General Drivers Of Extinction

Humans have caused or contributed to many plant and animal extinctions. Over the past 15,000 years, humans have contributed to extinctions

88 / Dov F. Sax and Steven D. Gaines of large fauna on most continents of the world (Barnosky, Chapter 12, this volume). Over the past few thousand years, human colonists and their commensals (such as the Polynesian rat) have contributed to the extinction of thousands of bird species across oceanic islands of the world (Steadman, 2006). Over the past 500 years, humans have reduced the amount of natural habitat worldwide, directly exploited species, introduced exotic species and exotic pathogens, and created many other conditions conducive to species extinction. The total number of recent extinctions is unknown, because many species have likely gone extinct before ever being recorded by science (Wilson, 1992). Estimates of global species loss vary, but based on rates of tropical deforestation and the species-area relationship a fairly typical estimate is 27,000 species lost per year; this is based on a species-area relationship with a z value of 0.15 and an estimate of 10 million species globally (Wilson, 1992). Even with a more conservative estimate of 5 million species on the planet (Primack, 2006), this would still equate to =13,500 species lost, or at least committed to extinction, per year. In sharp contrast to such estimates, the number of documented extinctions during the past 500 years is much lower; the International Union for Conservation of Nature, as of November 2007, reports 785 extinctions worldwide. Many other extinctions, not included in this number, have likely occurred, but they have not yet been documented adequately enough to be listed as extinct by the International Union for Conservation of Nature. Consequently, although species listed as extinct by the International Union for Conservation of Nature are likely an underestimate of the total number of extinctions over the past 500 years, those listed provide the most detailed evidence on extinction available.

There has been recent disagreement in the literature about how best to interpret extinction data from the International Union for Conservation of Nature, particularly with regards to the role species invasions play in causing extinctions (Gurevitch and Padilla, 2004; Ricciardi, 2004; Clavero and Garcia-Berthou, 2005). This disagreement has largely been due to the difficulty in ascribing precise causes to species extinctions. The precise mechanisms for any individual extinction are difficult to confidently determine for two reasons. First, extinctions are often caused by multiple factors, such as species invasions, habitat destruction, human exploitation, pollution, and infectious disease (KF Smith et al., 2006). Second, most ''documented'' extinctions actually involve some speculation about the factors responsible (because few species have been carefully monitored from the point of initial population decline to the point of final extinction). Additionally, it is worth noting that disagreement over species concepts, and disagreement over phylogenetic classifications of individual species, although not an issue for most extinct species, is an important point of debate in some cases. Given these limitations, it seems most appropriate to (i) consider whether general trends in the data exist, as opposed to focusing on the details of any one extinction, and (ii) focus analyses on those taxonomic groups that have been best studied and documented. Consequently, here we examine general trends in extinction in two of the best studied groups: terrestrial vertebrates and plants.

An analysis of the International Union for Conservation of Nature database on species extinctions reveals several emerging patterns for terrestrial vertebrate and plant species. First, most extinctions have been on islands as opposed to mainlands (Fig. 5.1A). This holds true generally when all causes of extinction are pooled (Fig. 5.1A) and specifically when only extinctions that exotic species are believed to have contributed to are considered (Fig. 5.1B). Second, terrestrial vertebrates have disproportionately gone extinct compared with plants (Fig. 5.1A), both in absolute terms and relative to the taxonomic richness of their respective groups. Third, the presumed causes of these extinctions are not evenly distributed among types of species interactions. Predation has been a far more important species interaction in causing extinctions than competition (Fig. 5.1 C). Indeed, predation alone, i.e., in the absence of other factors like habitat destruction or pollution, is listed as being responsible for the extinction of >30% of vertebrate species (Fig. 1C). In contrast, competition is never listed as being the sole factor responsible for species extinction (Fig. 5.1 C). Further, predation is listed as one of several contributing factors in >40% of terrestrial vertebrate extinctions, whereas competition is listed as a contributing factor in <10% of terrestrial vertebrate extinctions (Fig. 5.1C). This means that predation acting alone or in concert with other factors is believed to have contributed to the extinction of close to 80% of all terrestrial vertebrate species, whereas competition has contributed to <10% of these extinctions. This predominance of predation over competition in causing extinctions may be due in part to the broad range of processes that we classify here as "predation," e.g., both human hunting and parasitism (see Methods). However, even if we consider predation in the strictest sense to include only carnivorous animal interactions, the qualitative patterns described here are maintained. Further, differences between the role of predation and competition are conserved when considering only those cases where exotic species are believed to have played a contributing role in species extinction (Fig. 5.1D); in these cases, the combined influence of predation acting alone and predation acting in concert with other factors is believed to account for 98% of all extinctions. These patterns suggest that terrestrial vertebrates are much more likely to go extinct from predation than competition. This interpretation is consistent with observational and theoretical work by Davis (2003), who suggests that competition should rarely be an important factor in species extinctions. Finally, for plants, we did not evaluate the role of predation versus competition in causing spe-

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