IGCSE Biology 0610: Factors That Limit Population Growth and Carrying Capacity

Twelve rabbits were introduced onto a small island that had abundant grass but no rabbits and no natural predators. Over the next few years, the rabbit population grew rapidly, then levelled off at a roughly stable size.

(a) Name two factors, other than predation, that could limit the size of the rabbit population as it approaches its stable size on the island. [2]

(b) Explain how competition for food, in particular, would slow the rate of population growth as rabbit numbers increase. [3]

(c) Several years later, a small number of foxes were introduced to the island. Predict and explain the likely short-term effect of the foxes on the rabbit population. [2]

(d) Define the term "carrying capacity", as illustrated by the stable rabbit population size reached before the foxes were introduced. [1]

Part (a): Limiting factors other than predation

As a population grows in a fixed habitat, several factors besides predation can limit its size, including:

Food supply. A fixed amount of grass can only support a limited number of rabbits.
Competition for space, for burrows or nesting sites.
Disease or parasites, which can spread more readily and affect more individuals in a crowded population.

Any two of these are acceptable answers.

Part (b): Competition for food and growth rate

The island’s grass supply does not increase simply because the rabbit population is growing, so as rabbit numbers rise, the same amount of food must be shared among more individuals, meaning less food is available per rabbit on average. Rabbits that cannot get enough food are more likely to grow poorly, reproduce less successfully, or die, especially weaker or younger individuals. This raises the death rate and lowers the birth rate compared with when food was plentiful, which slows the rate of population growth. Eventually, births and deaths balance out, and the population size levels off.

Part (c): Introducing a predator

Before the foxes arrived, the rabbit population had no natural predators, so its size was limited mainly by food supply and other resource-based factors. Once foxes are introduced, they add a new source of mortality: rabbits are now also being eaten. This increases the overall death rate in the rabbit population, so in the short term the rabbit population is likely to grow more slowly than before, or to fall, rather than remaining at its previous stable size.

Part (d): Carrying capacity

Carrying capacity is the maximum population size that a habitat can sustainably support, given the resources it provides, such as food, water and space. In this case, the stable rabbit population size reached before the foxes arrived represents the carrying capacity of the island for rabbits at that time, since the available grass could not support any more individuals than this.

Final answers

(a) Any two of: food supply; competition for space; disease or parasites.
(b) Growing rabbit numbers must share a fixed food supply, so each rabbit gets less food on average, raising the death rate and lowering the birth rate, which slows population growth.
(c) The rabbit population’s growth is likely to slow, or the population may fall, because predation by foxes is a new factor increasing the death rate.
(d) Carrying capacity is the maximum population size a habitat can sustainably support with the resources available.