The new species may change the environment in such a way that it becomes…

The idea

Succession removes species as well as adding them, because the same change that opens a habitat to new colonisers can close it to the ones that made it possible. Mosses growing on the thin soil formed by lichens shade those lichens and hold water at the surface; taller herbs then overtop the mosses; shrubs and finally trees form a canopy that cuts light intensity at ground level to a small fraction of full sunlight. Light-demanding pioneers cannot photosynthesise fast enough in that shade, so they lose the interspecific competition for light, their population falls and they are excluded from the area. The same happens below ground, where deeper-rooted, faster-growing species absorb water and mineral ions more effectively, so the shallowly anchored pioneers are outcompeted for those resources too.

What earns the marks

• The new species changing the abiotic environment in a way that disadvantages the earlier species, for example by reducing light intensity through shading.
• Identifying the competition as interspecific and naming the resource competed for.
• The earlier species being outcompeted, so its population falls and it is excluded from that habitat.
• Stating that the earlier species survives elsewhere rather than becoming extinct.
• Linking this loss to the change in species composition seen at the climax community.

Common mistakes and how to put them right

• Writing that pioneer species die out or become extinct, rather than being outcompeted and excluded from that area.

• Saying the new species kills the earlier species, when the mechanism is competition for a resource.

• Omitting the named resource, so the competition mark is not earned.

• Describing intraspecific competition when the two species are different.

• Assuming the climax community must still contain all the earlier species.

Exam tips

• Write outcompeted for, then name the resource; the resource earns the mark, not the word competition.
• Explain both directions of the same change: it favours one species and disadvantages another.
• Use data on species present at each stage to say which species have disappeared.