2. Population can be defined as a group of
individuals belonging to the same
species that inhabit a specific
geographic location at a specific point in
time.
Population Ecology
It is the study of distribution, density,
numbers of individuals and structure
(gender, age), rates of mortality, as
well as the factors that affect the
growth of populations.
Depending on the nature of the species,
many factors may affect population
growth (food availability or quality,
predators, habitat change, etc.). 2
3. A knowledge of population growth may help us
find the best way to control the population.
Farmers, wildlife managers, foresters,
managers of fisheries and public health
practitioners – all must understand these
concepts.
The study of changes in population size and
the causes of these changes is called
population dynamics.
3
4. To understand population dynamics, first
important fact is to know the primary
characteristics that describe a population.
Any population has size, which is its total
number of individuals.
A population is also characterized by its
birth rate, death rate, and growth rate.
When one is interested only in the total
number of individuals in a population, these
three rates can be related to each other
simply: the growth rate equals the birth rate
minus the death rate.
growth rate = birth rate - death rate
4
5. Sometimes ecologists are not only interested in
the number of individuals, but in the amount of
living matter (or organic matter contained in
living organisms) called biomass.
To measure population growth by the number of
individuals or by the biomass depends on
following:-
to determine whether a species is endangered,
then the number of individuals is the measure
of interest. When the question concerns one
population that is a food source for another,
then the appropriate measure is often the
biomass.
5
6. The exponential growth curve:
a population that faces no predation, parasitism, or
competition, experiences no immigration , and exists in an
environment with unlimited resources is in ideal condition.
For any species the population growth rate under such ideal
conditions is enormous and is called exponential growth,
meaning that the percentage increase is constant.
For example, one pair of houseflies starting to breed in April
could have 191,010,000,000,000,000,000 descendants by
August if all their eggs hatched and if all the resulting young
survived to reproduce.
6
7. Although all biological populations have
the capability for exponential growth, this
is rarely achieved in nature, as control of
populations is the norm.
a. Exponential growth curve
7
8. The logistic growth curve:
No real population is expanding at infinite speed.
Studies show that a real population would grow
according to an S-shaped curve, called the logistic
growth curve.
This growth curve shows initial acceleration (rapid
expansion) of the population when it is at low densities
(size), then decelerating growth at higher densities,
and an eventual leveling off as the density
approaches what we call the carrying capacity of the
environment.
The carrying capacity is the maximum population
size that can exist in a habitat or ecosystem over a
long period of time without detrimental effects to either
the population or the habitat or ecosystem. 8
9. The logistic growth curve reflects a changing relationship
between births and deaths.
During the acceleration stage births greatly exceed deaths;
during the deceleration stage the birth rate is declining or
the death rate is rising ; and when the curve levels off, births
and deaths are in balance and the population is said to be at
its carrying capacity.
steady state
deceleration
acceleration
b. Logistic growth curve
9
10. If the population becomes larger
than the carrying capacity, deaths
exceed births, the growth is negative,
and the population declines back to
the carrying capacity.
If the population is reduced below
the carrying capacity, births once
again exceed deaths, and the
population grows back to the carrying
capacity.
10
11. Population regulation is the control of the size of a
population.
This regulation implies a tendency of the
population to achieve equilibrium or in harmony
with the surrounding environment.
If a population tends to remain about the same size,
then it is said to be stable.
There are basically two different types of
population regulation - classified according to the
types of factors that control the size of the population.
1. Density-dependent control
2. Density-independent control
11
12. 1. Density-dependent Control :
a) Definition: A density-dependent factor is one
where the effect of the factor on the size of the
population depends upon the original size of
the population.
◦ A disease is a good example of a density-
dependent factor.
◦ If a population is dense and the individuals
live close together, then there is a higher
probability of catching the disease. Greater
number and greater proportion of the
population will be affected.
12
13. b) Characteristic of the factor: In general,
density-dependent factors are biological
factors, such as diseases, parasites,
competition, and predation.
C) Characteristics of the ecosystems:
i) ecosystems where the communities have
many species,
ii) ecosystems not usually stressed periodically
by physical factors such as flooding, i.e.,
ecosystems that are usually more stable.
13
14. 2. Density-independent Control
a) Definition: A density-independent factor is one
where the effect of the factor on the size of the
population is independent of and does not depend
upon the original density or size of the population.
Examples:
◦ The effect of weather, like a severe storm and
flood coming through an area.
◦ a harmful pollutant put into the environment, e.g.,
a stream.
b) Characteristic of the factor:
In general, density-independent factors are
physical factors, such as weather factors (e.g.,
severe winter) or the presence of harmful
chemicals. 14
15. c) Characteristics of the populations:
◦ Many populations in this category have the growth
form called the "resource-limited" type.
◦ The population size often goes over the carrying
capacity before some other physical factor
decreases the population size.
◦ growth rates do not seem to show any trend at all
relative to population density.
Dispersion– the way in which the
individuals are arranged.
15
16. NOTE:
◦ Population regulation factors can be classified
as above into two types, but what usually
happens in nature is that a population is
actually controlled by a combination of
both density-dependent and density-
independent factors.
◦ Some populations will be primarily controlled
by one type and other populations will be
controlled primarily by the other type.
◦ Note that both types of factors are external
forces on the population.
16
17. Community ecology is the study
of how populations from different
species interact to mutually affect
each population's growth and
survival.
17
18. Community Composition and Diversity
Species composition is a list of the species within a
community; it does not reveal relative abundance.
Species diversity consists of two factors: richness
and evenness.
◦ Species richness is number of species; forest
with 20 tree species has more richness than a
forest with 12.
◦ Species evenness is the number of individuals
within each population; a forest with 76 Acacia
abyssinica and five doumpalms differs from a
forest with 40 of both species.
Number of species in a community increases as we
move from the poles to the equator.
18
19. Interaction within Communities:
No individual or population exists alone; the
essence of the continuity of life is the
interaction of species within communities.
It is possible that one population might
regulate another, or that two populations,
through their interactions, might regulate each
other.
Interactions include competition for resources,
predator-prey interaction, parasite-host
interaction, symbiotic interaction, etc.
19
20. Competition:
Competition can be defined as any
interaction between individuals, populations
or species that has somewhat negative
effects on birth, growth, or survival of both
species.
Competition occurs when different species
utilize a resource that is in short supply.
If the resource is not in limited supply, there
is no competition.
For example, different species of trees in a
forest compete for light, for water, for
nutrients in the soil and for a space to grow.
20
21. Predators, Parasites and Prey Interactions:
Most organisms are predator, parasite, prey or
host.
Predation occurs when one organism (predator)
feeds on another (prey), and usually the predator
kills the prey.
Parasitism occurs when an organism (parasite)
lives by feeding on (usually living within or on) a
prey (host) and does not usually kill the host.
Predation and parasitism increase the predator
population at the expense of the prey and host
populations.
21
22. Symbiotic Relationships:
Symbiosis is a close relationship between
members of two populations.
Not all relationships among organisms involve
food. Many organisms live together and share
resources in other ways. Any close relationship
between species is called symbiosis.
22
23. Commensalism
Mutualism
Parasitism
A symbiotic relationship where one benefits and the
other is neither harmed nor benefited is called
commensalism.
A type of a symbiotic relationship between two
species where both benefit is also called mutualism.
For example, nitrogen fixing bacteria and plants, and
E. coli living in the intestines of humans.
Parasitism – A symbiotic relationship in which one
organism benefits but the other is harmed.
24. Mutualism – A symbiotic relationship in which
both species benefit.
Examples:
Cowbirds and Large Animals
Termites and Trichonympha
Bees and Flowers
25. The cowbird benefits by
eating the ticks and mites off
the large animal. The large
animal benefits from have the
parasites removed from them.
The birds can also get the
food.
26. You probably think termites
eat wood; they do – in a way.
Termites can’t digest
cellulose, which is the main
component of wood.
Therefore, they get help
from a protozoan called
trichonympha. This
protozoan lives in the gut of
the termite. It breaks down
the cellulose for the termite.
The trichonympha gets a free
meal and shelter; the termite
is able to eat and receive
nutrients from the wood.
27.
28. Bees receive nectar
from the flowers in
order to make honey.
As the bees collect
nectar, they collect
pollen on their body.
As they fly to another
flower, they pollinate
it by dusting the
pollen on the flowers
stamen.
29. Commensalism – A symbiotic relationship in which
one organism benefits and the other is not affected.
Examples:
Clown fish and sea anemones
Shark and remora
30. The clown fish is immune to the
stings of the sea anemones
tentacles. The clown fish makes its
home in the tentacles for protection.
The clown fish gets shelter, but the
sea anemone gets nothing.
31. The remora hangs
around the shark
picking up any
scraps it may
leave. The
remora gets food
while the shark
gets nothing.
32. Parasitism – A symbiotic relationship in
which one organism benefits but the other is
harmed.
Examples:
Tapeworm and Humans
Cuckoo bird and warbler
Ticks
33.
34.
35.
36.
37. Ecosystems are defined as a interaction
among organisms of different species
and their surroundings.
Each ecosystem has structure and
function.
Structure includes the living (biotic)
and nonliving (abiotic) factors
Function includes the flow of energy
and cycling of nutrients.
38
38. Ecosystem ecology is the study of the
movement of energy and matter through
ecosystems. It deals with locally defined
ecosystems which exchange matter and
energy with their surroundings.
Ecosystem ecology is concerned with how
nutrients enter, are recycled within, and leave
the system; what inputs of energy do the
work in the system, how that energy is
passed from one part of the system to
another, and how energy leaves the system.
39
39. Ecosystem Structure:
The biotic components or organisms in an
ecosystem are either autotrophs or
heterotrophs:
Autotrophic organisms or Producers -
capture energy and incorporate it into organic
compounds.
Types:
Photoautotrophs
chemoautotrophs
Autotrophs are at the beginning or bottom of a
food chain. In terrestrial ecosystems, producers
are mostly plants; in aquatic ecosystems,
dominant producers are algae 40
40. Some organisms use the
Sun’s energy to create
energy rich molecules
through a process called
photosynthesis.
A producer is an
organism that uses an
outside energy source
like the Sun to make
energy-rich molecules.
41. Energy rich molecules,
usually sugars, serve as
food. They are made up
of oxygen, hydrogen, and
carbon atoms.
Energy is stored in the
chemical bonds of the
atoms. When the bond is
broken, energy is released
to fuel life processes.
42. Some producers make energy-rich molecules
through a process called chemosynthesis.
◦ These organisms are found near volcanic vents
in the ocean floor. Inorganic molecules in the
water provide the energy source for
chemosynthesis.
43. Heterotrophic organisms - need a
source of preformed nutrients and
consume tissues of other organisms.
These include:
◦ Herbivores
◦ Carnivores
◦ Omnivores
◦ Decomposers
All heterotrophic organisms are consumers.
44
44. A consumer is an
organism that cannot
make their own
energy-rich
molecules.
Consumers obtain
energy by eating
other organisms.
Wolves can’t
make their own
food. They are
consumers.
The Cape Buffalo can’t
make its own food. It
is a consumer.
45. Zebras eat grass.
They are
herbivores.
Cows are
herbivores.
Herbivores - animals that feed directly on
green plants, which are primary consumers.
48. Omnivores – Eat both plants and animals
Bears
Pigs
Humans
While the panda’s
digestive system is
that of a carnivore,
their diet consists
of 99% bamboo.
Raccoons are
omnivores. They
eat both plants and
animals.
49. Decomposers – Consume waste and dead
organisms. Decomposers help recycle once-
living matter by breaking it down into simple,
energy-rich substances. These substances
might serve as food for decomposers , be
absorbed by plant roots, or be consumed by
other organisms.
Fungi
Bacteria
Earthworms
51. Flow of energy:
Primary source of energy for ecosystems is sunlight.
Solar energy will be captured by producers, then
will flow through the different levels (trophic levels)
of an ecosystem and is governed by the first and
second laws of thermodynamics.
First law of thermodynamics - energy can
neither be created nor destroyed; it can only be
changed from one form of energy to another.
Second law of thermodynamics - when energy
is transformed from one form to another, there
is always some loss of energy from the system,
usually as low grade heat.
Therefore, ecosystems are unable to function
unless they receive a constant input of energy.
52
52. Energy flow through an
ecosystem in one
direction, from the sun to
autotrophs (producers)
and then to various
heterotrophs (consumers).
The complex feeding
relationships that exist in
nature are called food
chains or webs. A grazing
food web begins with
leaves, stems and seeds
eaten by herbivores and
omnivores.
Trophic levels
53. Shrubs are the
beginning of the
food chain. They
receive their
energy from
sunlight.
Because shrubs
make their own
food through
photosynthesis,
they are called
producers.
The deer is the
first organism of
the food chain to
eat the shrub. It is
the primary
consumer.
The mountain lion
is the second
organism of the
food chain. It eats
the deer. It is the
secondary
consumer.
54. Food Chains are a series of steps
in which organisms transfer energy
by eating or being eaten.
A straight line sequence of who
eats whom.
Simple food chains are rare in
nature.
55. 56
•based on the total
amount of energy.
•Amount of available energy
decreases for higher
consumers
•It takes a large number of
producers to support a
small number of primary
consumers
•It takes a large number of
primary consumers to
support a small number of
secondary consumers
Another way of showing
the transfer of energy in
an ecosystem is the
ENERGY PYRAMID.
56. Food webs show the
complex interactions
within an ecosystem.
Food Webs are Food
Chains that interact each
other. Food webs are
what really happens in
nature.
Each step in a food chain
or web is called a
trophic level. Producers
make up the first step,
consumers make up the
higher levels.
58. • Despite an inexhaustible influx of energy from the
sun, the continuation of life depends on the recycling
of essential chemical elements.
The flow of a nutrient from environment to living
organisms and back to environment.
•Main reservoir for the nutrient is in the environment.
Three Categories:
•Hydrologic cycle
–Water
•Atmospheric cycles
–Nitrogen and carbon
•Sedimentary cycles
–Phosphorus
59. In the hydrologic cycle, freshwater evaporates and condenses
into clouds in the atmosphere.
Oceans are the greatest source of evaporated water, but water
also evaporates from bodies of freshwater, and from land plants
(transpiration).
Rainfall sends water back to earth and it is picked up by
plant tissues, enters ecosystem, while the remaining water enters
the watershed.
60. Main reservoir is Earth’s crust; no gaseous phase.
Phosphorous made available by geological uplifting
and weathering, makes it into the soil, is picked up by
plants and enters the ecosystem.
Death and decay of organisms and decomposition of
animal wastes makes phosphate ions available again
in the environment.
61. 62
Carbon cycle
Terrestrial: Carbon dioxide in the atmosphere is picked
up by plants during photosynthesis, enters the
ecosystem, and is released back into the atmosphere by
respiration, burning, volcanic activity, and artificial
sources like combustion engines.
It can become buried when organisms die and are
covered by sediments, and later recovered in the form of
coal, oil or natural gas.
Aquatic: Carbon dioxide diffuses into the water, is
picked up by marine algae, enters the ecosystem and
when released back into the water through respiration it
can diffuse back into the atmosphere.
It also ends up in sediments due to death of organisms
that float to the floor of the ocean.
62.
63. Nitrogen in
the air
animal protein
dead plants & animals
urine & faeces
ammonia
nitrites
nitrates
plant made
protein
decomposition by bacteria & fungi
bacteria
(nitrifying bacteria)
nitrates absorbed
denitrifying
bacteria
root nodules
(containing nitrogen
fixing bacteria)
nitrogen fixing
plant
eg pea, clover
bacteria
Nitrogen cycle
