15.11: Competition can be hard to see, yet it influences community structure.

Interacting species’ agendas are not always aligned, as they are for plants and their pollinators. Often, species interact because both are trying to exploit the same resources. In such cases—that is, when species have similar niches—conflict usually occurs. This competition doesn’t last forever, though. Inevitably, one of two outcomes occurs: competitive exclusion or resource partitioning (FIGURE 15-22).

1. In competitive exclusion, two species battle for resources in the same niche until the more efficient one wins and the other species is driven to extinction in that location (“local extinction”). In the 1930s, this was demonstrated in simple laboratory experiments using Paramecium, a single-celled organism. Populations of two similar Paramecium species were grown either separately or together in test tubes containing water and their bacterial food source. When grown separately, each species thrived. When grown together, one species always drove the other to extinction.

2. Resource partitioning is an alternative outcome of niche overlap. Individual organisms and species can adapt to changing environmental conditions, and resource partitioning can result from an organism’s behavioral change or a change in its structure. When this occurs, one or both species become restricted in some aspect of their niche, dividing the resource. In other experiments with Paramecium, for example, researchers used one of the two species from the initial experiment and a new, third species. Again, each species thrived when grown alone. But when the two species were grown together in the same test tube, they ended up dividing the test tube “habitat.” One species fed exclusively at the bottom of the test tube, and the other fed only at the top. Simple behavioral change made coexistence possible.

In many situations, resource partitioning is accompanied by character displacement, an evolutionary divergence in one or both of the species that leads to a partitioning of the niche. A clear example occurs in two species of seed-eating finches on the Galápagos Islands. On islands where both species live, their beak sizes differ significantly. One species has a deeper beak, better for large seeds, while the other has a shallower beak, better for smaller seeds, and they do not compete. On islands where either species occurs alone, beak size is intermediate between the two sizes (FIGURE 15-23).

Competition between species has one very odd feature: it is hard to actually see it happening because it causes itself to disappear. After only a short period of competition, one of the species becomes locally extinct or leaves the area where the niches overlap, or character displacement occurs, largely reducing the competition. In either case, the net result is that the level of competition is significantly reduced or wiped out altogether. For this reason, biologists often have to look for character displacement—the “ghost of competition past”—to identify areas where competition once took place. Moreover, even while it is occurring, competition tends to be indirect, rather than head-to-head battles. Like a game of musical chairs, it’s more a question of both species trying to use a particular resource, with one being a bit better at it.

Biologists do have the opportunity to see competition in occasional natural experiments in which a new species is introduced to a community. For example, the American mink, a weasel-like mammal, was introduced into Europe, where previously only the European mink had lived. Within 10 years, in the area where their ranges overlapped, the European mink increased in size while the American mink decreased in size, as biologists documented the evolution. In the next chapter, we investigate in greater depth why newly introduced species frequently win the competition with native species.