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Area of island (km2)

Another hypothesis suggests that because plants can photo-synthesize year-round in the Tropics, more energy is available to support more organisms. The high richness of species in the Tropics, as shown in Figure 20-7, is likely the result of several factors.

Habitat Size and Species Richness

Another pattern of species richness is that larger areas usually contain more species than smaller areas do. This relationship is called the species-area effect. The species-area effect is most often applied to islands, where area is clearly limited by geography. In the Caribbean, for example, more species of reptiles and amphibians live on large islands, such as Cuba, than on small islands, such as Redonda, as shown in Figure 20-8. Because all of these islands are close together, differences in species richness cannot be due to differences in latitude. Why does species richness increase as area increases? Larger areas usually contain a greater diversity of habitats and thus can support more species.

The species-area effect has one very important practical consequence: reducing the size of a habitat reduces the number of species that the habitat can support. Today, natural habitats are shrinking rapidly under pressure from the evergrowing human population. About 2 percent of the world's tropical rain forests are destroyed each year, for example. The inevitable result of the destruction of habitats is the extinction of species.

Species Interactions and Species Richness

Interactions among species sometimes affect species richness. Several studies have demonstrated that predators can prevent competitive exclusion among their prey. In the 1960s, zoologist Robert Paine showed the importance of the sea star Pisaster ochraceus, shown in Figure 20-9, in maintaining the species richness of communities on the Washington coast. Paine removed all Pisaster individuals from one site and for several years prevented any new Pisaster individuals from settling there. This change caused a dramatic shift in the community. The mussel Mytilus californianus, which had previously coexisted with several other species, became much more abundant, spread over the habitat, and crowded out other species. The number of other species fell from almost 20 to fewer than 5 within a decade. Evidently, Mytilus was the superior competitor for space on the rocks, but its population was normally held in check by predation from Pisaster.

Community Stability and Species Richness

One of the most important characteristics of a community is how it responds to disturbance. Disturbances are events that change communities, remove or destroy organisms from communities, or alter resource availability. Examples of abiotic disturbances are droughts, fires, floods, volcanic eruptions, earthquakes, and storms. Examples of animal disturbances include elephants tearing up trees while feeding and prairie dogs moving soil around while digging their burrows. Human disturbances include bulldozing, clear-cutting, paving, plowing, and mowing land.

Disturbances affect practically all communities at some point. A number of organisms may even depend on a certain type of disturbance in order to survive. For example, the lodgepole pine, shown in Figure 20-10, depends on periodic fires to disperse its seeds. The cones of this tree contain a tough resin seal that is cracked open by intense heat. Disturbances may also create opportunities for species that have not previously occupied a habitat to become established.

Stability is the tendency of a community to maintain relatively constant conditions. Stability therefore relates to the community's resistance to disturbances. Ecologists suspect that a community's stability is also related to its species evenness and species richness, because communities that have more species contain more links between species. These links may spread out the effects of the disturbance and lessen disruption of the community. One line of evidence cited in support of this view is the vulnerability of agricultural fields to outbreaks of insect pests, given that these fields usually consist of one species of plants. Further evidence came from a study of grassland plots in the 1980s. Ecologists observed that during a drought, species-rich plots lost a smaller percentage of plant mass than did species-poor plots. The species-rich plots also took less time to recover from the drought.

figure 20-9

Predation by the sea star Pisaster ochraceus on the mussel Mytilus californianus promoted diversity of mussel species by controlling the Mytilus population.

figure 20-9

Predation by the sea star Pisaster ochraceus on the mussel Mytilus californianus promoted diversity of mussel species by controlling the Mytilus population.

figure 20-10

These young lodgepole pines have started growing after a devastating forest fire. The heat of the fire helped release the pine seeds from their cones, which allowed the seeds to germinate.

These young lodgepole pines have started growing after a devastating forest fire. The heat of the fire helped release the pine seeds from their cones, which allowed the seeds to germinate.

Word Roots and Origins succession from Latin succedere, meaning "to go beneath" or "to follow after"

figure 20-11

Successional changes in communities are most apparent after a major disturbance, such as a volcanic eruption. The aftermath of the 1980 Mount St. Helens eruption is shown in (a). The photo in (b) was taken 12 years after the eruption. Many herbaceous plants and young trees had grown up.

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