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FIGURE 1 Species-area curve. SOURCE: M.L. Shaffer. 1981. "Minimum Population Sizes for Species Conservation." BioScience 31.

other species. Fragmented habitats are clearly far less desirable for conservation purposes than continuous habitats, but the specific impacts of fragmentation on a given species can only be determined through careful study of that species.

Species addition or removal

The loss of keystone species from ecological communities can have profound effects on the system because these species play important functional or ecological roles. Keystone species can include (i) predators, herbivores, parasites, and pathogens that maintain diversity among competing organisms by reducing the abundance of dominant competitors and preventing competitive exclusion; (ii) mutualists that link the fate of many species, such as specialized pollinators; and (iii) species that provide resources critical to the survival of dependent populations during periods of low resource availability. It is not possible, however, to determine which species serve "keystone" roles in a biological community without detailed experiments. In particular, neither trophic level, abundance, nor body size of a species is a good predictor of keystone status.

Introductions of certain species can also profoundly change ecosystem services. For example, the introduction in southwestern U.S. of the deep-rooted Tamarix tree, which has the ability to draw water from unsaturated alluvial soil—an ability missing in native trees in the region—has altered ecosystem services



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