Ch24 Populations and Sustainability
(a) Factors that determine population size
A population is all the organisms of one species in a habitat
For example, all the foxes in a wood form a population. All the people in a town form a population
Population size is the total number of organism of one species in a habitat. This number changes over time as a
result of the effect of various factors
Typical growth of a population
1
, 1) Slow growth phase –
individuals are reproducing but as
there are only small numbers,
growth happens slowly
2) Rapid growth phase – as
the number of breeding individuals
increases, the total population
multiplies exponentially
3) Stable state – further
population growth is prevented by
limiting factors. During this time the
population size fluctuates but overall
its size is stable. Birth rates and death
rates are approximately equal.
Fluctuations are accounted for by
fluctuations in limiting factors
The stable state usually occurs when the environment reaches its carrying capacity. This is the maximum population
size that an environment can support
Limiting factors
Limiting factors prevent further growth of a population and in some cases cause it to decline
Abiotic: temperature, light, pH, availability of water or oxygen, humidity
Biotic: predators, disease, competition
(b) Interactions between populations
Competition
Competition is an example of a biotic limiting factor
Competition can either be:
• Interspecific competition
2
(a) Factors that determine population size
A population is all the organisms of one species in a habitat
For example, all the foxes in a wood form a population. All the people in a town form a population
Population size is the total number of organism of one species in a habitat. This number changes over time as a
result of the effect of various factors
Typical growth of a population
1
, 1) Slow growth phase –
individuals are reproducing but as
there are only small numbers,
growth happens slowly
2) Rapid growth phase – as
the number of breeding individuals
increases, the total population
multiplies exponentially
3) Stable state – further
population growth is prevented by
limiting factors. During this time the
population size fluctuates but overall
its size is stable. Birth rates and death
rates are approximately equal.
Fluctuations are accounted for by
fluctuations in limiting factors
The stable state usually occurs when the environment reaches its carrying capacity. This is the maximum population
size that an environment can support
Limiting factors
Limiting factors prevent further growth of a population and in some cases cause it to decline
Abiotic: temperature, light, pH, availability of water or oxygen, humidity
Biotic: predators, disease, competition
(b) Interactions between populations
Competition
Competition is an example of a biotic limiting factor
Competition can either be:
• Interspecific competition
2
