Concept 44.1: Communities Contain Species That Colonize and Persist

We saw in Chapter 43 that the abundances, distributions, and even phenotypes of species are affected by their interactions with other species. These interactions occur in the context of an ecological community—the group of species that occur together in a geographic area. Ecologists have practical reasons for wanting to know why communities contain the species that they do—we depend on them for all manner of natural resources and services. And like other dynamic systems, a community’s components (species) and their interactions determine how well it functions. Understanding how communities are put together and how they work is essential to conserving them, as we will see.

Despite the simple definition of community, ecologists must make some practical decisions about how to draw the boundaries of a community, or about which species to include in a study. Sometimes there are obvious changes in habitat that indicate a community boundary. A pond, for example, defines a group of aquatic species that interact much more with one another than with terrestrial species outside the pond. Often ecologists choose to study a representative portion of a single habitat, such as a 10- × 10-meter area of forest. But even a small area may contain so many species that ecologists decide to focus their attention on subsets of species. Those subsets may be defined taxonomically, by a habitat or location, by some character traits, or by the types of interactions the species engage in. Hence biologists may speak of the bird community of a certain island, the Daphnia community of rock pools (see Figure 42.10), the microbial community of the human gut, or the community of grazing herbivores in the marine rocky intertidal zone.

Regardless of how we delineate them, communities can be characterized in terms of which species they contain, or their species composition. Communities are also characterized by how many species they contain, and by how abundant each species is. These three attributes of communities—species composition, the number of species, and how abundant each species is—are components of community structure.

The mix of species in a community is determined by the same factors that explain the distributions of individual species; as we saw in Concept 42.1, a species can occur in a location only if it is able to colonize that location and persist there. Species may fail to colonize a community, or be lost from it, for any of several reasons. Individuals of a species may simply never reach the location—consider how few dispersers from a mainland are likely to reach a remote island before they perish! Once colonists have arrived, they may be unable to tolerate the environmental conditions, or a resource or an essential mutualistic partner may be missing. Species may be excluded by competitors, predators, or pathogens. Finally, some species may go locally extinct because their populations are so small that, by chance, all the individuals die at some point without reproducing (see Concept 42.5).

We can think of communities as being assembled through such gains and losses of individual species. The processes of community assembly are well illustrated by Krakatau, a small (17 km²) volcanic island in the Sunda Strait of Indonesia (see Figure 44.10). The volcano exploded in 1883, sterilizing what was left of the island and covering it with a thick layer of ash. Scientists quickly mounted expeditions to observe the return of life to the island, and they have surveyed it periodically ever since. By 1886, seeds of 10 plant species that grow on nearby tropical beaches had floated to Krakatau (FIGURE 44.1A), and wind had brought seeds or spores of 14 additional species of grasses and ferns. Tree seeds had arrived on the wind by the 1920s. As the forest canopy closed in, some pioneering plant species that require high levels of light disappeared from Krakatau’s now-shady interior. Once forests developed, fruiteating birds and bats began to be attracted to the island, bringing new animal-dispersed seeds with them.

Now, more than a century after Krakatau exploded, the island appears from afar to be covered by stable plant communities (FIGURE 44.1B). But a closer look reveals that their species compositions continue to change as new species colonize and earlier colonists go extinct.

Krakatau provides a dramatic illustration of a general pattern: the species composition of communities changes over time. Species composition also changes over space. In spite of this turnover of species in space and time, are there general rules that govern which species we find where and when?