Fig. 8.2 again shows the growth of a population that has been placed in a container with a limited supply of food. As the population becomes larger, the ind...

This question is about how a population growing in a closed container reaches a limit set by its resources. The S-shaped curve in Fig. 8.2 levels off because the environment can only support so many individuals.

(a) Individuals compete for anything in limited supply. Any two of: food, space, water, light or oxygen earn the marks [2]. Competition means more individuals are sharing the same fixed amount, so each gets less.

(b) As the population nears its maximum, the birth rate falls [1] and the death rate rises [1], and growth stops when the birth rate becomes equal to the death rate [1]. When births and deaths balance, the number added each generation equals the number lost, so the total stays roughly constant. This flat part of the curve is the carrying capacity [3].

(c) Waste products build up because there is no way for them to be removed in a closed container. The chain is: waste products become toxic / poisonous [1]; the conditions become unsuitable for the organisms [1]; so more individuals die or fewer reproduce, and growth stops [1]. The key idea is cause and effect: the waste itself does not use up food, it poisons the surroundings [3].

(d) Suggest any sensible factor with a reason, for example disease spreading more easily in a crowded population [1] which increases the death rate, or predators reducing the numbers [1]. The mark for the factor and the mark for its effect together give [2].

Examination tip: when a growth curve flattens, always explain it as birth rate falling towards death rate rising, not simply as "running out of food".