Ecologists have studied the distributions of birds on archipelagos. Some studies found an interesting "checkerboard" pattern within island groups. Sister species were never present on the same island, even when the islands were very close to each other. Which one of the following processes best explains this checkerboard pattern?

Question

Living in groups gives benefits but also brings costs. The graph below plots per capita benefits (solid line) and per capita costs (dashed line), both measured in the same units, against group size. Four group sizes are marked on the x axis: P, Q, R, and S.

Given these patterns in benefits and costs, which one of the group sizes P, Q, R, S best represents the optimal group size?

Answer 1

The pattern described here, closely related species that never turn up on the same island even when islands sit close together, is a well known finding from island biogeography, first worked out in detail for birds in New Guinea and its satellite islands. Let's test each process against this pattern.

Mutualism: This is a relationship where both species gain something from living near each other, such as a plant and its pollinator. If this were at work, we would expect sister species to often share islands, not avoid each other, so this does not fit.
Competition: Sister species share a very recent common ancestor, so they usually need the same kind of food and nesting space. When both land on the same small island, the limited resources cannot support both, and one species eventually excludes the other. This produces exactly the checkerboard, one species per island, pattern.
Predation: This would need one sister species to routinely hunt and eat the other. Closely related species with similar body plans and diets are rarely predator and prey of each other, so this is a poor fit.
Commensalism: Here one species benefits while the other is unaffected either way. This kind of relaxed relationship would not force species apart onto separate islands.

The mutual exclusion of sister species from the same island is the signature of competitive exclusion between ecologically similar rivals.

Let's summarize:

Sister species overlap heavily in their ecological needs.
When forced onto the same island, competition for shared resources pushes one species out, leaving the checkerboard pattern.

So competition between ecologically similar sister species best explains the checkerboard distribution.

Answer 2

The pattern described here, closely related species that never turn up on the same island even when islands sit close together, is a well known finding from island biogeography, first worked out in detail for birds in New Guinea and its satellite islands. Let's test each process against this pattern.

Mutualism: This is a relationship where both species gain something from living near each other, such as a plant and its pollinator. If this were at work, we would expect sister species to often share islands, not avoid each other, so this does not fit.
Competition: Sister species share a very recent common ancestor, so they usually need the same kind of food and nesting space. When both land on the same small island, the limited resources cannot support both, and one species eventually excludes the other. This produces exactly the checkerboard, one species per island, pattern.
Predation: This would need one sister species to routinely hunt and eat the other. Closely related species with similar body plans and diets are rarely predator and prey of each other, so this is a poor fit.
Commensalism: Here one species benefits while the other is unaffected either way. This kind of relaxed relationship would not force species apart onto separate islands.

The mutual exclusion of sister species from the same island is the signature of competitive exclusion between ecologically similar rivals.

Let's summarize:

Sister species overlap heavily in their ecological needs.
When forced onto the same island, competition for shared resources pushes one species out, leaving the checkerboard pattern.

So competition between ecologically similar sister species best explains the checkerboard distribution.

Show Hint

The Correct Option is B

Solution and Explanation

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