2.6 – Changes and some 2.5 Function
2.6.1 - 2.6.4
And 2.5.5 and 2.5.6
Topic 2 – The
2.6.2 – Describe and explain ‘S’ and ‘J’ population growth curves.
Population curves should be sketched, described, interpreted
and constructed from given data.
2.6.1 – Explain the concepts of limiting factors and carrying
capacity in the context of population growth.
• Nearly 1.6 million people
join the human population
• 84 million people join every
• In three years the human
population grows by an
amount nearly equivalent to
the entire U.S population.
• By 2025 the world
population could exceed 8
• We are going to concentrate on population
control of ecosystems but these theories can
also be applied to human populations.
• Studies on both human populations and smaller
ecosystem populations are carried out in depth.
• The study of any population is an important
aspect of science.
• The factors affecting a population
size may be biotic or abiotic.
• By taking samples and counting the
numbers of organisms in a
particular habitat, ecologists can
study the affects of any factor on
the size of a population.
• Together they affect the rate at
which population grows, and also
it’s final size.
• How many abiotic factors can you think of that might affect
• How many biotic factors can you think of that might affect
Biotic Factors Affecting Population Size
Biotic and Abiotic Factors
1. Temperature – higher
temperatures speed up
reactions and increase
2. Oxygen Availability – affect
the rate of energy
production by respiration.
3. Light Availability – for
breeding cycles in animals
4. Toxins and pollutants –
tissue growth may be
1. Food – both quantity
and quality of food are
2. Predators – refer back
to predator prey
3. Competitors – other
organisms may require
the same resources
from an environment.
4. Parasites – may cause
disease and slow down
the growth of an
All of these things come under the category of ‘Limiting Factors’
Biotic and Abiotic Factors
• Look at the graph of population growth.
• This shows how population growth is eventually
inhibited by environmental resistance and the
environment reaches it’s carrying capacity.
• This is usually because there is plenty of food and no
accumulation of poisonous wastes.
• When a small population grows in a particular
environment, the environmental resistance is almost
• Think of your brine shrimps!?
• Once the carrying capacity is reached, unless the
environmental resistance is changed, e.g. by a
new disease, the size of the population will only
• The carrying capacity (K) is the maximum
number of a species that the habitat can hold.
• This is the type of graph that is almost always seen in nature.
• As the energy resources become more scarce the population
size levels off at the carrying capacity (K).
• The graph we have just been looking at is an example of an ‘S’
• If the resources of the new habitat were endless
then the population would continue to increase
at this rate.
• This type of growth produces a J shaped growth
• Just as in the ‘S’ curve example, a population
establishing themselves in a new area will
undergo rapid exponential growth.
• Can you think of any examples where ‘J’ curve
population growth would be extremely
• Initially exponential growth will occur but
eventually the increase in numbers will not be
supported by the environment.
• This type of population growth is rarely seen in
• It is very natural to ask the linked questions - does humanity
have a carrying capacity and, if so, what is it - and when will
we reach or overshoot this
• Furthermore, experience with other species tells us that,
ultimately, resource limitations and/or habitat degradation
will force the human population curves to approach an upper
limit - the carrying capacity, often symbolized as " K" by
• ‘As we have seen, the human population growth curve is
currently following an exponential curve or a "J-shape”.
Common sense tells us that such growth cannot continue otherwise within a few hundred years every square foot of
the Earth's surface would be taken up by a human.
Is there a Carrying Capacity for Homo
2.6.3 – Describe the role of density-dependent and densityindependent factors, and internal and external factors, in the
regulation of populations.
• Complete the activity – The new zoos
• These factors exert their effect irrespective of
the size of the population when the catastrophe
• The following factors are classed as densityindependent factors:
• Forest Fires
Density Independent Factors
The drought ended in 1978, but even
with ample food once again available
the finch population recovered only
The decline (from 1400 to 200
individuals) occurred because of a
severe drought that reduced the
quantity of seeds on which this
This graph shows the decline in the
population of one of Darwin's finches
(Geospiza fortis) on Daphne Major, a
tiny (100-acre) member of the
Density Independent Factors
• Many rodent populations (e.g.,
lemmings in the Arctic) also go
through such boom-and-bust
• Read the information about the
• Intraspecific Competition competition between members of
the same species.
Density Dependant Factors
• This occurs when two species share overlapping
ecological niches, they may be forced into
competition for the resource(s) of that niche.
• This can include food, nesting sites, sunlight.
• Interspecific Competition – this is competition
between different species for different
Density Dependant Factors
• 2.6.4 – Describe the principles associated with survivorship
curves including K- and r -strategists.
• Ragweed is well adapted to
exploiting it’s environment
in a hurry – before
competitors can become
• “I once ploughed up an old
field and allowed it to lie
fallow. In the first season it
grew a large crop of
• Can you think of any other animals that may be r-strategists?
• We say that they have a high value of ‘r’
• They are called r-strategists
• Ragweed’s approach to continued survival is through rapid
In general, r-strategists share a number of
Usually found in disturbed and/or transitory
Have short life spans
Begin breeding early in life
Have short gestation times
Produce large numbers of offspring
Take little care of their offspring (infant
Have efficient means of dispersal to new
• K-strategists have a stable population that is
close to K.
• There is nothing to be gained from a high r.
• The species will benefit the most by a close
adaptation to the conditions of the environment.
• When a habitat become filled with a diverse
collection of creatures competing with one
another for resources, the advantage shifts to KStrategists
K-strategists share these qualities:
Found in a stable habitat
Long life spans
Begin breeding later in life
Long gestation times
Produce small numbers of offspring
Take good care of their young – infant
7. Have evolved to become increasingly efficient
at exploiting an ever-narrower slice of their
• Curve B – typical of populations in which factors such
as starvation and disease inhibit the effects of aging
and infant mortality is high.
• Curve A – characteristic of organisms that have low
mortality until late in life when aging takes its toll.
• Curve C – a theoretical curve for an organism whereby
the chance of death is equal at all stages
• Curve D – typical of organisms that produce huge
numbers of offspring accompanied by high rates of
• The Californian side-blotted lizard
• R-strategists usually have D survivorship curves.
• K-strategists usually have survivorship curves somewhere
between A and C.
• 2.6.7 – Describe factors affecting the nature of
• 2.6.6 – Explain the changes in energy flows,
gross and net productivity, diversity and mineral
cycling in different stages of succession.
• 2.6.5 – Describe the concept and processes of
succession in a named habitat.
Succession – An intro
• Succession takes places as a result of complex
interactions of biotic and abiotic factors.
• Early communities modify the physical environment
causing it to change.
• This in turn alters the biotic community which
further alters the physical environment and so on.
• A forest following a disturbance such as a fire.
• The gradual process by which the species population
of a community changes is called ecological
Succession – What happens?
• A succession (or sere) proceeds in seral stages, until the
formation of a climax community is reached.
• Each successive community makes the environment more
favourable for the establishment of new species.
• You will be studying glacial moraines in detail as well as the
succession occurring on bare rock.
• Can you think of examples where this would occur?
• Refers to colonization of regions where there is no pre-existing
• Study the information on glacial moraines and answer the
• Community changes on a glacial moraines
Explain what is meant by a climax community.
Explain each of the following changes which occur
Species diversity increases
Gross production increases
Stability of the ecosystem increases
Give two reasons why farmland in the UK does not
reach a climax community.
During succession there is a change in species
composition of a community. There are also changes
in species diversity, stability of the ecosystem, and in
gross and net production until a climax community is
Questions – Glacial Moraines
• Secondary Succession – occurs on sites that have previously
supported a community of some sort.
• Primary Succession – occurs on newly formed habitats that
have not previously supported a community.
Primary and Secondary Succession
After 100-200 years
Primary Succession – Bare
Example for a Northern Hemisphere lithosere: a succession on bare rock
• As these species begin to grow well, they produce shade.
Their own seedlings grow more poorly than shade-adapted
• Plants that grow well under full sun are replaced by plants
that germinate and grow better in deeper shade.
• Examples = lichens, grasses, herbs
• These are usually fast growing plants that photosynthesize
well in full sunlight.
• We call these pioneer species making up the pioneer
In Summary - the 1st Invaders!
• Humans may deflect the natural course of succession
in these circumstances (e.g. by mowing or farming).
• This leads to the development of a different climax
community than would otherwise develop naturally.
• These events do not involve loss of the soil.
• Secondary succession therefore occurs more rapidly
than primary succession.
• This type of succession takes place after a land
clearance (e.g. from fire or landslide).
and small trees
Time to develop: Years
Grasses and low
Secondary Succession – Cleared
150+ = climax community
• Animal species have a profound affect on the plant species
occurring within a habitat.
• Changing conditions in the present community allows for new
species to become established (the future community).
• Succession continues until the climax community is reached.
• Decomposers will join the community as well as animal
• As the plant community changes, the soil will also undergo
changes (abiotic factors will change).
• Wetland areas present a special case of ecological succession.
• Wetlands are constantly changing:
• Wetland ecosystem may develop in a variety of ways:
• In special circumstances, a an acid peat bog may develop.
(may take 5000+ years).
• In non-acidic, poorly drained areas, a swamp will eventually
develop into a fen.
• In well drained areas, pasture or heath may develop as a
result of succession from fresh water to dry land.
• This is called productivity
• Think back to the work on food webs/chains
• It is often useful to know how much energy is
passing through a trophic level over a period of
• Productivity is a measure of the amount of
energy incorporated into the organisms in a
trophic level, in an area, over a certain period of
• 2.5.6 – Define and Calculate the values of gross and net
productivity from given data
• 2.5.5 – Define the terms gross productivity, net productivity,
primary productivity, secondary productivity, gross primary
productivity and net primary productivity.
• The rate at which producers convert light energy into chemical
energy is called primary productivity.
• It is therefore measured in units of kilojoules per square metre
per year (kJm-2year-1)
• The area is normally one square metre and the time is usually
• The producers use some of this energy during respiration and
energy needs which is eventually lost to the environment as
• The remaining energy is available to the herbivores and is
known as net primary productivity (NPP)
• It is related to the total amount of chemical energy
incorporated into the producers.
• This is sometimes shown as GPP – Gross Primary Productivity
• Gross Productivity (GP) – is the total gain in energy or
biomass per unit time.
Recap of Definitions!
• Gross Productivity (GP) = the total gain in energy/biomass per unit time.
• Gross Primary Productivity (GPP) = the total gain in energy of the producers.
• Net Productivity (NP) = the gain in energy/biomass per unit time remaining
after allowing for respiration (R) loses.
• Net Primary Productivity (NPP) = the gain in energy/biomass per unit time
remaining after allowing for respiration loses which is passed onto the
• Secondary Productivity = The rate at which primary material is synthesised
into animal tissue per unit area in a given time.
• Primary Productivity = The rate at which energy/biomass is formed through
• Productivity = production per unit time
• The rate at which plants can convert light energy
into chemical energy is affected by many factors:
• Amount of nutrients
• Primary productivity varies greatly in different
• In the oceans however, the most productive areas are in cold
regions due to the up-welling of water bringing plant nutrients
• This is due to good light levels and high temperatures in the
• In natural ecosystems primary productivity tends to be highest
in tropical regions.
• We can calculate NPP for both producers and
NPP = GPP – energy used in respiration
• We can calculate GPP as follows:
GPP = NPP + R
• In addition, the equation for consumers only is:
GP = food eaten – faecal losses
Calculating Productivity Values
• What is the GPP of an ecosystem if the NPP is 1660 kJm2yr-1 and the energy lost during respiration is 573 kJm-2yr1?
• What is the NPP if the GPP is 2700 kJm-2yr-1 and the
energy used in respiration is 1850 kJm-2yr-1?
• What is the % energy from sunlight that is fixed as GPP if
the total energy from the sun in 3 x 106 and the gross
primary productivity = 2.8 x 104?
• Some easy ones to start you off!
Now for some slightly harder ones!
Calculating Productivity Values
Therefore, for photoautotrophs, photosynthetic
efficiency is determined as:
Photosynthetic Efficiency =
Net production ÷ Light Energy Absorbed
Ecological Efficiency is the net production of new
biomass at each trophic level as a percentage of the
total energy flowing through that trophic level
Energy flow diagrams illustrate energy flow
through communities and include values for
respiratory losses and energy flow through
Information from energy flow diagrams can be used to
calculate ecological efficiencies
Energy Flow Diagrams
Use information from the energy flow diagram to:
• Explain the meaning of the term Gross Primary Production
• Explain the meaning of the term Net Primary Production
• Calculate the Photosynthetic Efficiency of the phytoplankton
Gross Primary Production is the total energy fixed
by photoautotrophs during photosynthesis
Net Primary Production is the energy stored as
biomass (gross production – energy lost as heat
3.7 x 104
------------ x 100
172 x 104
• This is because climax communities are better
adapted to an efficient rate of utilisation of their
• They become stable.
• As ecosystems become more diverse, the overall
GPP is also going to increase.
• The NPP and GPP of any ecosystem is going to
fluctuate. This is especially the case during each
Finally Back To Succession!
The Early Stages
• This is due to low respiration rates of the initial
producers and therefore a lot of energy available to be
• This allows the system to grow and biomass to
• Net Productivity = High
• This is due to the initial conditions and the relatively low
density of producers.
• Gross Productivity = Low
The Later Stages
• Net Productivity = Low
• Increased rates of respiration and other energy
sapping activities by consumers means that NP
will begin approaching zero.
• This is due to an increase in the consumer
community who can synthesise a lot of energy
from the food they eat.
• Gross Productivity = High
• Ultimately, the climate will be responsible for
affecting the nature of the climax community
unless human or other factors maintain an
equilibrium at a sub-climax community.
• Climax communities are more stable that the
seral stages that preceded them.
• Succession comes to an end with the
establishment of a mature, relatively stable
community – the climax