Who Eats Whom, and What It Costs

Five communities, one index

Every community below holds a hundred individuals. Pick one and the index is recomputed in front of you from the raw counts — not looked up. Two of them contain exactly the same number of species.

Term by term

The index is one subtraction away from a sum of squares. Squaring is what makes it notice dominance: look at how much a single common species contributes compared with a rare one.

Richness against evenness

All five communities at once. Richness runs left to right, evenness bottom to top, and the size of each marker is its diversity index. Neither axis alone predicts the marker size.

Every interaction is a pair of signs

Ask what the relationship does to each population — helps it, harms it, or neither — and you have classified it. Almost. Watch what happens to two of these rows.

Neither population is steering

Predators and prey, run from two different starting points. The model is identical in both runs — same birth rate, same predation rate, same conversion, same death rate. Only the starting distance from equilibrium differs, and the textbook answers hold in one run and not the other.

Whose number is it?

Both populations hover around an equilibrium. Work out which parameters set which equilibrium and the answer is not the one you would guess.

Remove the top predator

The same rules, the same removal, run on a four-level chain and on a three-level one. Choose a chain and read the directions downward.

Two advantages, one of which is useless

Two species, one resource, used in exactly the same way. In the first run species A grows faster. In the second it tolerates more crowding. Only one of those is an advantage that ends the competition.

Then separate the niches

The same pair, the same advantage, run at four degrees of niche overlap. Overlap 1.0 means they are using the resource identically; 0.3 means they mostly are not competing at all.

Check yourself

Answer these in your own words before you open the answers.

1. Define a community, and name the two things its structure is described in terms of.
2. Write Simpson's Diversity Index and define each symbol in it.
3. Explain why two communities with the same number of species can have very different diversity indices.
4. Explain what squaring each proportion does, and what the index would be if the square were left out.
5. Name the three symbioses and give the pair of effects for each.
6. Explain why the signs alone cannot distinguish predation from parasitism, and give a difference that does.
7. Explain why predator and prey populations cycle rather than settling at fixed numbers.
8. The predator's peak is often said to lag the prey's by a quarter of a cycle. State the condition under which that is true.
9. Explain what a trophic cascade is, and why removing a top predator lowers the plants in a three-level chain but raises them in a four-level one.
10. State the competitive exclusion principle, and say what kind of advantage decides which species wins.
11. Explain how niche partitioning permits coexistence, and why partitioned niches can support more individuals in total.