The Importance of Species by Kareiva Peter;Levin Simon A.;

Author:Kareiva, Peter;Levin, Simon A.;
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2014-08-15T00:00:00+00:00

Local Extinctions or Near-Extinctions

The removal of livestock from Santa Cruz Island, California, in 1989 resulted in an explosive, unexpected increase in exotic weeds, especially fennel (Foeniculum vulgare), which had been introduced in the 1850s and was not previously a prominent weed before the livestock operation (Dash and Gliessman 1994; R. Klinger pers. comm., 2001). The fennel, in turn, replaced native plant species on much of the island. Other impacts, such as those of herbivores on fennel and the plants it replaces, or on the fire regime, have not been studied, but large parts of the island that were probably a mixed, diverse prairie of native plants before European settlement are now a sea of tall fennel. Surely the total ecosystem consequences of the removal of cattle would be measured as major. Similarly, feral pigs and goats have recently been eradicated from Sarigan Island, a 5-km2 island in the Mariana Islands (Kessler 2002), with the unexpected subsequent explosive growth of an introduced vine, Operculina ventricosa. The impact of Operculina on the regeneration and expansion of native forest is as yet unquantified. However, vast stretches of ground and trees are now thickly carpeted with this vine, a formerly minor component of the flora, and it is hard to believe that this vegetational change has not greatly affected individual population abundances as well as various ecosystem traits. These examples suggest that the extinction of single species of livestock, and perhaps of ungulates generally, can have enormous ecosystem consequences.

A similar decline of mammalian grazers, but one for which one might have expected functional redundancy to have dampened the impact of near-extinction, was caused by the eruption of rinderpest, a disease introduced with Indian cattle to Africa in the 1880s (Sinclair 1979, 1995). This introduction led to an epizootic in the 1890s that raged through much of the continent until the 1960s, when cattle vaccination largely stemmed the disease in wildlife. Although many ungulates were affected by rinderpest, some species (e.g., buffalo and wildebeest) suffered as much as 90% mortality, while the impact on others was much less severe. The disappearance of all these animals, followed by the disappearance of the disease and the dramatic resurgence of some of the ungulates, constitutes a double example of the impact of near-extinctions. The effects in both instances were farreaching and dramatic (Sinclair 1979, 1995). The earlier decline led not only to great changes in relative abundances of the ungulates but also to major shifts in the distribution and abundance of their food plants, their predators, and various ecosystem processes (e.g., fire). With respect to functional redundancy, the fact that many ungulates were much less affected by the virus did not prevent a huge impact from the tremendous decline experienced by certain species. Of course, the various species are not exact functional equivalents, but the main evidence for this statement is probably the suite of impacts of the near-extinctions of the buffalo and the wildebeest. In other words, the whole notion of functional equivalence, like that of keystone

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