Species do not live in isolation; they interact with one another in many ways. Symbiosis refers to a close, long-term biological interaction between two different species living in direct contact. Mutualism is a symbiotic relationship in which both species benefit, such as bees pollinating flowers while obtaining nectar, or mycorrhizal fungi colonizing plant roots to enhance water and mineral absorption while receiving carbohydrates produced by photosynthesis. Commensalism benefits one species while leaving the other neither helped nor harmed, as when barnacles attach to a whale. Parasitism benefits one organism, the parasite, at the expense of the host organism.
Each species also occupies an ecological niche, defined as its role and position in the environment, including its habitat, diet, activity patterns, and interactions with other species. The competitive exclusion principle, also known as Gause's law, states that two species competing for the exact same resources cannot coexist indefinitely, since one will eventually outcompete the other. To reduce this competition, similar species often engage in resource partitioning, dividing limited resources so that they can coexist in the same habitat. The theory of island biogeography extends these ideas by stating that species richness on an island is determined by a balance between immigration and extinction rates, influenced by island size and distance from the mainland.
Ecological communities change over time through ecological succession, the gradual process by which ecosystems develop through a series of community stages. Primary succession occurs on newly exposed surfaces with no prior soil, such as bare rock from volcanic lava or retreating glaciers, beginning with pioneer species like lichens and mosses that begin soil formation. Secondary succession occurs in areas where a disturbance has destroyed an existing community but soil and seeds remain, allowing faster recovery than primary succession. Eventually, succession may produce a climax community, a stable, mature ecological community that has reached equilibrium and remains relatively unchanged unless disturbed.
Beyond these interactions, ecologists have identified several important community-level concepts. A keystone species has a disproportionately large effect on its ecosystem relative to its abundance, such as sea otters controlling sea urchin populations. An indicator species reflects the health or specific environmental conditions of an ecosystem, with lichens indicating air quality being a classic example. The edge effect refers to changes in population or community structures that occur at the boundary of two or more habitats, often increasing species diversity at the interface. Ecological resilience describes the capacity of an ecosystem to absorb disturbance and reorganize while maintaining essentially the same function, structure, and feedbacks.