Population ecology and

environmental studiesMrs Slabbert

1

What we will learn ...

• 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;

2

What we will learn ... (CONTINUED)

• 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.

3

What you will be able to do ...

• 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

4

What you will be able to do ... (continued)

• 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).

5

Terminology

Definition and relationships amongst:

species,

community,

ecological niche,

habitat,

ecosystem,

population,

ecology

6

Terminology

• 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

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The relationship between species, populations community and the ecosystem

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Terminology

• 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

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Terminology

• 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

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Ecological niche12

11

Habitat niche 14

Population parameters

• There are four factors that

affect the size of the

population

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Decrease populatio

n sizeIncrease population size

1. Natality2. Immigratio

n

3. Mortality4. Emigration

Population size

• 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

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Population size

A stable population:

B + I = D + E

To calculate population size:

N = (B + I) – (D + E)

Activity 1 page 270

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Time

Num

be

r of in

div

uals

–popula

tion s

ize

Graph to show a geometric (J-shaped) growth

form

Lag

phase

Accelerating

growth

phase

Repetition in

mosquitoes

(summer

growth,

winter death

phase)

© Lizette Neethling

0828246343

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Population growth forms

0

200

400

600

800

1000

1200

0 20 40

growth of bacterial population over 24 hour

period

growth ofbacterialpopulationover 24hour period

The characteristic pattern of increase of a population is referred to as its growth form

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GEOMETRIC GROWTH FORM

OR J-SHAPED CURVE

• Populations grow slowly

at first

• It then increases

exponentially over a

short period of time

• The population may

decrease very rapidly

once it reaches the

maximum carrying

capacity

Time

Nu

mb

er

of in

div

ua

ls –

po

pu

latio

n s

ize Carrying capacity

Lag

phase

Accelerating

growth

phase

Decelerating

growth

phase

Equilibrium

phase

(stationary

phase)

Environmental resistance

Graph to show a logistic (S-shaped) growth form

© Lizette Neethling

0828246343

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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

independent factors) and (b) inherent regulating factors

(density-independent factors)

Po

pu

lati

on

siz

e21

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Aspects of population fluctuation and regulation

Carrying capacity

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.

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Under ideal conditions, a 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.

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Population growth forms

0

5

10

15

20

25

30

35

40

45

0 20 40 60

growth of kudu population over a 50 year period

growth ofkudupopulationover a 50year period

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

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LOGISTIC GROWTH FORM

OR S-SHAPED CURVE• Populations grow slowly at first

– A. lag phase

• It then increases exponentially

over a short period of time – B.

acceleration /log phase

• As the population approaches

the carrying capacity the

population growth slows down

– C. deceleration phase

• Once it reaches the asymptote

the population levels off and

fluctuates around the carrying

capacity - D. equilibrium

phase

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If a population reaches carrying capacity it can remain stable or fluctuateIf 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.

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Carrying capacity of rabbits in a specific area

1. What does the blue line represent? What does the purple 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?

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Environmental Resistance

• Environmental resistance is caused by limiting factors. These regulate the population size

• Carrying capacity

• Density-dependent

• Density-independent

• Competition

• Territoriality

• Predation

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• 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.

30Population regulating factorsDensity: number of individuals in a population

• 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.

31Population regulating factorsDensity: number of individuals in a population

• 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.

32Interactions in a communitycompeting for resources

Interactions in a communitycompeting 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.

33

Territoriality

• 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

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Territoriality

• 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

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Ecological niche

• 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

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Interactions in the environment

• Predation

• Competition

• Parasitism

• Mutualism

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PREDATION 38

Predation

• 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

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Predator-prey interactions

• 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

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COMPETITION

• Struggle for resources

• Between different species –interspecific competition

• Affects breeding, distribution and evolution

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Competitive exclusionreferred to as Gause's Law of competitive exclusion or just Gause's Law, states that two species that compete for the 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

42

43

Competitive coexistence

• 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

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Resource partitioning 45

• Temporal partitioning• Two species use the

same resource but at different times

• 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

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PARASITISM

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• 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

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MUTUALISM

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• 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

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COMMENSALISM

52

• 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

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Symbiosisa close relationship and interaction between two organisms of different 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

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Symbiosis

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

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symbiosis

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

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Counting Populations

• 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

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Population estimation

• 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

58

Methods to determine population size

• Direct technique (census – counting individuals)

.

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Population estimation

• 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

60

Mark recapture

A number of animals are caught, marked and then released to mix thoroughly with the unmarked individuals 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.

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Peterson or Mark – recapture method

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.

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Precautions to take to ensure a reliable result

• 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

48

Simple sampling method

• 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

64

Quadrat sampling method

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This method is useful in sampling plants and

sessile or slow-moving organisms

Choose your samples randomly

Take several samples and take the

average estimate

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Social organisation of populationsvarious structured interaction to improve the survival of the 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

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Ecological Succession

• 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

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Stage 1

Origin –

bare rock

Stage 2

Lichens

-

pioneer

s

Stage 3

Mosses-

pioneers

Stage 5

Small

flowerin

g plants

Stage 6

climax

communit

y- shrubs

& trees

Stage 4

Ferns-

pioneers

Ecological succession

• 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

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Primary succession

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

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Primary succession

• Each stage is accompanied by animals and decomposers

• Pioneer species are tough and resilient

• Climax communities are rarely reached

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Secondary succession

• Takes place in areas where an existing ecosystem has been disturbed

• Examples: forest fires, abandoned farmland and deforestation

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Secondary succession

• Secondary succession is usually

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

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Stage 1

Bare

ground

Stage 2

Pioneer plants

–

Grass & weeds

Stage 3

Small shrubs

& trees

Stage 4

Climax

community

Ecological footprint

• 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 produce the resources it consumes, and to absorb the waste it generates

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Human needs versus conservation

• 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.

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Human populations

• 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

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Human populations

• 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

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Understanding humanpopulation growth

• 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

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Age and gender distribution for different countries

• 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

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Consequences of further human population growth for the natural 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)

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