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1.Factors Affecting
Population Size
OBJECTIVES
> To understand what is meant by environmental resistance
> To list examples of biotic and abiotic factors which limit population growth
> To explain the form of a typical growth curve
A population is a group of organisms of one Important abiotic factors affecting population
species, living in the same area at the same time. growth include:
Environmental resistance Temperature - higher temperatures speed
By taking samples and counting the numbers of up enzyme-catalysed reactions and
organisms in a particular habitat, an ecologist can increase growth.
study how any factor affects the size of the Oxygen availability - affects the rate of
population. For example, a woodland manager energy production by respiration.
might wish to know whether thinning the trees in the Light availability - for photosynthesis.
wood affects the population of low-growing plants, Light may also control breeding cycles in
or a farmer might wish to know whether the time of animals and plants.
grass-cutting affects the population of bank voles. Toxins and pollutants - tissue growth can
These factors may be living (biotic) or non-living be reduced by the presence of, for
(abiotic). Together they affect the rate at which a example, sulfur dioxide, and reproductive
population grows, and also its final size. All the success may be affected by pollutants
factors that affect population growth and size such as oestrogen-like substances.
together make up the environmental resistance.
Some significant biotic factors that affect population Growth curves and carrying capacity
growth include: When a small population begins to grow in a
Food - both the quantity and the quality of food particular environment, the environmental
are important. Snails, for example, cannot resistance is almost non-existent - there may
reproduce successfully in an environment low in be plenty of food and no accumulation of
calcium, no matter how much food there is, poisonous wastes. The diagram opposite
because they need this mineral for shell growth. shows how environmental resistance
Predators - as a prey population becomes eventually limits population growth, and the
larger, it becomes easier for predators to find environment reaches its carrying capacity.
the prey. If the number of predators suddenly Unless the environmental resistance is
falls, the prey species might increase in number changed, perhaps by a new disease
extremely quickly. organism, the size of the population will only
Competitors - other organisms may require the fluctuate slightly. Organisms that are able to
same resources, such as food, from the maintain their population, or even increase it,
environment, and so reduce the growth of a must be well adapted to their particular
population. environment.
Disease - often caused by parasites, and may
slow down the growth and reproductive rate of
organisms within a population.
1 Learning Module Organisms and Their Environment | Class 11
