A community is the set of all populations found in a given place. This definition seems straightforward, but it raises some large questions about the nature of communities. Two different views were proposed nearly a century ago, and ecologists have debated their merits ever since. Frederic Clements, a prominent American ecologist of the early twentieth century, likened plant communities to a “superorganism” in which species interact strongly and predictably, like the organs within a body. In contrast, Henry Gleason, his contemporary, viewed plant communities as simply the products of species’ acting individually in time and space. Not surprisingly, research inspired by their disagreement suggests that the plants in most communities lie somewhere between these extremes.

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Populations in a community, whether a salt marsh, mountaintop, or desert, are tied together by the various interactions that secure their spot in a food web (Chapter 25), as well as by their physical location. The populations in a local community share the same rainfall, climate, and soil, and often encounter one another physically as they grow, reproduce, and die. Some provide food or shelter for others, and all end up being recycled by the many small decomposers in the soil. We can often characterize communities by the principal plants and animals they harbor, such as the cacti and lizards of deserts, the tiny, rock-loving mosses and marmots of a mountaintop, or the marsh grasses and mosquitoes of the coastal salt marsh. Indeed, communities often seem quite distinctive to us and easily recognized, and they are important as the local instances of biodiversity. At first glance, it would even appear that the plants and animals and fungi that live in one kind of place might compete for the very same local resources.

When we look at the details of where particular species occur, almost no two species have exactly the same geographic distribution. This is partly a result of competitive exclusion (unless the species are obligate associates of each other, like aphids and their bacteria). The northern bogs of New England may appear to have distinctive species, such as Black Spruce and Bog Club Moss, but while these and other bog-loving species overlap in their mutual preference for the water-saturated, acidic, sandy soils found in northern bogs, they do not occur in the same places over most of their geographic ranges. In contrast to populations or species, then, communities don’t have clearly defined boundaries.

The activities of different populations in a community also vary in time. For example, flies are active during the day, pollinating flowers that open in the morning and falling prey to dragonflies, birds, and other predators. In contrast, moths are important pollinators at night, when spiders and bats that prey on them are also active. Similarly, in coastal marine communities, oysters, crabs, and many other animals follow the daily rise and fall of tides. And, of course, the activities of many species change seasonally in responses to sunlight, temperature, and rainfall. Protein-rich insects that birds prey on to feed their young are most abundant in the spring, and fruits and seeds rich in the oils that migratory birds need to supply fat reserves are mostly available in late summer and fall.