In this chapter, we have examined the ways in which species interact with one other and with their environments, noting the dependence of species on each other. But not all species have equal dependence on or influence over others. Within a community, the presence of some species, called keystone species, greatly influences which other species are present and which are not (FIGURE 15-31). If a keystone species is removed, the species mix in the community changes dramatically. The removal of other species causes relatively little change.

Figure 15-31Preserving biodiversity. Keystone species can keep aggressive species in check, allowing more species to coexist.

Bison are a keystone species. Experiments were set up in which a herd of bison was allowed to graze in certain parts of a prairie in Kansas, while being excluded from other areas. The areas without the bison were soon dominated by a single species of tall grass that was like a bully to other plant species. The areas where the bison grazed, on the other hand, had significantly greater species diversity. When kept in check by the grazing bison, the bully grass was unable to outcompete the other plant species, which were able to thrive.

Sea stars, too, are a keystone species (see Figure 15-31). On the rocky seashores of the Pacific Northwest, they consume mussels, which tend to crowd out other species. In a five-year study, sea stars were removed and kept out of certain areas of the intertidal zone but allowed in other areas. As with the bison study, species diversity dropped dramatically where sea stars were excluded. In fact, ultimately, just the mussels remained. In the areas with sea stars, mussels also remained but in reduced numbers. More importantly, 28 other species of animals and algae were also able to live there.

Keytone species make it possible to get more “bang for your buck” when your aim is to conserve biodiversity. Consequently, identifying keystone species is an important part of conservation biology. Besides bison and sea stars, some other keystone species are dam-building beavers, elephants of the African savanna, and lichens in the desert. We explore these questions of conservation in greater detail in the next chapter.