International A Level Biology / Ecology

How the niche explains distribution and abundance

Edexcel International A Level Biology (YBI11), chapter 15: Ecology, specification point 5.13. Free lesson.

Read the revision notes · International A Level Biology past papers

A species’ niche helps explain where it lives and how many individuals are found there. Competition for shared resources can restrict its distribution, while differences in niches allow species to share a habitat.

What distribution and abundance mean

These terms answer different questions about a species. Distribution tells you where it occurs in a habitat; abundance tells you how many individuals there are. A species can be widespread but still rare. Find distribution by recording where a species occurs, for example along a transect, which is a line across a habitat. Measure abundance using density, frequency or percentage cover.

How niches lead to competitive exclusion

A niche is a species’ role in its habitat. It includes how the species interacts with other organisms, and the conditions and resources it needs. If two species have the same niche, they need the same food, space and conditions, so they compete. Even a small advantage can mean one is better adapted and more of its offspring survive and breed. Over generations, the other species declines. It may die out there, or survive only where the winner cannot live. This is the competitive exclusion principle: one species per niche in the long term. Species can share a habitat, but not the same…

Why barnacles live at different heights

Joseph Connell studied two barnacle species on a rocky shore in Scotland. Barnacles cement themselves to rock and filter food from the sea. On the natural shore, small Chthamalus lives high up, while larger Semibalanus (also called Balanus in older books) lives in the middle and lower zones. Their distribution is explained by both drying out and competition for space. Chthamalus can survive long periods out of the water at low tide. Semibalanus dies if exposed for too long, so it cannot live high on the shore. Lower down, Semibalanus grows faster. It crowds Chthamalus out by growing…

How warblers share a habitat

Robert MacArthur showed that several species can survive in the same North American spruce forest when their niches differ. Three warblers eat insects, but feed in different zones of the trees. Because they do not all feed in the same place, they compete less and can share the habitat. Pine warbler feeds in the upper canopy. Yellow-rumped warbler feeds in the middle section and also takes berries. Ovenbird forages in the leaf litter at the base of the tree.

Applying the niche idea

If two species have the same niche, identify the resource they share, then explain how a difference in adaptation affects which one survives. For example, two species of Paramecium grown together in one culture both feed on bacteria. They compete for that food; the better-adapted species has more surviving offspring, while the other dies out. This is competitive exclusion.

More lessons in Ecology

• Population, community, habitat and ecosystem
• Succession: from colonisation to climax
• Abiotic factors and population size
• Biotic factors: predation, competition and disease
• Core practical 11: random sampling with quadrats
• Core practical 11: transects and abiotic measurements
• Null hypotheses and choosing a statistical test
• Spearman's rank correlation coefficient
• Student's t-test
• The chi-squared test