How does competition shape communities?

In the short term, what we would call “ecological time scales”, competition shapes communities primarily by limiting population growth. Every population requires and excess of required resources to grow larger, but if another population is taking those resources instead, then their growth is limited. If your competitor is significantly better at securing resources, and is growing larger, your population may find that it is actually too large for the amount of resources it is capable of obtaining, and so will have to shrink down to a level that can be sustained by the amount of resources it can get.

Competition with individuals of your own species also limits the size of your population, even more so than competition with other species. This is because members of the same species have more overlapping sets of resources that they require. Two humans both need houses and water, so they compete for them both, but they don’t have to compete with birds for houses, only water.

In the longer term, called “evolutionary time scales”, competition can change the characteristics of the species that are in competition with each other. Species may evolve to need certain resources less if their competitors are better at obtaining them. This allows those two species to evolve to be more compatible, allowing them to better coexist by making them less competitive with one another.

Evolution won’t change a single species to be able to compete less with members of its own species, but in some cases it can lead to the creation of new species.