1
Population
ecology and
environmental
studies
Mrs Slabbert
What we will learn ... 2
• Population Size
• Interactions in the Environment
* predation: two South African examples of predator
prey
• Competition:
* interspecific and intraspecific competition
• Specialisation
* competitive exclusion and resource partitioning;
* parasitism: two examples from South Africa;
* mutualism: two examples from South Africa;
* Commensalism: two examples from South Africa
• discuss one example of coexistence in
animals and one example in plants;
What we will learn ...
(CONTINUED) 3
• Social Organisation: The benefits of herds/flocks
* (avoidance); packs (hunting); dominance; and the division of
tasks (castes)
• Community change over time: Succession
* Primary and secondary succession and possible endpoints
depending on environmental fluctuations
• Human Population
* Reasons for exponential growth: age and gender distributions
for different countries, including South Africa;
* Forecast of South Africa’s population growth over the next
twenty years and predict possible consequences for the
environment.
What you will be able to do ... 4
• Determine the size of a population by quadrant or
simple sampling e.g., simulated mark/recapture.
• Collect and record data,
• Interpret data
• Calculate/estimate the population size.
• Case study: Rationale for culling, e.g. elephants in the
Kruger National Park as an example of an application
of estimating population size (link to researched
reasons for culling).
• Draw up a public survey form to test the public opinion
about culling. Show results in a pie graph
What you will be able to do ...
(continued) 5
• Draw a life cycle of the Bilharzias' (linked with
excretion) parasite or tapeworm (simplify
larval stages). (Links to animal biodiversity)
• Identify an area in or close to the school
grounds where succession is taking/has taken
place. (e.g., in the goal area on the sports
field at the end of a season or a roadside that
has been scraped).
Terminology
6
Definition and relationships amongst:
species,
community,
ecological niche,
habitat,
ecosystem,
population,
ecology
Terminology
8
• SPECIES: a group of organisms that share many
similar characteristics, are able to interbreed
and produce fertile offspring
• POPULATION: a group of individuals of a
particular species that live in the same habitat
at the same time and can randomly interbreed
• COMMUNITY: two or more different
populations living together in the same habitat
9
The
relationship
between
species,
populations
community
and the
ecosystem
Terminology 10
• ECOSYSTEM: is a community of living
organisms (plants, animals and microbes)
in conjunction with the non-living
components of their environment (things
like air, water and mineral soil),
interacting as a system.[
• ECOLOGY: the study of living things and
their relationship to each other and to the
environment
l
Terminology 11
• ECOLOGICAL NICHE: the functional position of
an organism in its environment
• ENVIRONMENTAL RESISTANCE: the combined
effect of all the limiting factors that limit the
growth of a population
• LIMITING FACTOR: factors (density dependent
or density independent) that limit the rate of
population growth
Ecological niche
12
11
Habitat niche 14
Population parameters 15
• There are four factors that
affect the size of the
population Decrease
populatio
1. Natality n size 3. Mortality
2. Immigratio Increase 4. Emigration
n populatio
n size
Population size 16
• If the balance is positive, populations will increase
• If the population is negative, population will decrease
• If they are equal the population size remains the same
(stable or in equilibrium)
• N = population size
• B = births
• D = deaths
• I = immigration
• E = emigration
Population size 17
A stable population:
B+I=D+E
To calculate population size:
N = (B + I) – (D + E)
Activity 1 page 270
Graph to show a geometric (J-shaped) growth
form
Number of indivuals – population size
18
Lag Accelerating Repetition in
phase growth
phase mosquitoes
(summer
growth,
winter death
phase)
Time
© Lizette Neethling
0828246343
growth of bacterial
population over 24 hour
Population growth forms period
19
1200
GEOMETRIC GROWTH FORM 1000
OR J-SHAPED CURVE growth of
800
• Populations grow slowly bacterial
600 population
at first
400 over 24
• It then increases hour period
exponentially over a 200
short period of time 0
• The population may 0 20 40
decrease very rapidly
once it reaches the The characteristic
maximum carrying pattern of increase of a
population is referred
capacity to as its growth form
Graph to show a logistic (S-shaped) growth form
Environmental resistance
Carrying capacity 20
Number of indivuals – population size
Lag Accelerating
phase growth
phase
Decelerating Equilibrium
growth phase
phase (stationary
phase)
© Lizette Neethling Time
0828246343
The figure below illustrates a population that has reached
its carrying capacity level. It may remain constant, but
may fluctuate in response to the (a) environment (density 21
independent factors) and (b) inherent regulating factors
(density-independent factors)
Population size
22
Aspects of population fluctuation and regulation
Carrying capacity
23
Carrying capacity is the maximum population size a certain
environment can support for an extended period of time, for a
population of a particular species.
Under ideal conditions, a 24
population naturally
increases until it overshoots
the carrying capacity. At this
point, the environment can
no longer provide for the
species, due to a number of
different environmental
resistances, including food,
crowding, competition,
etc. The population, due to
lack of resources, will begin
to die out, allowing the
environment to recover.
Carrying capacity is the number of individuals a habitat can sustain without
being permanently damaged due to over population.
Carrying capacity is regulated by environmental resistance which
are inhibiting factors fundamental to the habitat
Population growth forms 25
LOGISTIC GROWTH FORM
OR S-SHAPED CURVE
• Populations grow slowly at first
– A. lag phase growth of kudu population
• It then increases exponentially over a 50 year period
over a short period of time – B. 45
acceleration /log phase 40
• As the population approaches 35
the carrying capacity the 30 growth of
population growth slows down 25 kudu
– C. deceleration phase 20 population
• Once it reaches the asymptote 15 over a 50
the population levels off and 10 year period
fluctuates around the carrying 5
capacity - D. equilibrium 0
phase 0 20 40 60
26
27
If a population reaches carrying
capacity it can remain stable or
fluctuate
If there is more rainfall and more
food available the carrying
capacity increases and the
population will increase until it
reaches the new carrying
capacity before it levels off
again.
If there is habitat destruction or a
draught the carrying capacity
decreases and the population will
decrease until it reaches the new
carrying capacity and levels off
again.
Carrying capacity of rabbits in a specific area
1. What does the blue line
represent? What does the purple 28
line represent? What does it
mean when the purple line rises
above the blue line?
2. Which of the following
situations might cause the purple
line to decrease below the blue
line: abundant food sources, lack
of competition, a young
population, or plentiful roaming
space?
3. Can you think of any
events that would cause
the purple line to stay
above the blue line
indefinitely?
Environmental Resistance 29
• Environmental resistance is caused by limiting
factors. These regulate the population size
• Carrying capacity
• Density-dependent
• Density-independent
• Competition
• Territoriality
• Predation
Population regulating factors
Density: number of individuals in a 30
population
• Environmental factors
that are directly
dependent on the
numbers in the population
• E.g. food, water, space,
shelter, predators,
disease etc.
• So the more zebra there
are in a population the
more competition there
will be for food.
Population regulating factors
Density: number of individuals in a population 31
• Environmental factors that
will affect the population
no matter how dense the
population is
• E.g. drought, fire, floods,
tornados, hurricanes,
earthquakes, tsunamis etc.
• So no matter how many
mice in a field there are if
a fire sweeps through the
habitat the population will
be affected.
Interactions in a community
competing for resources 32
• Competition within the
species for resources as the
population gets bigger and
reaches carrying capacity
• e.g. males competing for
mating partners, competition
for food, nutrients in the soil,
light, water etc.
• This is overcome by strategies
like territoriality or ensuring
seeds are dispersed far from
the parent plant etc.
Interactions in a community 33
competing for resources
• Competition between
organisms from different
species.
• e.g. herbivores
competing for the same
food resources
• This is often alleviated by
resource partitioning i.e.
dividing the food
resource and having
different levels of
feeding like grazers and
browsers.
Territoriality 34
• Many animals demarcate a
piece of the habitat as
their territory.
• In this demarcated territory
they protect their
resources i.e. mating
partners and offspring
• This is one of the most
important methods of
regulating populations of
birds and large mammals as
natality rate is kept low
Territoriality 35
• Territory is chosen by the male to secure and
protect resources e.g. food and breeding
space
• Defend their territories
• Birds announce their territory by singing
• Failure to establish a territory means they
won’t breed successfully or move out of the
area
Ecological niche 36
• Individuals in populations compete for resources such as
food, space, shelter, water and access to mates.
• The resources that a population needs are determined
by the ecological niche of that species.
• This refers to the habitat and the role of a species in an
ecosystem i.e. being a producer or a predator or
decomposer
• Factors like optimum (preferred) temperatures,
moisture and pH are also important
• The individuals that successfully compete for the
required resources survive and those that don’t obtain
the resources may die or emigrate
Interactions in the environment 37
• Predation
• Competition
• Parasitism
• Mutualism
PREDATION 38
Predation 39
• This is a feeding relationship in which one animal catches and kills
another animal for food.
• The animal that does the killing is called the Predator
• the animal that is killed is called the Prey
• Generally the weaker animals are killed, leaving the stronger ones to
survive and reproduce
• Predation is a population
limiting factor which
prevents prey populations
from becoming too big
• The prey population helps
to regulate the predator
population
Predator-prey interactions 40
• Populations fluctuate (high and low)
• Called population cycles (e.g., large specie 8-11 year
cycle)
• If predator relies on single prey species – cyclical
fluctuation (Fig 3.1.15 pg 285)
• Predators usually maintain prey population below
carrying capacity
• Predators may have no impact on the prey population
(attack old, young and sick)
• Lions are opportunistic predators
COMPETITION
• Struggle for resources
• Between different species –
interspecific competition
• Affects breeding, distribution
and evolution
41
Competitive exclusion
referred to as Gause's Law of competitive exclusion or just
Gause's Law, states that two species that compete for the 42
exact same resources cannot stably coexist.
• Competition between two species
which results in one species surviving
and the other species disappearing
• One of the two competitors will
always have an ever so slight
advantage over the other that leads to
extinction of the second competitor in
the long run
• Has resulted in the extinction of most
of the organisms that have ever
existed on earth
• therefor played an important part in
the process of evolution
• Can survive together if they use
different parts of the environment –
niche differentiation
43
Competitive coexistence 44
• Competition
between two species
that results in both
species surviving but
with smaller
populations than if
they lived on their
own
• This is brought about
by resource
partitioning
• Temporal partitioning
• Two species use the
same resource but at
Resource partitioning different times
45
• Diurnal and nocturnal
animals feeding on the
same leaves at different
times
• Plants growing at
different times of the
year
• Spatial partitioning
• When two species use the
same resources but different
parts
• Plant roots that grow at
different depths in the soil
• Animals feeding on the same
tree at different heights
Resource partitioning 46
47
PARASITISM
48
49
• Parasites live on or in their hosts, take what
they need from the host and damage the host
in the process.
• Usually smaller than host
• Specialised for their lifestyle
• Reproduce quickly and in great numbers
• Often cause disease e.g., tapeworm and
bilharzia
MUTUALISM
50
51
• Relationship between two organisms of different
species in which both benefit from the association
• Eg., bees pollinate flowers and in return feed on
nectar and pollen
• Nitrogen-fixing bacteria on roots
COMMENSALISM
52
53
• A relationship where one organism benefits
and the other neither benefits or is harmed
• Egrets and grazers (buffalo, rhino, cattle)
• Lichen hanging from a tree
Symbiosis
a close relationship and interaction
between two organisms of different
54
species
• There are three types of symbiosis
• 1. Mutualism: where two individuals of different
species live together and both benefit from this
interaction
• Examples
• pollination in flowers and bees
• Myrmecophily is an interaction between a
species of plant or animal and ants e.g. Brenton
Blue butterfly and ants & aphids and ants
• Removal of parasites e.g. oxpecker and
herbivores
Symbiosis 55
2. Commensalism: where two individuals of different
species live together and one benefits from this
interaction and the other is not affected
• Examples
• Kelp and Limpet on Western Cape coast
• Epiphytes like orchids and trees
• Remora fish and Shark
• Cattle Egret
• Anemone & Clownfish
symbiosis 56
3. Parasitism: where two individuals of different species live together
and one benefits from this interaction and the other called the host
is harmed.
• Examples
• Dodder gets organic nutrients, water and minerals from
the host
• Malaria & Bilharzia
Counting Populations 57
• Difficult to count populations in nature for
various reasons, for example, an ecologist may
disturb the population which will affect the
results
• Population sizes are almost always estimated
instead of obtaining an absolute count
• Direct counting
• Indirect counting
Population estimation 58
• Population size is determined by physically
counting all individuals in the population
• E.g. census, aerial photographs
• This can be used to count humans, slow
moving or sessile animals such as Molluscs or
Barnacles as well plants
Methods to determine population size
• Direct technique (census – counting individuals) 37
.
Population estimation 60
• This is allows us to estimate the size of a
population using different techniques
• E.g. mark – recapture, sampling, quadrant method
• These methods need to be repeated to improve
reliability
• Usually used to estimate the number of plants in
an area or the size of aquatic animal populations in
lakes and dams
Mark recapture
A number of animals are caught, marked and then
released to mix thoroughly with the unmarked individuals
45
of the same species. Later, after allowing sufficient time
for the dispersal, a
second sample is taken and the
number of marked and
unmarked animals is counted.
Peterson or Mark – recapture
method 62
For the mark – recapture method the formula below must be
used
• N = MXC N: estimated population size
R M:1st sample captured, marked and
released
C: 2nd sample caught
R: total number of marked
individuals in the second
capture
• Estimate the population of carp fish in Centurion lake if the
1st capture was 27 and 10 days later 43 were caught, of
which 16 were marked.
Precautions to take to ensure a
reliable result 48
• Only a short time should pass between the first and the
second sampling so that no births and deaths can occur
(or immigration/emigration)
• Sampling should be repeated several times and an
average population calculated
• The marking must not damage the individual (no harm)
• Marking must not affect its movement or behaviour
• The marked animal must mix freely with the rest of the
population before a new sample is taken
• The 2nd catch sample should be larger than the 1st
© Lizette Neethling
0828246343
Simple sampling method 64
• Count all the individuals in a small area of the habitat
and calculate the total population using the following
formula
• Population = no. of organisms in sample x habitat size
sample size
• Estimate the size of a population of dandelions if there
are 3 in a sample area 10 m². The whole habitat is
5000m²
41
Quadrat sampling method
This method is useful in sampling plants and
sessile or slow-moving organisms 42
Choose your samples randomly
Take several samples and take the
average estimate
Social organisation of populations
various structured interaction to improve the survival of the 67
population and increase ability to breed
• Herds – makes it difficult for predators
to attack individuals e.g. buffalo
• Flocks/schools – to avoid predators
e.g. flamingos, sardines
• Communal breeding – alerts all to
possible danger e.g. weaver birds
• Cooperative hunting in packs e.g. Wild
Dogs & Lions (ambush, overpowering)
• Breeding hierarchies where only
certain individuals are allowed to
breed e.g. Hyenas & Wild Dogs,
• Organised societies with labour
division e.g. castes in Bees & Termites
Ecological Succession
68
• the process of change or
transformation over time as one type
of community or ecosystem takes
over another
• This is an orderly change that we can
predict and that usually occurs over
long periods of time (from pioneer
organisms to climax community)
• Fig 3.2.29 pg 301
Stage 2 Stage 6
Stage 5
Stage 1 Lichens Stage 3 Stage 4 climax
Small
Origin – - Mosses- Ferns- communit
flowerin
bare rock pioneer pioneers pioneers y- shrubs
g plants
s & trees
Ecological succession 69
• As a habitat moves from one stage
to the next there is an increase in biodiversity.
• Nutrients are retuned to the soil and food chains are
established.
• The sequence of vegetation types that occur during
succession – from pioneer to climax is called a sere
• There are a number of different seres according to the
environment be colonised
• E.g. hydro sere: succession in an aquatic environment
• Halo sere: succession in a salt marsh
Primary succession 70
Primary succession
occurs on entirely
new areas where
there have been no
previous (plant)
communities
Example: sand
dunes, newly
quarried rock faces,
newly formed
islands due to
volcanic activity and
even mine dumps
Primary succession 71
• Each stage is accompanied by animals
and decomposers
• Pioneer species are tough and resilient
• Climax communities are rarely reached
Secondary succession 72
• Takes place in areas where an existing
ecosystem has been disturbed
• Examples: forest fires, abandoned farmland
and deforestation
Secondary succession
• Secondary succession is usually 73
a faster process because:
• Soil already exists
• There are nutrients in soil
• There are already plant seeds in the soil
• Root system are undisturbed in the soil
• Tree stumps, roots and other parts can
regenerate quickly
• Fertility and structure of the soil modified by
previous organisms make it suitable for
growth and recolonisation
Stage 2
Stage 1 Stage 3 Stage 4
Pioneer plants
Bare Small shrubs Climax
–
ground & trees community
Grass & weeds
74
• We can measure human
demands on the environment
using the ecological
footprint
• It measures the amount of
biologically productive land
and sea humans need to
Ecological footprint produce the resources it
consumes, and to absorb the
waste it generates
Human needs versus
conservation 75
• The delicate balance is very often
damaged by human activity,
• degradation of the environment.
• A rapidly increasing population exploits
natural resources.
• Developed world - 20% of the world’s
population but consumes more than
50% of the world’s resources
• The individual footprint of a person in
a developed country would be heavier
than that of a person in a developing
country
• Sustainability = Management of natural
resources
• Conserving ecosystems = increasing
biodiversity.
Human populations 76
• Estimation of population
sizes
• The first census was done in
1650
• Earlier estimates were based
on radiocarbon dating -
fossil evidence and other
artefacts such as weapons
and tools
• ± 5 million in 8000 BC to
±200& 300 million at the
beginning of this era
• Determination of human
population since 1650 has
been through censuses
Human populations 77
• Estimation of population size
for the future is made as
follows:
• Studying present trends of
population growth and calculation
of the present growth rate
• Extending this trend into the future
to project population figures for
years to come
• One such projection based on the
current increase of approximately
1,6% pa that the world population
would be more than 7 billion by
2010 and about 8 billion by 2019
Understanding human
population growth 78
• The human population
grows when the birth rate
is higher than the death
rate
• Factors that influenced the
growth of the human
population:
• Prior to 1650 hunter
gathering way of life
• 1650 agricultural revolution
• 1750 industrial revolution
• 1800 medical revolution
• 1850 urbanisation
Age and gender distribution for
different countries 79
• Age-genders population pyramids
• Illustrate the age and sex distribution in a population
• Show trends and predictions
• Can plan for schooling etc.
• The study of population trends is called demography
• Demographers predict population sizes using age and
gender pyramids
• Pg 305: stationary, expanding, constrictive pyramids
Consequences of further human
population growth for the natural 80
environment.
• In SA, more young than old
• Life expectancy is dropping due to HIV/AIDS
• Due to many young people, we will need greater
environmental and economical resources in the future
• We have to use our natural resources wisely to sustain:
• Human development
• Health facilities
• Industrial and economical development
• Transport
• Energy requirements (global warming)
