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1. Chapter 3
Populations, Communities
and Species Interaction
Prof. Dr. Ali El-Naqa
Hashemite University, Department of Water
Management & Environment
2022/2023

2. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chapter 3
Lecture
Outline

3. Learning Outcomes
After studying this chapter, you should be able to answer the
following questions:
• How does species diversity arise?
• Why do species live in different locations?
• How do interactions among species affect their fates and that of
communities?
• If a species has unlimited growth potential, why doesn’t it fi ll the
earth?
• What special properties does a community of species have, and
why are they
important?
• What is the relationship between species diversity and community
stability?
• What is disturbance, and how does it affect communities?
3-3

4. Chapter 3 - Topics
• Who Lives Where, and Why?
• Species Interactions
• Community Properties
• Communities in Transition

5. Learning Objectives
• describe how environmental factors
determine which species live in a given
ecosystem and where or how they live.
• understand how random genetic variation and
natural selection lead to evolution,
adaptation, niche specialization, and
partitioning of resources in biological
communities.
• compare and contrast interspecific predation,
competition, symbiosis, commensalism,
mutualism, and coevolution.

6. Learning Objectives
• explain population growth rates, carrying
capacity, and factors that limit population
growth.
• discuss productivity, diversity, complexity,
and structure of biological communities and
how these characteristics might be connected
to resilience and stability.
• explain how ecological succession results in
ecosystem development and allows one
species to replace another.
• list some examples of exotic species
introduced into biological communities, and
describe the effects such introductions can
have on indigenous species.

7. Evolution
Jean-Baptiste Lamarck (French: late 1700s-early
1800s): predominant theory of “evolution” prior to
Darwin:
• During their lifetimes, individual organisms
acquire structures or skills useful in dealing with
environment
• These acquired structures are passed on to their
descendants
• Over time, the accumulation of acquired structures
changes one type of organism to another
• Flaw with Lamarckian Evolution: characters
acquired during the lifetime of an organism are
not passed on to descendants!

8. During early-mid 1800s, two natural historians
independently developed an alternative, and superior
model: Natural Selection.
Two individuals were Alfred Russell Wallace and
Charles Darwin:
• Both British
• Both trained in England, but traveled to far countries
(Darwin to South America and Galápagos,Wallace to
Amazon River basin and Malaysia)
• Independently made same basic observations and
conclusions
• Mutual friends decided to present papers of both (in
1858) on their behalf, so both could get credit

9. • The following year (1859) Darwin
published On the Origin of Species by
Means of Natural Selection: an instant
"best-seller“ and source of decades of
controversy.

10. Their basic observations:
I. Organisms in all populations possess
heritable variations - size, color, agility, speed,
digestion …
II. Some variations are more favorable then
others.
III. More young are born to every population
than can POSSIBLY survive
IV. Those with favorable variations are more
likely to survive and produce offspring with
their favorable variation.

11. • Thus, IF some variation gives the individual
a slight advantage (bigger, stronger,
smaller, smarter, less tasty, whatever) at
surviving; and IF that variation is inherited;
THEN there is a somewhat better than
average chance that organisms with that
variation will survive to bear the next
generation. Over the long expanse of
geologic time, the accumulation of these
variations will change the population from
one form to another: the origin of species.
= Natural Selection

12. • This process is analogous to artificial
selection (i.e., domestication), and thus
called natural selection.
• NOTE: Natural Selection is NOT "survival
of the fittest", as implied by your textbook
• NOTE ALSO: Darwin did not use the word
"evolution" very often; instead, preferred
the phrase "descent with modification".

13. Part 1: Who Lives Where,
and Why?

14. Tolerance Limits
Each environmental factor (temperature, nutrient supply,
etc.) has both minimum and maximum levels beyond which
a species cannot survive or is unable to reproduce.

15. Abundance and Distribution of Species
• Liebig - proposed that the
single environmental factor in
shortest supply relative to
demand is the critical
determinant in species
distribution
• Shelfold - added to Liebig’s
work by proposing that the
single environmental factor
closest to tolerance limits
determines where a particular
organism can live

16. • Today we know that for
many species the
interaction of several
factors, rather than a single
limiting factor, determines
biogeographical distribution.
• Sometimes, the
requirements and
tolerances of species are
useful indicators of specific
environmental
characteristics.

17. Adaptation
• term refers to when species acquire traits that
allow them to survive in a specific
environment.
– Acclimation - available to individual
organisms during their lifetime - not
hereditable
– Population level adaptation - brought about
by the inheritance of specific genetic traits
that allow a species to live in a particular
environment

18. Adaptation
Two types of adaptation:
• Acclimation - changes in an individual
organism due to non-permanent
physiological modifications
• Evolution - gradual changes in a species
due to changes in genetic material and
competition
Theory of evolution - developed by Charles
Darwin and Alfred Wallace.

19. Natural Selection
• Due to a variety of environmental pressures
certain traits are favored
– Limited resources/space exert selective
pressure on a population
• Physiological stress - moisture,
temperature, pH, etc.
• Predation -
• Competition -
• Luck -

20. Natural selection - genetic combinations best
adapted for present environmental conditions
tend to become abundant
• Spontaneous, random mutations
• Selective pressure - physiological stress,
predation,competition, luck
• Speciation - Production of a new species
from a previously existing species.
– Isolated sub-populations
– Extinction - Loss of an entire species.
– Common feature of evolution

21. Tolerance Limits
• The limits to the environmental conditions
that a organism can endure.
– temperate
– moisture
– nutrient supply
– soil and water chemistry
– available space
• There are maximum and minimum tolerance
levels for organisms

22. • Used to think that there was one critical
limiting factor that determined the distribution
of an organism
– Saguaro cactus
• Found out that it is usually a combination of
several different factors
• Juvenile forms almost always more sensitive
than adult forms

23. • Can also be used backwards - the presence
or absence of certain species can be used to
say something about the environment.
– lichen and eastern white pine are very
sensitive to acid precipitation -
their absence is indicative of acid rain
& sulfuric acid
Indicator species: is a term for organisms
whose sensitivities can tell about
environmental conditions in an area.

24. Speciation
• Given enough time generations of species
may gradually evolve to become better suited
to their environment (or much change
because the environment changed)
• The development of new species is known as
speciation. It results from
• new opportunities
• new risks
• isolation (Darwin’s finches)
– divergence - when a small population
become isolated and its genetic
characteristics become distinct from the
original population

25. Convergence
• When different
species develop
functionally similar
adaptations but
come from different
genetic stock

26. Speciation
These 13 species of finches are descendents of a single
seed-eating species.

27. The Taxonomic Naming System

28. •Habitat - the place or set of environmental
conditions in which a particular organism lives
•Niche - Functional role an organism plays in
its surroundings.
•Ecological niche - the role played by a
species in a biological community. Niches as
community roles, describe how a species
obtain food, what relationships it has with
other species and the services it provides its
community
The Ecological Niche

30. Resource
Partitioning
Over time, niches can
evolve as species
develop new strategies
to exploit resources.
Law of Competitive
Exclusion:
No two species will
occupy the same niche
and compete for the
same resources in the
same habitat for very
long.

31. Part 2: Species Interactions
Predation and competition - antagonistic relationships

32. Organism Interactions
• Predation - One animal kills/eats another.
– Predator benefits from food.
– Prey species may benefit by eliminating non-
adaptive genes from the gene pool.

33. • Competition - Two organisms compete to obtain
the same limited resource.
For what do organisms compete?
• Energy and matter in usable forms.
• Space
• Specific sites for life activities
Intraspecific - same species
Interspecific - different species
• In general, the more similar the competing
species, the more intense the competition.

34. • Symbiotic - Close, physical relationship
between two different species. At least one
species benefits from the interaction.
Three Types of Symbiosis:
• Commensalism - one member benefits, while
the other is neither benefited nor harmed
(Remoras and Sharks)
• Mutualism - both members of the partnership
benefit (Yuccas and Yucca Moths)

35. • Parasitism - one species benefits and the
other is harmed
• Parasitism - One organism (parasite) living
in or on another organism (host), from which
it derives nourishment.
 Ectoparasites - Live outside host
 Endoparasites - Live inside host
Lichens: combination of
algae and fungi in a
classical example of
mutualistic symbiosis

36. Types of Symbiosis: Intimate relations
among species
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37. Types of Symbiosis:
Intimate relations among species
• Commensalism is a type of symbiosis in which one member clearly
benefits and the other apparently is neither benefited nor harmed.
• Mutualism is a type of symbiosis in which both members clearly benefit.
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38. Keystone species –
Keystone species: species or
set of species whose impact
on its community or
ecosystem is much larger and
more influential
than would be expected from
mere abundance.
species that play essential
community
roles (examples: mycorrhizae,
giant kelp, Giant Algae in
California, it provides food,
shelter and structure for a
whole community)

39. Population Dynamics
Population – number of individuals of a species that
live in the same area at the same time.
• Population Density - # of individuals per unit area
or volume.
• Distribution – general pattern in which individuals
are distributed.
• Growth rate – change in population density per
unit time.
3 Properties of Populations

40. Part 3: Population
Growth
Exponential growth - the unrestricted
increase in a population (also called
the biotic potential of a population)
Carrying capacity - the maximum
number of individuals of any species
that can be supported by a particular
ecosystem on a sustainable basis

41. Population Growth
• The mathematical formula for exponential
growth is:
• dN/dt = rN
This is the change in numbers of individuals
(dN) per change in time (dt) equals the rate of
growth (r) times the number of individuals in
the population (N). The r representing the
average individual contributing to population
growth. If r less than 1, then the poulation is
shrinking, if r is exactly 1, then there is no
change and dN/dt = 0.

42. Overshoots and Diebacks
•Overshoot: when the population exceeding the carrying
capacity
•Dieback led to population crash

43. • There are limits to
growth.
• Malthusian growth –
pattern of population
explosion followed
by a crash.
• Dieback –
population
decreases faster
than it grows.
• Overshoot – extent
to which population
exceeds carrying
capacity.
Malthusian Growth
carrying capacity
time
#
of
individuals
overshoot dieback

44. Population Oscillations

45. • Some species may grow exponentially when
resources unlimited, but their growth slows as
they approach the carrying capacity of the
environment. This pattern is called logistic
growth.
• Mathematically, this growth pattern is
described by the following equation, which
adds a term for carrying capacity (K) to the
biotic potential growth equation:
)
1
(
K
N
rN
dt
dN



46. • Logistic growth –
exponential growth
when resources are
unlimited, slowed
growth as species
approach carrying
capacity.
• Carrying capacity -
# of individuals that
can be sustained by
the resources in a
given area.
• Carrying capacity
can vary over time.
Logistic Growth
initial carrying capacity
new carrying capacity
time
#
of
individuals
S-shaped curve characteristic
of logistic growth

47. • This equation says that the change in
numbers over time (dN/dt) equals the
exponential growth rate ( r times the portion
of the carrying capacity (K) represented by
the population size (N).
• The term (1-N/K) represents the relationship
between N at any given time step and K, the
number of individuals the environment can
support. If N less than K, say 100 compared
to 120 then (1-N/K) is a positive number
(1-100/120) = 0.17 and population growth
dN/dt is slow but positive

48. • If N is > K, that the population is greater than
the environment can support, then 1-N/K) is a
negative number.
• The logistic model describes a population that
decreases if its number exceeds carrying
capacity.
• The following figure shows the difference
between exponential and logistic growth. The
J-curve represents the maximum number of
species might possibly attain. The S-curve
represents the logistic growth (sigmoidal
curve)

49. Growth to a Stable Population

50. Environmental Resistance
Environmental resistance - factors that tend to
reduce population growth rates
• Density-dependent
• - linked to population size
• - disease, lack of food,
stress, predation, parasites
• Density-independent
• - often environmental
• - droughts, floods, habitat
• destruction,landslides,
human activities
• Intrinsic
• - Intense competions
within a species are both
attributes of a species
themselves
• - slow reproduction
• Extrinsic
• - external to a species
• - predators, competitors,
• environmental risks

51. r-adapted species
• r-adapted species: tend to have rapid
reproduction and high mortality offspring, and
they have frequently overshoot carrying
capacity and die back. They belong to
exponential growth pattern.
• Malthusian strategies – externally controlled
growth.Insects, rodents, marine invertebrates,
parasites, annual plants.

52. k-adapted species
• K-adapted: organisms tend to reproduce
more slowly as they approach carrying
capacity. These species are referred to k-
adapted.
• Logistic strategies – intrinsically controlled
growth.
• Wolves, elephants, whales, and primates.

53. Characteristics of Reproductive Strategies
1) Short life
2) Rapid growth
3) Early maturity
4) Many small offspring
5) Little parental care
6) Little investment in offspring
7) Adapted to unstable env’t
8) Pioneers, colonizers
9) Niche generalists
10)Prey
11)Regulated by extrinsic
factors
12)Low trophic levels
1) Long life
2) Slow growth
3) Late maturity
4) Few large offspring
5) High parental care
6) High investment in
offspring
7) Adapted to stable env’t
8) Late stages of succession
9) Niche specialists
10)Predators
11)Regulated by intrinsic
factors
12)High trophic levels
K-adapted species
r-adapted species

54. Part 3: Community Properties
• Primary productivity - a community’s rate of
biomass production, or the conversion of solar
energy into chemical energy stored in living (or
once-living organisms)
• Net primary productivity - primary
productivity minus the energy lost in
respiration
• Productivity depends on light levels,
temperature, moisture, and nutrient availability.

55. Relative biomass accumulation
of major world ecosystems.

56. Abundance and Diversity
Abundance - the number of individuals of a species
in an area
Diversity - the number of different species in an area
• A useful measure of the variety of ecological
niches or genetic variation in a community
• Decreases as we go from the equator towards the
poles
Abundance and diversity depend on total resource
availability in an ecosystem.

57. Antarctic
Marine Food
Web
Complexity - the
number of species
at each trophic
level and the
number of trophic
levels in a
community

58. Stability and Resilience
Stability - a dynamic equilibrium among the physical
and biological factors in an ecosystem or a
community
Resiliency - the ability to recover from disturbance
Three kinds of stability or resiliency in ecosystems:
• Constancy - lack of fluctuations in composition or
functions
• Inertia - resistance to perturbations
• Renewal - ability to repair damage after disturbance

59. Community structure
Distribution of members of a
population in a given space can be:
• Random - individuals live wherever
resources are available
• Ordered - often the result of
biological competition
• Clustered - individuals of a species
cluster together for protection,
mutual assistance, reproduction, or
to gain access to a particular
environmental resource

60. Edges and Boundaries
• Ecotones- the
boundaries
between adjacent
habitats
• Often rich in
species diversity
• Example: the
boundary between
a forest and a
meadow

61. Edge effects - the environmental and biotic conditions
at the edge of a habitat
• Temperature, moisture levels, predator species, etc.
• Edge effects associated with habitat fragmentation
are generally detrimental to species diversity.
Core habitat - the interior area of a habitat
• Habitat not impacted by edge effects
• Some species avoid edges and ecotones and prefer
interior environments.
Edge vs. Core

62. Part 4: Communities in
Transition
Ecological succession - the process by which
organisms occupy a site and gradually change
environmental conditions by creating soil, shelter,
shade, or increasing humidity
• Primary succession - occurs when a
community begins to develop on a site
previously unoccupied by living organisms
• Secondary succession - occurs when an
existing community is disrupted and a new one
subsequently develops at the site

63. Terrestrial Primary Succession
 Pioneer Community: Collection of
organisms able to colonize bare rock (ie
lichens).
 Lichens help break down rock and
accumulate debris, helping to form a thin
soil layer.
 Soil layer begins to support small life
forms.

64.  Life forms replace lichen community.
 New community replaced by perennial plant
community.
 Perennial plant community replaced by shrubs.
 Shrubs replaced by shade intolerant trees.
 Shade intolerant trees replaced by shade
tolerant trees.
 Eventually a climax community is reached -
Stable, long-lasting.

65.  In general, climax communities are more
stable and have larger, more diverse
populations of species than earlier
stages of succession.
 Successional (seral) stage - each step in
the process.

66. Primary
Succession
on Land

68. Secondary Succession
 Occurs when an existing community is
disturbed or destroyed.
 With most disturbances, most of the soil
remains, and many nutrients necessary
for plant growth may be available for
reestablishment of the previous
ecosystem.
 Tends to be more rapid than primary
growth.

70. Climax Community
• Climax community - a community that develops in
primary or secondary succession and seemingly
resists further change
• Clements: succession to a climax community is like a
parade or relay, in which species replace each other
in predictable groups and in a fixed, regular order
• Gleason: succession is individualistic and
unpredictable

71. Climax Communities - Biomes
 Terrestrial climax communities with wide
geographic distributions. Usually defined
by undisturbed natural plant communities.
 Two main non-biological factors
determining biomes:
 Temperature
 Precipitation

72. Disclimax (equilibrium)
community - community
characterized by, and
adapted to, periodic
disruption
Example: fire-climax
communities

73. Aquatic Primary Succession
 Except for oceans, most aquatic
systems are considered temporary.
 All aquatic systems receive inputs of
soil particles and organic matter
from land. - results in gradual filling
of shallow bodies of water.
 Plants can begin to take root.

74. Introduced Species and Community
Change

75. Exotic Species
• Sometimes communities can be completely
altered by the introduction of exotic species.
• Exotic species are often introduced by
humans.
• Successful exotics tend to be prolific,
opportunistic species, such as goats, cats,
and pigs.
• Many ecologists consider exotic species
invasions the most pressing hazard for
biological communities in the coming
century.