Environmental gradient

Environmental gradient

Figure 14.19 Population distributions along a hypothetical environmental gradient for both closed and open communities.

Closed communities are more easily defined as communities, since their boundaries are marked more clearly. Sharp boundaries between closed communities are called ecotones. They are especially prominent when the environmental gradient is fairly rapid, such as the transition between a wetland and adjacent woodlands. Nevertheless, they can also be present along gradual boundaries, such as the transition between grasslands and forest along long-range moisture gradients. In this case the boundaries are maintained by mutual inhibition, as when grasses prevent tree seeds from germinating and trees prevent grasses from growing by shading them.

Ecotones have greater species diversity not only because they may have representatives of two communities, but because some species benefit directly from both communities. This is called the edge effect, a concept that is attributed to Aldo Leopold. Many birds, such as the American robin, for example, nest in the forest but feed in fields. Human alterations to the landscape, such as by clearing fields or putting roads through wilderness areas, result in increasing the edges. This also has the effect of decreasing the area of each community, which can reduce the number of species each can support [equation (14.34)], and particular species with large area requirements may be eliminated.

We saw previously how the number of species tends to increase with the area of the ecosystem. However, each species may not necessarily range over the entire area, but rather, be limited by environmental conditions to a subset. It turns out, that most species have small ranges, and fewer have large ranges. Species with larger ranges also tend to have greater population density. These tendencies may be due to the differing dispersion capabilities of species or (as some think) to the artifacts of sampling, which are biased against rarer species.

Individuals within a population tend to distribute themselves more or less evenly due to intraspecies competition. Individuals or groups of vertebrates tend to restrict themselves to an area known as the home range. If the home range is defended aggressively and does not overlap with those of neighbors, it is called a territory. Species with complicated reproductive behavior, such as nest building and extended care of young, are more likely to be territorial. Birds form territories more often than other groups. Territoriality is a way to avoid the need for aggressiveness, since once a territory is formed and marked, others of the same species tend to respect it.

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