POPULATION ECOLOGY RECAP

Population ecology is concerned with fluctuations in the size of a population and

the factors, both physical and social that regulates these fluctuations.

TERMS

Organism is an individual form of life.

Biosphere is the part of the earth where living organisms are found.

Ecosystem is made up of groups of different species of organisms that interact

with each other and with the environment. Community is a group of different
species that inhabit and interact in a particular area.

Species is a group of closely related organisms that are very similar to each other
and are usually capable of interbreeding and producing fertile offspring.

Population is a group of organisms of the same species, in the same area that
can interbreed and produce fertile offspring.

What affects the size of a population?

• Natality – birth rate in animals or the production of seeds in plants

• Mortality – death rate
• Immigration – individuals move into a population and stay
• Emigration – individuals leave a population and do not return

For humans the:

• birth rate is the number of births per 1000 people in a year.

• death rate is the number of deaths per 1000 people per year.

In a closed population = no immigration or emigration.

How is the growth of a population regulated?

Food in abundance, no predators- organisms reproduce exponentially- results in
shortage of resources- build environmental resistance- resulting in decrease in birth
rate and immigration and an increase in death rate.

environmental resistance = the total number of factors that stop a population from
reproducing at its maximum rate

Eventually a balance is reached- population stabilised at carrying capacity of the

ecosystem.

carrying capacity = the population density that the environment can support

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Limiting factors- Regulate the growth of a population

• Density independent factors that limit the growth of a population as a result

of natural factors

–physical factors, eg rainfall, temperature, humidity, acidity, salinity

–catastrophic events, eg floods, fire, drought, volcanic eruptions, tsunami,

earthquakes.

• Density dependent factors that have a greater effect when the population
density is high.

–compete more for resources such as food, light, oxygen, water, space and
shelter

–are more easily found by predators

–spread disease and parasites more readily.

These limiting factors collectively build up environmental resistance.

Stable and unstable populations

A stable population -one that fluctuates around the carrying capacity.

An unstable population- far exceeds the carrying capacity.

How is population size estimated?

1. Direct methods
Census and aerial photographs

2. Indirect methods
1. Quadrat method
Total population = (average) numbers in sample x size of whole habitat

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size of quadrat

2. Mark-recapture method
P = TOTAL MARKED x TOTAL CAUGHT IN SECOND SAMPLE

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TOTAL MARKED IN SECOND SAMPLE

What precautions must be taken for a reliable result?

• Only a short time should pass between the first and second sampling, so that
no births and deaths can occur.
• Sampling should be repeated several times and an average population
calculated.
• The marking must not damage the individual or affect its movement or
behaviour.
• The marked animal must mix freely with the rest of the population before a
new sample is taken.
• No immigration or emigration is allowed, i.e. the population must be closed.

Predator-prey relationships
The feeding relationship between the predator and the prey determines the size of
the two populations by means of a negative feedback mechanism.

How does this relationship work?

As the prey population decreases due to predator killing, the food available for the
predators is less, and so their numbers subsequently decline.

With the predator pressure reduced, the numbers of the prey can increase once
again and the cycle goes on.

Competition
competition = when two or more individuals compete for the same resources that are
in short supply

A. Intraspecific competition- same species.

B. Interspecific competition- different species

Ecological niches- all the conditions necessary for an organism to survive and
reproduce.
specialisation = the structural and behavioural adaptations that enable individuals of
different species to co-exist.

Competitive exclusion - one of the two competing species is much more successful

than the other. The successful species survives and the other disappears.

Competitive co-existence- two competing species co-exist in the same habitat.

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Resource partitioning- species with similar requirements, living in the same
habitat, evolve specialised traits that enable them to utilise the resources
differently.

Ecological succession is a predictable pattern of gradual change over time in the

types of species in a community following a disturbance.

Primary succession begins on sites that have not previously had plants growing on
them, such as beaches, larva flows, severe landslips, ponds and bare rock – No soil.

Secondary succession begins in areas where a disturbance removes some or all

species, but the soil remains.

Pioneer species stage- Hardy vegetation that establishes rapidly.

Primary succession- Lichens, Mosses, ferns and grasses.

Secondary succession- grasses, herbs and weeds

Climax community
This is the last semi-stable stage or the endpoint of succession. Ω

1. Herds or flocks as a predator

Safety of the group is increased by avoiding and defending against predators-
dilution effect.

As a predator tries to single out its prey, the herd scatters in all directions, confusing
the predator- confusion and distraction effect.

2. Packs as a successful hunting strategy

African Wild dogs are highly social animals with complex methods of
communication that keep the group functioning together, especially when hunting.

How do they catch their prey?

They hunt in very closely-knit packs of up to fifteen adults. Their prey includes
antelope, zebra and warthog.

• They are tireless runners, prey tires they immobilise it; young eats first
• Wild dogs left behind in the den – the dominant female and the pups, or those
unable to hunt – the sick, injured and very old, are fed on regurgitated meat.

3. Animals with a dominant breeding pair

Dominant male and female breeding pairs, e.g. wild dogs.

Pack led by dominant alpha male and female- mate for life- only breeding pair.

What happens to the offspring?

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 • The females reach sexual maturity at eighteen months to two years, at which
point they leave to join a new pack.
• The males remain with their pack for the rest of their lives, which last, on
average, eleven years.

What are the benefits of this type of social organization?

• The dominant pair keeps the pack under control which operates as a highly
successful unit to ensure the survival of the species.
• Raising the pups of the dominant breeding pair and caring for old or sick
individuals is a group task.
• Subordinate members of the group benefit too in that they have access to
mates and other resources shared by the group. At a later stage in their lives,

these members may become dominant as well.

4. Division of tasks among castes- termites, bees, wasps and naked mole rats-
Each individual in the group has a role to play that is important for the success of the
group as a whole.
Eusocial animals are the most advanced form of social organisation. These
animals live in colonies in which:

• there is a dominant breeding pair or a single female (queen).

• the non-breeding animals have different tasks to perform, i.e. there is a
strict caste system. Tasks include collecting food, caring for the young and
building, maintaining and protecting the nest.

eusocial = species that exhibit the highest level of social organisation

Termites – eusocial insects

Their caste system includes:

Human population dynamics

Currently growing exponentially.

What has caused this exponential growth?

The recent alarming population growth is mainly due to the fact that humans have:

• reduced environmental resistance and

• increased the carrying capacity of the world’s food-producing regions.

This has been done in two ways:

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1.Food production has increased substantially.

2.Methods of treating diseases have improved greatly.

Do countries of the world differ?

1.More developed countries (MDCs) that have a:

• slow population growth, i.e. 0.1% per year

• high standard of living, e.g. North America, Europe, Japan and Australia.

2.Less developed countries (LDCs) that have a:

• rapid population growth, i.e. 1.6% per year

• lower standard of living, e.g. Latin America, Africa and Asia.

What is a population pyramid?

A bar graph that shows the composition, by age and sex, of a nation's population at
the time of a census.

A population contains three major age/sex groups:

• pre-reproductive
• reproductive
• post-reproductive.

What factors can cause the make-up of a population to change?

The most dramatic changes in the make-up of a population may be due to:

• HIV/AIDS causing the deaths of many sexually active young men and women
particularly in developing countries.
• high proportions of young immigrants being rapidly absorbed, or losses due
to emigration of able-bodied young adults.
• losses due to able bodied men fighting wars.
• reduced birth rates during times of economic crisis.

EXAM PRACTICE QUESTIONS 2023 PAPER I

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