Just as individuals live within a population, populations exist within communities. A community refers to all the populations that interact with each other in a given environment and geographical area. The specific role and way of life of each population is called a niche. When populations have overlapping niches, a variety of types of interaction may occur, including competition, symbiosis, predation, and other food relationships. Communities are shaped over time by ecological succession.

Each population in a community plays a unique role in the community. This role, the population’s niche, ranges from where the members of a population live, what they eat, when they sleep, how they reproduce, and every other characteristic that defines a population’s lifestyle within a community. You can think of the niche as a sort of node in the network of interactions that make up a community. Wherever the niches of two populations overlap, interaction follows.

When two populations share some aspect of a niche, such as a nesting site or a food source, competition results. There are two basic outcomes of competition between populations:

Symbiosis refers to an intimate association between organisms called symbionts. The symbiotic relationship may or may not be beneficial to the organisms involved. There are three kinds of symbiosis: parasitism, commensalism, and mutualism. Each type of symbiosis describes a different relationship of benefits between the two symbionts. A tapeworm is a parasite that lacks a digestive tract and therefore infects a host and steals predigested food; parasites benefit while their hosts suffer. In commensalism, one species benefits and the other remains unaffected. Barnacles and whales live in a commensal relationship. Finally, in mutualism, both species benefit from the presence of and interactions with each other. Lichens, which consist of a fungus and alga that provide for each other, respectively, moisture and food through photosynthesis, are a good example of a mutualist relationship.

Predation refers to one organism eating another. Predation does not only refer to carnivores. Just as an eagle eating a rodent is a form of predation, so is a rodent munching on some grass. In fact, predation doesn’t always result in the death of the prey. An antelope that gets eaten by a lion will die, but a tree that loses a few leaves to a hungry giraffe will go right on living.

Carrying capacity shifts in a periodic manner based on the cycles of predation. When the population of rabbits increases, the population of coyotes that eat the rabbits will also increase, as there’s more food for the coyotes. However, at some point, there will be so many coyotes eating so many rabbits that the rabbit population will fall in number. The coyotes’ great success in eating rabbits has eliminated their food source, and as the rabbit population declines, so will the coyote population. But as the coyote population dwindles, the lack of predators allows the rabbit population to grow again, and so the cycle continues.

The change in a population due to a shift in environment is one of the engines of evolution. Imagine the rabbits and their predators, the coyotes. As the coyotes increase in number, the rabbit population ceases to grow, and many rabbits are caught and eaten. As the coyotes increase in number, the carrying capacity of the rabbit population shrinks. But it is important to notice that not all rabbits are caught by the coyotes. The faster rabbits escape capture by the coyotes far more often than the slower rabbits. Fast rabbits survive and breed and have offspring, while slower rabbits get eaten. The next generation of rabbits will therefore be faster because they are descended from faster parents—this is directional selection in action. The population of increasingly fast rabbits means that the coyotes must be faster in order to catch the rabbits. More fast coyotes catch rabbits and live to reproduce, creating a next generation of faster coyotes. When two populations affect their mutual evolution in this manner it is called coevolution.

It is arguable that predation is actually helpful to the prey population. Since predators want to capture prey with the least possible effort, the weakest members of the prey population are usually targeted. In this way, the predators often remove from the gene pool of a population those prey animals that have the weakest and least fit alleles.