVERSITY
ON LAND AND IN WATER
1. Latitude (distance from equator) in terrestrial
communities – species diversity decrease steadily
with distance from the equator toward either pole,
resulting in the highest species diversity in tropical
areas e.g. tropical rain forests and lowest in polar
areassuchasarctic tundra. The main effect of latitude
is on temperature, which later affects life.
2. Depth in aquatic systems - in marine communities,
speciesdiversity increasesfrom the surface to a depth
of 2,000 metresand then begins to decline with depth
until the deep-sea bottom is reached, where species
diversity is very high. This change is attributed to
light penetration which affects photosynthesis,
availability of oxygen and availability of dead
organisms at the sea bottom.
3. Pollution in aquatic systems – increased pollution
kills off or impairs the reproductively of various
aquatic species hence reducing species diversity and
abundance.
4. Increased solar radiation increases species diversity
in terrestrial communities.
5. Increased precipitation in terrestrial communities
increases species diversity.
6. Increased elevation decreases species diversity.
7. Pronounced seasonal changes increase species
diversity.
FACTORS THAT AFFECT SPECIES DIVERSITY
IN AN ISLAND ECOSYSTEM
Robert MacArthur and Edward O. Wilson (1960s)
studied communities on islands after which they
proposed the species equilibrium model or the theory of
island biogeography.
According to this model, the number of species found on
an island is determined by a balance betweentwo factors:
(i) The rate atwhich new speciesimmigrate to the island
and
(ii) The rate at which species become extinct on the
island.
The model predicts that at some point the rates of
immigration and extinction will reach an equilibrium
point that determines the island’s average number of
different species (species diversity)
The model also predicts that immigration and extinction
rates (and thus species diversity) are affected by two
important features of the island:
i) Size of the island.
ii) Distance of the island from the nearest main land.

Size of the island
Larger islands tend to have higher species diversity than
Small islands because of two reasons:
Small islands generally have lower immigration rates
since they are a smaller target for potential colonizers
(ii) Smaller islands should have a higher extinction rate
because they generally have fewer resources and less
diverse habitats for colonizing species.
Distance of the island from the nearest main land:
For two islands of about equal size and other factors,the
island closest to the main land which is a source of
immigration, species will have the higher immigration
rate and thus a higher species diversity (assuming that
extinction rates on both islands are about the same. EXPLANATIONS FROM THE OBSERVATIONS
MADE FROM THE GRAPHS
a) Immigration and extinction rates:
The rate of immigration decreases with increase in
species number, while the extinction rate increases with
increase in species number on the island.
The equilibrium number of species on the island is
reached when immigration rate and extinction rate equal.
Extinction rate increases with increasing species number
because of interspecific and intraspecific competition for
the limited available resources.
b) Effect of island size on immigration and extinction
rates:
The rate of extinction increases with increase in species
number on the island on both small and large islands, but
it is higher on small islands than on large islands. The
higher extinction rate on small islands is because of the
fewer resources and less diverse habitats for colonizing
species.
The rate of immigration decreases with increase in
species number on both small and large islands. But with
a large island having a higher immigration rate than a
small island. Small islands generally have lower
immigration rates because they are a smaller target for
potential colonizers, while a large island has more
resources and becomes a large target for the incoming
species of animals.

c) Effect of distance from mainland on immigration
and extinction rates:
For both near and far islands, immigration rate decreases
with increase in species number, but immigration rate is
higher on near island than on the distant island. The
higher immigration rate on near island is because of the
easy reach by organisms enabled by its proximity to the
main land.
Since extinction rate increases with increasing species
number that exert interspecific and intraspecific
competition, extinction rate is far higher on small islands
due to the fierce competition caused by the higher
immigration rate because of easy reach by organisms
enabled by its proximity to the main land. (UNEB 2006
P2)
BIOGEOCHEMICAL CYCLING (NUTRIENT
CYCLING)
This the process by which chemical compounds of a
particular element that constitutes living matter are
transferred between living organisms (biotic phase) and
non-living environment (abiotic phase).
These cycles driven directly or indirectly by incoming
solar energy and gravity include the carbon, nitrogen,
phosphorus, oxygen, Sulphur and hydrological (water)
cycles, but a few have been considered below.
The earth’s chemical cycles also connect past, present
and future forms of life. Just imagine:
i) Some of the carbon atoms in your skin may once have
been part of a leaf.
ii) Some of the oxygen molecules you just inhaled may
have been inhaled by a person a billion years ago!
1. HYDROLOGICAL (WATER) CYCLE
The water cycle is powered by energy from the sun and
by gravity, and it involves the following main processes:
a) Evaporation (conversion of water into water vapour)
b) Transpiration (evaporation from leaves of the water
extracted from soil by roots and transported
throughout the plant)
c) Condensation (conversion of water vapour into
droplets of liquid water)
d) Precipitation (rain, hail, snow and sleet)
e) Infiltration (movement of water into soil)
f) Percolation (downward flow of water through soil
and permeable rocks to ground storage areas called
aquifers)
g) Runoff (downslope surface movement backto the sea
to resume the cycle)
2. NITROGEN CYCLE
Nitrogen is the atmosphere’s most abundant element,
with chemically unreactive nitrogen gas making up 78% of
the volume of the troposphere. However, N2 cannot be
absorbed and metabolized directly by multicellular plants
and animals.
Atmospheric electrical discharges in the form of
lightning causes nitrogen and oxygen in the atmosphere to
react and produce oxides of nitrogen, which dissolve in
rainwater and fall to the ground as weakly acidic solutions
.
Nitrogen fixation occurs when the nitrogen in soil is
reduced to ammonium ions, catalysed by nitrogen-fixing
bacteria which may be free-living e.g. Azotobacter and
Clostridium; symbiotic bacteria in root nodules e.g.
Rhizobium or blue- green algae e.g. Nostoc.
Nitrification occurs when ammonium compounds in soil
are converted first to nitrite ions (highly toxic to plants) by
Nitrosomonas bacteria and later to nitrate ions by
Nitrobacter bacteria.
Ammonification (putrefaction) occurs when
decomposers e.g. saprophytic bacteria and fungi convert
nitrogen-rich organic compounds, wastes like urea and
dead bodies of organisms into ammonia and ammonium
ion-containing salts.
Assimilation occurs when inorganic ammonia,
ammonium and nitrate ions are absorbed by plant roots to
make nucleic acids, amino acids and protein.

 Denitrification occurs when mostly anaerobic bacteria
e.g. Pseudomonas denitrificans and Thiobacillus
denitrificans in water logged soil and deep in ocean, lake
and swamp bottoms convert ammonia and ammonium
ions back into nitrite and nitrate ions, and then into
nitrogen gas and oxygen. Nitrogen gas is released into
the atmosphere while oxygen is used for the respiration
of these bacteria.
How human activities affect the nitrogen cycle
1. Burning of fuels forms nitric oxide, which reacts
with atmospheric oxygen to form nitrogen
dioxide gas that reacts with water vapour to form
acid rain containing nitric acid. Nitric acid
together with other air pollutants:
i Damages trees
ii Corrodes metals
iii Upsets aquatic ecosystems.
2. The inorganic fertilizers applied to soil are acted
upon by anaerobic bacteria to release nitrous
oxide into the stratosphere, where it;
i Contributes to ozone depletion
ii Contributes to greenhouse effect.
3. Nitrogen is removed from top soil when we;
harvest nitrogen-rich crops
i irrigate crops
ii burn or clear grasslands and forests before
planting crops
4. Adding nitrogen compounds to aquatic
ecosystems e.g. sewage algal blooming, which
upon death, their decomposition causes oxygen
shortage resulting into death of aerobic
organisms e.g. some fish.
5. The accelerated deposition of acidic nitrogen
containing compounds e.g. NO2 and HNO3 onto
terrestrial ecosystems stimulates growth of
weeds, which outcompete other plants that
cannot take up nitrogen as efficiently.
CARBON CYCLE
 Based on carbon dioxide gas, making up 0.036% of
the volume of the troposphere and is also dissolved
in water.
 Carbon fixation involves the reduction of carbon
dioxide to large organic molecules during
photosynthesis and chemosynthesis.
 During aerobic respiration by all organisms, carbon
dioxide is returned to the atmosphere or dissolves in
water.
 Over millions of years, buried deposits of dead plant
debris and bacteria are compressed between layers of
sediment to form the carbon-containing fossil fuels
e.g. coal, oil and natural gas, which when burnt
release carbon dioxide into air.
 In aquatic ecosystems, carbon dioxide may;
(i) remain dissolved
(ii) be utilized in photosynthesis
(iii) React with water to form carbonate ions and
bicarbonate ions. As water warms, more

dissolved carbon dioxide returns to the
atmosphere.
 In marine ecosystems, some organisms take up
dissolved carbon dioxide molecules, carbonate ions
and bicarbonate ions and these ions react with
calcium ions to form calcium carbonate (CaCO3) to
build their shells and skeletons.
 When the animals with calcium in shells and
skeletons die and drift into deep bottom sediments of
oceans, immense pressure causes limestone and
chalk to form after a very long period of time.
 Weathering processes release a small percentage of
carbon dioxide from limestone into the atmosphere.
How human activities affect the carbon cycle
(i) Cutting trees and other plants that absorb CO2
through photosynthesis increases carbon dioxide in
the atmosphere.
(ii) Burning of fossil fuels like coal, petroleum oil etc
and wood adds large amounts of CO2 into the
troposphere.
THE BIOGEOLOGICAL CYCLES IN SUMMARY
1. Hydrologic cycle (water cycle).
• Reservoirs: oceans,air (as water vapor), groundwater,
glaciers. (Evaporation, wind, and precipitation move
water from oceans to land.)
• Assimilation:plants absorb waterfrom the soil; animals
drink water or eat other organisms (which are mostly
water).
• Release: plants transpire; animals and plants
decompose.
2. Carbon cycle. Carbon is required for the building of
all organic compounds.
• Reservoirs: atmosphere (as CO2), fossil fuels (coal,
oil), peat, durable organic material (cellulose, for
example).
• Assimilation: plants use CO2 in photosynthesis;
animals consume plants or other animals.
• Release: plants and animals release CO2 through
respiration and decomposition; CO2 is released when
organic material (such as wood and fossil fuels) is
burned.
3. Nitrogen cycle. Nitrogen is required for the
manufacture of all amino acids and nucleic acids.
• Reservoirs: atmosphere (N2); soil (ammonium,
ammonia, or nitrite, nitrate).
• Assimilation: plants absorb nitrogen either as NO3 – or
asNH4+
;animals obtain nitrogen by eating plants or other
animals. The stages in the assimilation of nitrogen are as
follows:
Nitrogen fixation: N2 to NH4
+
by prokaryotes (in soil and
root nodules); N2 to NO3 by lightning and UV radiation.
Nitrification: NH4+to NO2
–
andNO2– toNO3– by various
nitrifying bacteria.
NH4+ or NO3– to organic compounds by plant
metabolism.
• Release: denitrifying bacteria convert NO3
–
back to N2
(denitrification); detrivorous bacteria convert organic

compounds back to NH4
+
(ammonification); animals
excrete NH4+ (or NH3), urea, or uric acid.
Qn. (a) Describe the flowofenergy and the cycling of
carbon and nitrogen in any named ecosystem.
(b). Suggest reasons why felling and removal of
forest trees result in changes in the levels ofnutrients
in the soil.
HOW BIOTIC FACTORS AFFECT THE
DISTRIBUTION AND ABUNDANCY OF
ORGANISMS
Biotic factors are those that arise in organisms
interacting with each other. Examples include (i)
diseases (ii) competition, (iii) parasitism, (iv) pollution,
(v) pollination &dispersal,(vi) antibiosis (vii) mimicry.
. (a) Human influence.
 Of all living organisms, humans exert most
influence on the distribution and survival of other species
through a multitude of activities like pollution,
deforestation, farming, construction etc
Man is also a predator hunting down many animals to a
point of extinction.
b) Competition
 This is a relationship whereby two individuals of
the same species or different species struggle to obtain
resources which are in limited supply. E.g plants
competing for light, carbon dioxide, water, minerals,
pollinators, and sites for spores and seeds to germinate
while animals compete for food, mates, breeding sites
and shelter from predators.
(i) Intraspecific competition
 Is the competition between members of the same
species for the same resources.
 Intraspecific competition tends to have a stabilizing
influence on population size.
 If the population getstoo big, intraspecific population
increases, so the population falls again.
 If the population gets too small, intraspecific
population decreases, so the population increases
again.
(ii) Interspecific competition
Is the competition between members of two or more
different species for food, space, good hiding place,
water, sunlight, nesting sites or any other limited
resource.
Competition is very intense when there is significant
overlap of niches, and in this case one of the competing
species must;
1 Migrate to another area if possible
2 Shift its feeding habits or behaviour through
natural selection and evolution
3 Suffer a sharp population decline or
4 Become extinct in that area, otherwise two
species can never occupy exactly the same ecological
niche.
According to Gause’s (Russian biologist) competive
exclusion principle “no two species can occupy the same
ecological niche”
e.g (i). Two species of flour beetles, Triboliumcastenum
and T. confusum were kept in the laboratory in bottles of
flour acting as a habitat and providing food for them,
under variable temperature conditions(24-34) and humid
conditions (very humid , 70%RH& 30% RH).
Observation. At high temperatures and in very humid
conditions, Triboliumcastenum succeded better,while at
low temperatures and very dry conditions T. confusum
did better. Whatever the conditions, only one of the
species eventually survived.
(ii). Two speciesof ParameciumAurelia and P.caudatum
were grown separately in the same culture, then later
cultured together.

Observation
1) When cultured separately, each species has
maximum population, only coming almost
constant with time due to;
(i) -Presence of toxic wastes which can poison
paramecium.
(ii) -Heat generated during respiration may kill
some paramecium.
(iii) -Decrease in food measures.
2) When the two species of paramecium are
cultured together, paramecium aurelia gets
competitive advantage over P. caudatum and
after several days, population of P. caudatum
gradually decreases and later decreases rapidly
until its excluded hence competitive exclusion
principle. P.caudatum therefore,goes to
extinction. Competitive advantagesof P.aurelia
are;
(i) High rate of reproduction.
(ii) High growth rate.
(iii) Good nutrient absorptive
capacity/greater efficiency in obtaining
food.
(iv) Being small, it requires less food hence
can easily survive when food is scarce.
- Survivorship, long life span.
EXPERIMENTS WITH FLOUR BEETLES:
Tribolium, beetles of the family Tenebrionidae, attack
stored grains and grain products. Thomas Park (1948)
explored interspecific (interspecies) competition between
Tribolium confusum and Tribolium castaneum. The
variables studied included climate, initial density, food,
volume of flour and presence or absence of a parasite
called Adelina.
One such experiment was conducted under six
environmental conditions below:
The results of the experiment were summarized as
follows:
 Hot-moist (340
C, 70% RH) Single- Both
populations persisted over the entire duration of
the experiment in equal proportions, hence T.
confusum population is equal to that of T.
castaneum
 Hot- moist (340
C, 70% RH) Mixed, Tribolium
castaneum excludes Tribolium confusum
 Cool-dry (240
C, 30% RH) Single, Tribolium
castaneumdies off after a short while therefore,
T. confusum is greater than T. castaneum
 Cool-dry (240
castaneum was excluded.
toget
her
.

 Temperate- moist (290
C, 70% RH) single, Both
populations persisted, but T. castaneum thrived
better, hence, T. confusum was less than T.
castaneum
 Temperate- moist (290
C, 70% RH) mixed, T.
castaneum excluded T. confusum more times
 Hot-dry (340
C, 30% RH) Single, Both
populations persisted, but T. confusum shrived
better, hence, T. confusum is greater than T.
castaneum
 Hot-dry (340
confusum excluded Tribolium castaneum more
times
 Temperate-dry (290
C, 30% RH) single, Both
populations persisted, but T. confusum thrived
better, hence, T. confusum is greater than T.
castaneum
 Temperate-dry (290
C, 30% RH) mixed,
Triboliumconfusum won Tribolium castaneum
 Cool- moist (240
C, 70% RH) Single, Both
populations persisted, but T. castaneum thrived
better hence T. confusum is less than T.
castaneum
 Cool- moist (240
confusum won Tribolium castaneum.
Deductions and interpretations of results:
i) Cool-dry conditions appear to favour Tribolium
confusum
ii) Under a particular set of conditions, either Tribolium
confusum or Tribolium castaneum was usually
favoured, but not always.
iii) Under intermediate environment conditions, each
species did well when grown alone but the outcome
of interspecific competition was not completely
predictable. Sometimes T. confusum won,
sometimes T. castaneum won
iv) Growing the two species separately showed that the
fundamental niche of Tribolium castaneum includes
five of the six environmental conditions in the
experiment, while the fundamental niche of
Tribolium confusum includes all the six
environmental conditions.
v) Growing the two species together suggests that
interspecies competition restricts the realized niches
of both species to fewer environmental conditions.
vi) Interspecific competition restricts the realized niches
of species in nature.
HOW SPECIES REDUCE OR AVOID
COMPETITION THROUGH RESOURCE
PARTITIONING
Resource partitioning is the dividing up of scarce
resources so that species with similar needs use them (i)
at different times (ii) in different ways or (iii) in different
places.
 Some species that are in competition for the same
resources have evolved adaptations that reduce or avoid
competition or an overlap of their fundamental niches.
 Resource partitioning decreases competition
between two species leading to increased niche
specialization
Examples of resource partitioning:
1. When living in the same area, lions prey mostly on
larger animals while leopards on smaller ones.
2. Hawksand owls feedon similar prey,but hawks hunt
during the day and owls hunt at night.
3. Each of the five species of common warblers (insect-
eating birds) minimizes competition with the others
by
(i) Spending at least half its feeding time in a
different part of spruce tree branches e.g. some
hunt at the extreme top, others at the lower
portion, some mid-way etc
(ii) Consuming somewhat different insect species.
4. Different speciesof eagles in a forestfeed at different
times of the day e.g. bald headed eagles are most
active early mornings and evenings while the white-
breasted eagles feed vigorously towards noon.
5. When three species of ground finches of Galapagos
Islands occur on separate islands, their bills tend to
be the same intermediate size, enabling each to feed
on a wider range of seeds, but where they co-occur,
there is divergence in beak size to suit each finch
species to feeding on seeds of either small, medium
or large size, but not all sizes.

6. In an abandoned field, drought tolerant grasses with
shallow, fibrous root system grow near the soil
surface to absorb moisture; plants with a taproot
system grow in deeper soil while those with a taproot
system that even branches to the topsoil and below
the roots of other species grow where soil is
continuously moist.
NB:
i) The more than two species in the same habitat
differ in their use of resources,the more likely they
can coexist.
ii) Two competing species also may coexist by
sharing the same resource in different ways or at
different times.
iii) The tendency for characteristics to be more
divergent when populations belong to the same
community than when they are isolated is termed
character displacement e.g Galapagos finches.
(c.) Predation.
This is a relationship whereby members of one species
(the predator) feed on all or part of a living organism of
another species (the prey). Therefore, predators are only
found where there is prey e.g.herbivores are found where
there is suitable plant material.
A predator is an animal that feeds on another live
organism.
A prey is the live organism that is fed on by the predator.
PREDATOR-PREY INTERACTIONS IN
ECOSYSTEMS
Description of the changes in population numbers:
Initially, the population of the prey is higher that the
population of the predator.
Within a short time, both populations of prey and
predator increase rapidly.
The population of the prey reachesa maximum earlier the
predator.
As the prey population decreases rapidly, the predator
population continues to increase gradually for a short
than time to a maximum then also decreases rapidly. As
the predator population continues to decrease, the prey
population starts to increase rapidly, followed by a rapid
increase in predator population. The cycle is repeated.
Explanation for the observed changes in populations:
At the beginning, there are more prey than predator to
provide food to the predators.
When the predator population is low, they get enough
food and few preys are eaten so they both increase
rapidly.
The large number of preys provides food to predators, so
they reproduce fast and increase in numbers.
The increased predator population eats many preys and
the prey population crashes.
The decrease in prey numbers causes the predators to
starve and even their reproduction reduces, so the
predator numbers crash. Finally, the very low number of
predators allows the prey population to recover, causing
the cycle to start again.
Evolutionary significance of predator –prey
Predation usually eliminates the unfit (aged, sick, weak).
This gives the remaining prey accesstothe available food
supply and also improves their genetic stock hence,
enhances the chances of reproductive success and
longtime survival, thus pass on their good traits to their
off springs which can improve their evolution.
How are the predator suited for capturing prey?
1. Have keen eyes for locating prey eg wolves, African
lions hunt in groups.
2. Preying mantis, chameleon have cryptic
coloration/camouflage that enable them to walk to
prey unnoticed..
3. Nocturnal predators eg bats have highly developed
sense for detecting sound made by prey.
4. Some snakes which have glands to secrete poison
(venom) which the fangs inject into prey to
immobilize it (prey).
5. Web-spinning spiders use their silky cob webs to
catch small sized ground walking or flying insects.
6. Ant-lions lay traps by making pits in the ground
where preys fall
7. Some have soft pads at the bottom of their feetso that
they are not easily detectedasthey walk towards prey
8. Some have stinging cells which paralyze their prey
e.g sea anemones

9. Have long and sharp canines which pierce and kill
prey
10. Well-developed limbs which increase the speed of
locomotion to chase and capture prey.
How are prey species suited to avoid predation?
1) Ability to run, swim or fly faster.
2) Possession of highly developed sense of sight or
smell alerting the presence of predators.
3) Possession of protective shells eg in tortoise and
snails for rolling into armour-plated ball
4) Possession of spines to prick the predators.
5) In some lizards, the tail breaks off when attacked
giving the animal (lizard) time to escape.
6) Possession of spines (porcupines) or thorns (cactiand
rose-bushes) for pricking predators.
7) In some lizards tails break off when attacked,giving
the animal enough time to escape.
8) Some prey camouflage by changing colour e.g.
chameleon and cuttlefish, or having deceptive
colours that blend with the background e.g. arctic
hare in its winter fur blends into snow.
9) Some prey species discourage predators with
chemicals that are poisonous (e.g. oleander plants),
irritating (e.g. bombardier beetles), foul smelling
(e.g. stinkbugs and skunk cabbages) or bad tasting
(e.g. monarch butterflies and buttercups)
10) Some prey species have evolved warning coloration
– contrasting pattern of advertising colours that
enable predators to recognize and avoid such prey
e.g. the poisonous frogs, some snakes, monarch
butterflies and some grasshoppers.
11) Some species gain protection to avoid predation by
mimicking (looking and acting like) other species
that are distasteful to the predator e.g. the non-
poisonous viceroy butterfly mimics the poisonous
monarch butterfly.
12) Batesian mimicry occurs when the palatable species
mimics other distasteful species e.g . Viceroy
butterfly mimics the poisonous monarch butterfly,
the harmless hoverfly mimics the painful stinging
wasp while:
13) Mullerianmimicry occurswhen both the mimic and
mimicked are unpalatable and dangerous e.g. the five
spot Burnet and related moths.
14) Other preys gain some protection by living in large
groups e.g. schools of fish, herd of antelope, flocks
of birds.
15) Some prey scare predators by puffing up e.g.
blowfish, or spreading wings e.g. peacock.
16) The flesh of some slow-moving fish is poisonous e.g.
porcupine fish.
17) Some preys secrete poisonous or repellant substances
e.g. scorpions, caterpillars, some grasshoppers, culex
mosquito eggs
18) The electric fish Malapterurus (a cat fish) produces
high voltage discharge of up to 350v that shocks any
predator that makes contact with it.
19) Other preys employ alarm signals and calls e.g. ants,
various fish, small birds and mammals.
20) Group defense, occurring among those that live and
feed in herds.
NB. Camouflage is the use of any combination of
materials, coloration, or illumination for concealment,
either by making animals difficult to see,or by disguising
them as something else.
Exists in various forms;
(i) Warning coloration,conspicuous colouring that
warnsa predator that ananimal is unplalable or poisonous
e.g poisonous frogs, some snakes, monarch butterflies,
and some grasshoppers
(ii) Disruptive colouration/patterning, works by
breaking up the outlines of an animal with a strongly
contrasting pattern, thus decreasing detectability e.g.
group of zebras
(iii) Cryptic colouration allows an organism to
match its background and hence become less vulnerable
to predation e.g chameleon.
NB: Predation
1. -Determines distribution and abundance of the prey
because predators will always be found in places of
their potential prey.

2. -leads to dispersal of animals which reduces
competition, since it involves movement of animals
from place to place.
3. Is a biological control method.
(d) Pollination and dispersal
1. Pollination is an ecological interaction because plants
and animals interact with each other. Insects transfer
pollen grains from anthers to stigma.
2. Dispersal of seeds and fruits introduces new plants to
new habitats and this minimizes competition among
species.
3. Both interactions between the flowering plants and
animals like insects, birds & bats may be highly
elaborate and species specific.
4. This co- evolution ensures that the distribution of the
plants with their pollinations or agents of dispersal
are related e.g arum lily flowers are pollinated by
dung flies.
NB. Co evolution is a long term evolutionary adjustment
of two or more groups of organisms that facilitate those
organisms living with one another.
Examples include;
(i) Many features of flowering plants have evolved
as a result of dispersal of plant’s gametes by
insects and insects have in turn evolved special
traits for obtaining nectar
(ii) Grasses have evolved the ability to deposit silica
in their leaves and stems to reduce their risks of
being grazed, large herbivores have in turn
evolved complex molars with enamel ridges for
grinding up the plant material.
(e) Antibiosis; is the secretion by organisms chemical
substances into their surrounding that may be repellant to
members of the same species or different species e.g.
penicillium (a fungus) secretes antibiotics that inhibit
bacterial growth, ants release pheromones to warn off
other members of a species in case of danger.
Two types exist i.e
(i) Intraspecific antibiosis secretion by organisms
chemical substances into their surrounding that
may be repellant to members of the same species
e,g male rabbits secrete pheromones from their
submandibular salivary glands that are used to
mark territory as a warning to other bucks that
the territory is occupied
(ii) Interspecific antibiosis secretion by organisms
chemical substances into their surrounding that
may be repellant to members of the different
species e.g penicillium (a fungus) secretes
antibiotics that kill or prevent the bacterial
growth.
(f) Parasitism
An organism called parasite obtains part or all its
nutrients from the body of another organism of different
species called host.
The parasite is usually smaller than its host in size.
Parasites do not usually kill their hosts, but the host
suffers harm.
Many parasites live permanently on (ectoparasites) or in
their hosts (endo parasite) while some visit their hosts
only to feed.
Some parasites are facultative, live on or in the host for
some time e.g.Pythium(a fungus) that causesdamping off
seedlings, on killing the seedlings, lives as a saprophyte
on their dead remains and others are obligate (live on or
in the host for their entire lives.)
(g) Mutualism. Is an interspecific association in which
both organisms benefit.
Examples include
(i) Cellulose digesting bacteria in gut of
ruminants such as goats, cattle& sheep.
Ruminants obtain sugars, amino acids while
bacteria obtains shelter and food.

(ii) Leguminous plants e.g clover and nitrogen fixing
bacteria (Rhizobium). The plants obtain nitrates
while bacteria obtains shelter, sugars, vitamins.
(iii) Microorganisms and cellulose digestion.
Interspecific mutualism is demonstrated by the
flagellate protozoan, Trichonympha an obligate
anaerobe in the gut of several species of wood
eating termites where it digests cellulose.
Trichonympha also occurs in the alimentary
canal of wood eating roach Cryptocerus. The
termite and roach reduce the wood to small
fragments, passing them through the alimentary
canalto hind gut, where the protozoans digest the
cellulose, changing it into sugar. The host
benefits the protozoa by removing harmful
metabolic waste products and maintaining
anaerobic conditions in the intestine.
(iv) Mycorrhizae (fungus and root of higher plants)
In ectotrophic mycorrhiza, the fungus forms a
sheath covering lateral roots of forest trees such
as oaks, beech, conifers, while depending on
photosynthesis by the tree to provide organic
materials.
Endotrophic mycorrhiza involves most of
fungi inside the root of orchids with the fungi
digesting lignin and cellulose in the soil; and
passing the end products into the roots of plants.
(v) Lichens; algae and fungus. Algae carries out
photosynthesis, providing nutrients to the fungus
while the fungi it is protected by the fungi from
intense sunlight and dessication, minerals
absorbed by the fungus are passed onto it.
(vi) Hermit Crab And Sea Anemones, with the
hermit crab (Eupagurus berhardus) obtaining
defence from the stinging cells of anemones
(Adamsia) & camouflaging from its predators.
Sea anemones feed on food remains of the crab
& obtains free transport from one area to another
NB: Some ecologists place the interaction of sea
anemone and hermit crab under commensalism,
yet some books also describe it as
protocooperation.
(h) Commensalism.Isanassociation between organisms
of different species in which one benefits while the other
neither benefits nor its harmed e.g
(i) Cow and white egrets; egrets are associated with
large herbivores which during grazing, attract
insects which are eaten by the birds.
(ii) Epiphytes and host plant. Many epiphytes
develop a thick network of roots upon which
windblown dust accumulates and provides the
necessary edaphic environment where it obtains
nutrients for growth and development.
(i) Many harmless protozoans occur in the intestinal
tract of mammals, including man. Some
microorganisms such as bacterium Escherchia
coli is found in human colon.
ECOLOGICAL SUCCESSION
 This is a long- term directional change in the
composition of a community from its origin to its climax
through a number of stages brought about by the actions
of the organisms themselves.
 It is a process by which plants and animal
communities in a given area change gradually over time,
becoming replaced by different and usually more
complex communities.
 Pioneers are first sets of organisms to occupy the
area, collectively such organisms constitute the pioneer
community.
 Climax community: the final community at the
end of succession, which a particular environment can
sustain. Climax community is characterisedby (i) diverse
species (ii) complex feeding relationships and (iii)
progressive increases in biomass.
 The process of succession continues through
stages known as seral stages and there are a number of
sere (complete succession) according to the environment
being colonized:
(i) Hydrosere; succession in aquatic environment
(ii) Halosere; succession in salty environment
(iii) Xerosere; succession dry envirionments e.g
deserts
(iii) Lithosere; succession on a rocky surface.
Types of succession
a) Primary succession
b) Secondary succession

a) Primary succession
This is the gradual change in species composition of an
area that has never had any vegetation growing on it.
It occurson Bare rocks exposed by erosion, newly cooled
lava, newly created shallow ponds, Sand dunes,
Abandoned highway or parking yard.
Description of Primary succession on land
 If a bare rock is left undisturbed for a long period
of time, it creates a favorable environment for the
colonization of the area by primitive plants like the
lichens and mosses.
 Lichens and mosses attach to bare rocks and start
forming soil by trapping windblown soil particles,
producing tiny bits of organic matter due to their death
and secreting mild acids that slowly breakdown the rock.
Mosses also survives/withstands desiccation by
absorbing moisture in the air. Alternate heating and
cooling also causes break down of rocks. Mosses and
lichens are therefore the pioneer community in the area.
 As patches of soil build up and spread, it creates
a suitable environment for more other species of plants
and eventually the pioneer species are replaced by the
early successional plants like small grasses and ferns,
whose seeds and spores respectively germinate after
arriving by wind or in droppings of birds.
 Some of their roots penetrate and break rocks
into soil particles, and death and decay of small grasses
and ferns increases nutrients in soil.
 After a long period of time, the soil becomes
deep, moist and fertile enough to support the growth of
mid successional plant species like herbs, large grasses,
low shrubs and small trees that need a lot of sunlight.
 Late successionalplant species (mostly trees that
tolerate shade) later replace the mid successional plant
species.
 Unless natural or human processes disturb the
area, a complex forest community remains
Characteristics of the stages of primary succession;
a) Early succession
 Species grow very close to the ground and have
low biomass.
 Species have short life span.
 Species are simple and small sized.
 Species diversity (number of species present in a
habitat) is very low.
 Community is open ie allows space for other
colonizers.
 Species may show symbiotic relationships to aid
their establishment.
 Species are poor competitors and hence get
replaced by higher, more demanding plants like grasses,
shrubs and trees.
 The community is mostly is mostly composed of
producers and a few decomposers.
 Net productivity is high.
 Feeding relationships are simple, mostly
herbivores feeding on plant with few decomposers.
b) Late succession
 Plants are of large size and complex.
 Species diversity is high

 Community is a mixture of producers,consumers
and decomposers.
 Biomass is high
 Net productivity is low
 Community takes a longtime to establish.
 Climax community is often determined by one
dominant species.
 There is increased soil depth and nutrients.
 Interspecific competition is very high.
 There is little space for new species
 The climax community is stable and is in
equilibrium with its environment.
 Feeding relationships are complex, dominated
by decomposers.
PRIMARY SUCCESSION IN A WATER BODY
The first community to occupy the lake consists of
pioneer species with r-selected characteristics. These
characteristics include good dispersal ability, rapid
growth, and rapid reproduction of many offspring.
The lake is first populated by algae and protists, then
followed by rotifers, mollusks, insects, and other
arthropods.
Various vegetation, such as grasses, sedges, rushes, and
cattails, grows at the perimeter of the lake. Submerged
vegetation (growing on the lake bottom) is replaced by
vegetation that emerges from the surface, perhaps
covering the surface with leaves.
As the plants and animals die, they add to the organic
matter that fills the lake. In addition, sediment is
deposited by water from streams that enter the lake.
Eventually, the lake becomes marshy as it is overrun by
vegetation.
When, the lake is completely filled, it becomes a
meadow, occupied by plants and animals that are adapted
to a dry, rather than marshy, habitat. Subsequently, the
meadow is invaded by shrubs and trees from the
surrounding area.
In a temperate mountain habitat, the climax community
may be a deciduous forest consisting of oaks or maples.
In colder regions, the climax community is often a
coniferous forest, consisting of pines, firs, and hemlocks.
SECONDARY SUCCESSION.
This is the gradual change in species composition of an
area where the natural community of organisms has been
disturbed, removed or destroyed but some soil or bottom
sediment remains.
It occurs on abandoned farmlands, burnt or cut forests,
heavily polluted streams, flooded land.
Due to some soil or sediment present, vegetation usually
begins to germinate within a few weeks.
Seeds and spores can be present in the soil and can be
carried from nearby plants by wind, birds and insects.
The ground may even contain resistant plants/vegetative
organs of the colonizing plants that survived the changes.
Reasons why invasive species are often successful in
colonizing new habitats.
1. No natural predators, parasites, pathogens;
2. Effective aggressive mechanism of invasive
organism;
3. No limitation on resources.
4. No environmental inhibitors (e.g., pollutants).
5. R-selected species; increased season for
reproduction; large or logarithmic populations.
6. Variation in phenotype of large population.
7. Available niche not occupied by any other species,
hence no successful competitors.
8. Prey lack effective defense mechanism against
introduced species.
9. Appropriate environmental conditions (e.g., rainfall,
temperature).
TYPICAL EXAMINATION QUESTION
Explain the sequence of changes that will occur in a
previously burnt piece of land from its initial stages
until a climax community. (11 marks)
Approach
Its secondary succession ; pioneer organisms are
fast growing annual herb plants ; like Bidens pilosa/
commelina species ; and animals such as
Insects/detritivores(earthworms) ;theseorganismsdie
,decomposeandadd organic matterinto thesoil ; a few
years later, perennial herbs ; such as Lantana camara

beginstoreplace theannualherbs andestablishthemin
thearea; Manyyearsafter,shrubslikeacacia replace the
perennial herbs ; birds begin to inhabit areas where
acacia is present ; the litter from the falling Leaves
accumulate in the area whose decompositionadd more
organicmatterintothesoil;thicknessofsoilisincreased
;thiscreatesfavourable conditionsforthetreestogrow
and larger animals live in and climax community is
attained ; ;@1/2 mark = total 11 marks
During succession:
1) Each species facilitates the emergence of other
species by modifying the environment, making it
more suitable for new species with different niche
requirements.
2) Early species inhibit / hinder the establishment and
growth of other species by releasing toxic chemicals
that reduce competition from other plants.
3) Late successional plants are largely unaffected by
plants at earlier stages of succession, explaining why
late successional plants can thrive in mature
communities without eliminating some early
successional and midsuccessional plants.
4) Plagioclimax climax community: one that is
gradually established after human interference,and it
appears very different from the original climax. This
is termed deflected succession:
POPULATION DYNAMICS
These are changes in population in response to
environmental stress or environmental conditions.
A population is a group of organisms of the same species
living together in a given place at a particular time.
TERMS USED IN POPULATION STUDIES:
Population size: Number of individuals in a population.
Population density: Total number of organisms of a
species per unit area (land) or per unit volume (water)
Population growth: A change in the number of
individuals (increase-positive or decrease-negative)
Population growth rate; Change in number of individuals
per unit time
Birth rate (natality): Number of new individuals
produced by one organism per unit time (Humans: per
year). Expressed as the number of individuals born in a
given period for every 1000 individuals e.g 36 births per
1000 people per year.
Deathrate (mortality): Number of individuals dying per
unit of time per unit of population (humans: number of
deaths per 1000 per year e.g. 20 deaths per 1000 people
per year)
Environmental resistance: All the environmental
factors acting jointly to limit the growth of a population.
Carrying capacity: Maximum number of individuals of
a given species that can be sustained indefinitely in a
given area of land or volume of water.
Age structure/distribution; is the proportion of
individuals of each age in a population.
The young-age group before reproduction
Middle age- reproductive age
Old age-age after reproductive stage
Biotic potential:Maximum rate at which the members of
a given population can reproduce given unlimited
resources and favourable environmental conditions.
Immigration: Movement of individuals into a population
from neighboring populations.
Emigration: Departure of individuals from a population.
Rare species: Species with small populations either
restricted geographically with localized habitats or with
widely scattered individuals.
Endangered species: Species with low population
numbers that are in considerable danger of becoming
extinct.
Extinct species: Species,which cannot be found in areas
they previously inhabited nor in other likely habitats
Population distribution/dispersion - distribution of
organisms in a habitat.
The nature of distribution of organisms of a given
population in a particular area depends on:
1. The nature of distribution of physical resources
and conditions necessary for the survival of the
organisms e.g snailsin an area with a stream
passing through will have a linear displacement
on the banks of the stream.

2. The distribution of other organisms with which
the species interacts e.g predators, prey, mutual
commensals, parasites, hosts and competitors.
3. The mode of life of the organisms, particularly
the intraspecific relationships between the
number of the same species e.g social organisms
such as bees, termites, humans tend to develop
clumped distribution, while antisocial organisms
or solitary organisms like hyenaand keckos tend
to have sparse distribution.
Five main types exist i.e
(i) Uniform/even distribution this is where the
individuals are distributed uniformly in an area
or volume.
This distribution arises if the environmental
resources and conditions are evenly distributed over
an area.
Conditions for a uniform distribution to occur
1) Severe struggle for resources
2) Some plants produce natural growth
limiter/inhibitor e.g. terpenes released by Salvia
3) Territoriality ensuresthat animals are spacedout.
(ii) Linear distribution; this is where the
individuals of a population form a single file in
the area. This distribution arises when the
resources and conditions are distributed linearly
e.g snails on the banks of a stream, human
settlements around roads and railways.
(iii) Clumped/clustered distribution organisms
aggregate into groups to gain better protection,
feeding, reproduction etc. Clumped dispersion is
the most common pattern of population
distribution.
NB. Main characteristics of a population are (i) density
(ii) dispersion (iii) age structure (iv ) natality (v)mortality
(vi) population size.
Conditions for clustered distribution to occur
 Localization of /patchy/uneven resources;
 Inability to move independently from habitat
e.g. Eaglets;
 Social interactions between individuals;
 Dispersal mechanisms;
 Need for protection from predation
 Increasing predation chances
 Extinct/threatened species that share traits.
 Related taxa share habitat types where
human-induced threats are concentrated
(iv) Random distribution organisms are dispersed
by chance with neither forces of attraction nor
repulsion and the environmental resources are
randomly distributed in the area.
Conditions for random distribution to occur
1) Individuals are arranged without any apparent
pattern
2) Homogeneous environments
3) Environmental conditions / resources are
consistent
4) No/weak social interaction within species
(v) Sparse population distribution; this occurs
when the individuals are sparsely distributed

over an area. It arises when environmental
resources and conditions are sparsely distributed
over the area or due to the solitary nature of the
organisms e,g in cacoons
Population- age structure
The ration of different age groups (pre-, reproductive and
post reproductive) are important in predicting the rate of
population growth.
The higher the number of pre-reproductive age group
individuals compared to the other age groups, the higher
the population growth rate.as shown below.
Age structure:
i) The young – age group before reproduction.
ii) Middle age – reproductive age.
iii) Old age – age after reproductive stage.
POPULATION GROWTH PATTERNS
 Population grows when(i) natality is greater than
mortality (ii) immigration is greater than emigration
 Population growth may form a curve which is
either (i) exponential population growth curve (J-shaped)
(ii) logistic population growth curve (Sigmoid/S-shaped)
(i) Exponential population growth (J-shaped curve)
 It is a theoretical population growth curve in
which the population growth rate increases with time
indefinitely.
 Population growth starts out slowly and then
proceeds faster and faster as the population increases.
 It occurs when resources are unlimited and the
population can grow at its intrinsic rate of growth.( rate
at which a population would grow if it had unlimited
resources)
 Howeverthis is rare in nature because of limiting
factors (environmental resistance).
Description
Number of individuals (population) is small. Their
number increases gradually/slowly with time along AB.
Later the population size increases
rapidly/sharply/drastically with time along CB.
Explanation
Stable
Productivity
Increasing
Productivity
Declining
Productivity
C
B

Initially, the number of individuals increases gradually
with time because the population size is small, thus few
reproducing individuals,reproducing individuals are
scattered within the environment, some may not have
reached reproductive age, organisms are still getting used
to their environment. Later on, number of individuals
increases rapidly because many individuals have now
reached reproductive age, & number reproducing
individuals now gets bigger
(ii) Logistic population growth curve sigmoid / S-
shaped) .
 Population growth starts out slowly and then
proceeds faster to a maximum (carrying capacity)
and then levels off.
 Population then fluctuates slightly above and below
the carrying capacity with time.
 The population stabilizes at or near the carrying
capacity (K) of its environment due to environmental
resistance(any factors that may prevent a population
from increasing as expected eg predation, parasitism,
and accumulation of toxic substances)
The actualfactorsresponsible for the shape of eachphase
depend on the ecosystem, and this can be illustrated by
considering two contrasting examples: yeast in a flask
(reproducing asexually), and rabbits in a field
(reproducing sexually).
YEAST IN A FLASK RABBITS IN
GRASSLAND
Phases
1. Lag phase Little growth while
yeast starts
synthesizing
appropriate
enzymes
Little growth due to
small population.
Individuals may rarely
meet, so few matings.
Long gestation so few
births.
Acceleration
phase
Slow growth
because cells are
getting used to
conditions in the
environment
Slow growth because
of few reproducing
individuals
Log phase
(Logarithmic
phase)
Rapid exponential
growth.
No limiting factors
since relatively low
Density.
Rapid growth.
Few limiting factors
since relatively low
Density.

Deceleration
phase
(Negative
acceleration
phase)
Slow growth due to
accumulation of
toxic waste
products (e.g.
ethanol) or lack of
Sugar.
Slow growth due to
intraspecific
competition for
food/territory,
predation, etc.
Stationary
phase
Population is stable
(fluctuates slightly
above and below
the carrying
capacity).
Cell death is
equivalent to cells
formed
Population is stable
(fluctuates slightly
above and below the
carrying capacity).
Death rate is
equivalent to the birth
rate
(REFER to growth and development for more
information)
CARRYING CAPACITY
Definition: number of individuals of a population
(species) sustainable by an environment (as long as the
environment remains the same)
Examples: predator/prey; rabbits in Australia; deer on
Kaibab; human population;
Limiting factor(s) determine carrying capacity
(competition, waste, and predation)
LIMITING FACTORS
Any factor operating to restrict population growth
e.g.
Biotic - population density, competition, predation
Abiotic - moisture, temperature, weather/climate, wind,
sunlight, soil, topography, geographic location, nutrients.
Density-dependent - change birth/death rate as density
changes
Density-independent - change birth/death rate regardless
of density
FACTORS THAT TEND TO INCREASE OR
DECREASE POPULATION THE SIZE OF A
POPULATION
Factors that cause a population to grow (Biotic
potential)
i Favourable light – mostly for plants.
ii Favourable temperature.
iii Favourable chemical environment (optimal level of
critical nutrients and toxic wastes).
iv High reproductive rate.
v Adequate food supply
vi Ability to compete for resources.
vii Ability to hide from or defend against predators.
viii Ability to resist disease and parasites.
ix Ability to adapt to environmental changes.
x Ability to migrate and live n other habitats.
xi Suitable habitat.
xii Generalised niche
Factors that cause population size to decrease
(Environmental resistance)
1. Too much or too little light – mostly for plants.
2. Too much or too little temperature.
3. Unfavorable chemical environment (too much or too
little of critical nutrients and high waste
accumulation).
4. Low reproductive rate.
5. Inadequate food supply
6. Too many competitors for resources.
7. Insufficient ability to hide from or defend against
predators.
8. Inability to resist disease and parasites.
9. Inability to adapt to environmental changes.
10. Inability to migrate and live n other habitats.
11. Unsuitable or destroyed habitat.
12. Specialized niche
How Population Density Affects Population Growth
(a) Density dependent factors, are those factors
whose effectiveness depends on number of individua ls
present in a unit space. The more individuals there are in
the population, the greater the percentage of population
that dies or fails to reproduce. These include; diseases,
predation, competition for food, parasitism, pollution
(accumulation of wastes etc.

(b) Density independent factors, are those whose
effectiveness is not related to the density of the
population. Any change in the factor affects the same
proportion of the population regardless of population
density. They include; temperature,rainfall, light, floods,
soil nutrients, fires, drought, hurricanes and habitat
destruction e.g. clearing a forest or fishing in a wetland,
pesticide spraying. They are mainly abiotic factors.
SURVIVORSHIP
This is the percentage of an original population that
survives to a given age.
Survivorship curve:is a graph which shows the number
(or percentage) of surviving individuals of each age
group of a population for a particular species.
Importance of plotting survivorship curves:
1) Enables determination of mortality rates of
individuals of different ages and hence to determine
at which age they are most vulnerable.
2) Enables identification of factors causing death at
different ages so as to plan regulation of population
size.
1 Late loss curves
Occurs in Humans, elephants, rhinoceroses,
mountain sheep
These are organisms with stable populations close to
carrying capacity of the environment (K).
They produce few young ones which are cared for
until reproductive age, thus reducing juvenile
mortality and therefore enabling high survivorship to
a certain age, then high mortality at later age in life.
2 Early loss curves
Occursin annual plants, most invertebrates and most
bony fish species; with a high intrinsic rate of
increase.
They produce many offspring which are poorly cared
for resulting into high juvenile mortality. There is
high survivorship once the surviving young reach a
certain age and size.
3 Constant loss
Many song birds,lizards, small mammals and hydra
This is characteristic of species with intermediate
reproductive patterns with a fairly constant rate of
mortality in all age classes and thus a steadily
declining survivorship curve.
There is an equal chance of dying at all ages.
These organisms face a fairly constant threat from
starvation, predation and disease throughout their
lives.
Survivorship curves for some countries in the
world compared
QUANTITATIVE ECOLOGY
Methods of collecting organisms
There are various methods used to collect organisms
and these depend on;
1 The size of the organism
2 The nature of the organism
III II
Age in years

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