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A guided journey · 4 stops

Who gets the resource?

Why similar species do not simply eliminate each other — and why the textbook answer has never been tested on the textbook example.

The route

  1. Ecological niche
  2. Competitive exclusion
  3. Niche partitioning
  4. Character displacement

Two species needing the same thing should not be able to share it indefinitely. That is a result about arithmetic, and it is true under conditions that nature rarely supplies.

This route works through what happens instead. It starts with the niche, and with the experiment that split the concept in two — a shore, two barnacles, and a scraper. Then the exclusion principle with its assumptions attached, and the case that broke the tidy version. Then the standard explanation for coexistence, which is beautifully documented and has never been tested on its most famous example. And finally the cases where competition was caught changing the animals themselves.

The theme is not competition. It is what counts as evidence that competition is doing something.

  1. Stop 1 of 4. Ecology

    Start with what a niche is, and is not

    Because the teaching shortcut — a niche is an animal’s job — quietly does damage. A job exists whether or not anybody fills it; a niche does not. And the barnacle experiment here is the foundation of everything that follows: one boundary set by drying out, the other by another animal, and no way to tell them apart without removing one.

    Read Ecological nicheA barnacle’s upper limit is the sea. Its lower limit is another barnacle — and from a photograph the two edges look identical.

  2. Stop 2 of 4. Ecology

    Then the principle, with its conditions attached

    Stated as a law it is contradicted by any pond in summer; stated with its assumptions it is exactly right and rarely applicable. Gause’s own experiments produced both outcomes — exclusion when two paramecia fed the same way, coexistence when they fed differently in the same tube — and the second is the one that gets left out.

    Read Competitive exclusionThe principle is not wrong. It is conditional — and a lake in summer meets almost none of its conditions.

  3. Stop 3 of 4. Ecology

    The standard explanation, and the missing experiment

    This is where the route earns itself. Five warbler species using different parts of the same tree is the textbook demonstration, it is careful observational work, and it cannot show that competition put them there. Coming to it after the barnacles makes the gap obvious: one system had the competitor removed, and this one never has.

    Read Niche partitioningFive warblers in one tree is the textbook case. Nobody has ever removed one to see what the others do.

  4. Stop 4 of 4. Ecology

    And where competition was caught in the act

    Two studies that did not rely on a pattern. Sticklebacks in ponds with and without a competitor, where the fish resembling it grew worst — selection measured directly. And a finch population where a competitor arrived, a drought struck, and beak size shifted within a generation. This is what the warbler case would need, and it closes the route on the evidence rather than the idea.

    Read Character displacementA competitor arrived, a drought struck, and beak size shifted within one generation — watched rather than reconstructed.

Where this leaves you

You end knowing the difference between a pattern that competition would produce and evidence that it did. Species with adjoining ranges, or using a habitat differently, are consistent with competition — and equally consistent with the places differing, or with the species having been unalike long before they met.

The stop that stays with you is the warblers. Five species dividing one spruce tree is in every textbook as the demonstration of niche partitioning, and nobody has ever removed one to see what the others do.

What the route does not settle: how much of the partitioning documented across ecology would survive that test. Nobody knows, because for most of it the test has not been run.

Last reviewed 2026-09-04. All guided journeys