The previous chapter dealt with populations, the first tier of ecological organization. We examined how populations grow and how various biotic and abiotic factors influence population growth and size. Now we turn our attention to the second tier of organization, the community. Instead of focusing on the characteristics of a single population and what helps determine them, we now look at the way populations interact. A community is defined by the set of populations that coexist in a given area. Communities are no more static than are individual populations. Populations interact in complex ways, from predator-prey interactions and parasitism, to mutually beneficial interactions. Over evolutionary time, many populations have adapted and adjusted to each other. This coevolution has led to remarkable chemical and morphological defenses, countering predatory strategies, warning and mimicry colorations, specialized morphologies and behaviors, and symbiotic relationships. Just as "no man is an island," no population is an island, existing in isolation. Each population is part of a community and interacts with other populations.