19.2 Niche

19.2 Niche

Role, not address

A niche is the organism's ecological role: what it eats, when it is active, where in the habitat it forages, and which conditions it tolerates. The habitat is the address. The niche is the way of making a living at that address. The two words are not synonyms. Two species can share a forest and still occupy different niches, and one species can be missing from part of a suitable-looking place because a competitor already fills that role.

Describe a niche with verbs. A nocturnal insect-eater that hunts on lower trunks, in the same trees where a daytime seed-eater feeds in the canopy, has a different niche. Include timing, the foraging spot, and the conditions the species can tolerate. A hole in a rock is a place. The niche is the role played in the place.

Fundamental niche

The fundamental niche is the full range of conditions and resources the species could use if enemies were absent. Enemies here means competitors and predators. In a lab, or on a shore where a dominant competitor has been removed, the species may feed or settle across this wider range. The fundamental niche is a potential. It is not the smaller zone you usually see in the field.

Realized niche

The realized niche is the smaller range the species actually uses when competitors and predators are present. It sits inside the fundamental niche. It is not larger. Predators can confine foraging to safe patches. Competitors can take the best food or the best settlement sites. If a stem says the realized niche is the bigger one, it has the direction backward.

Barnacles make the contrast concrete. Some high-shore barnacles can survive lower on the rocks once competitors are removed, so the lower band is part of the fundamental niche. With those competitors, or with predators, the barnacles stay mostly on the upper rocks. That occupied band is the realized niche. Drying still caps how high they can live. Competition and predation shrink the role they actually play. If removing the enemy lets the species spread, the old boundary was realized, not the edge of tolerance.

Competitive exclusion

Competitive exclusion says two species cannot occupy the exact same niche indefinitely. If they need the same limiting resource, in the same place, at the same time, and neither can switch, one is expected to exclude the other. G. F. Gause grew Paramecium aurelia and Paramecium caudatum alone on the same bacterial food, and each persisted. Grown together on that food, one drove the other out. Similar species may share a habitat. They cannot share an identical niche forever. Coexistence needs a difference that reduces the overlap.

Exclusion takes time. Early in a culture both species may still be present, and then the poorer competitor declines. Reject a claim that both increase without limit while using one identical niche. Competition also does not, by itself, turn the two species into mutualists. Mutualism means each partner benefits. Exclusion means one competitor is lost.

Gause showed the other outcome in the same kind of jar. Paramecium caudatum and Paramecium bursaria could coexist because they used food differently, one feeding more in the open liquid and the other nearer the bottom. Same container, different roles. That result does not cancel exclusion. It shows that exclusion is not predicted when the niches are not the same.

Resource partitioning

Resource partitioning lets similar species coexist by dividing a resource. Robert MacArthur's wood warblers in spruce forests are the classic case. Several closely related species fed in the same trees but concentrated on different heights and different parts of the tree, such as outer tips versus inner branches. The birds shared a habitat and avoided one identical niche. Partitioning can split food type, feeding time, or foraging location.

Partitioning is often a consequence of competition, not evidence that competition never occurred. Individuals that overlapped most strongly left fewer offspring, so each population shifted toward the slice with less overlap. Different heights are different roles. Do not describe those warblers as five species with one niche.

Character displacement

Character displacement is an evolutionary change that can follow when similar species live together. Traits tied to resource use diverge more where the species co-occur than where each lives alone. In some Galapagos ground finches, the difference in beak depth is greater on islands where two seed-eating species both live than on islands where each lives without the other. Deeper and shallower beaks handle different seeds, so diet overlap shrinks. Partitioning is the ecological split in resource use and can happen without a change in body shape. Displacement is the evolved exaggeration of a difference where the species co-occur.

Habitat beside niche

What to predict

If two species use the same resource in the same way at the same time, and the overlap does not shrink, predict competitive exclusion. Do not predict that either realized niche grows past the fundamental niche. If they feed at different heights, times, or foods, predict that resource partitioning can allow coexistence. MacArthur's warblers are that pattern: one forest, different foraging positions.

If a feeding trait is more different where two species live together than where each lives apart, predict character displacement. A difference in location alone, with no evolved trait difference, is partitioning or habitat use, not displacement. Enemies present means the realized niche, and it is the smaller range. Enemies removed means the species may spread toward its fundamental niche.