Was extinction selective and if so how so

Our knowledge of the environments in which the extinct species lived and of the chronology of extinction events derives largely from sediment cores but also directly from the fossils and associated evidence of human occupation. Burney et al. (2004) provide a review of the chronology of late prehistoric Madagascar, and of hypotheses regarding the extinction process (see also Burney, 1999). A few observations are worth repeating here. First, most if not all of the extinct lemurs (as well as giant ratites, pygmy hippos, and giant tortoises) were living on Madagascar when humans first colonized the island over 2000 years ago. Second, there was a major insult to these taxa shortly after humans arrived, as evidenced by a precipitous decline (in sediment cores) in the spores of the coprophilous fungus, Sporormiella (Burney et al., 2003). This fungus is an excellent proxy for megafaunal biomass because it cannot complete its life cycle in the absence of dung of large animals. That decline, possibly due to megafaunal hunting by humans, did not lead to their immediate extinction, as many survived into the last millennium, and some were still alive only a few hundred years ago if not later (Burney et al., 2004). A peak in charcoal microparticles (signalling an increase in the number and intensity of fires) occurs after the initial megafaunal decline. Ultimately, many factors (including natural aridification, hunting, fires, and habitat disturbance) contributed to megafaunal extinction. There is direct evidence in the form of butchery marks that giant lemurs were hunted and eaten by humans in Madagascar (Perez et al., 2005), but we do not know the relative contribution of hunting (as opposed to habitat disturbance, etc.) to their disappearance.

Most relevant for our purposes is the selectivity of the extinction process. The extinctions were not limited to particular regions or habitat types, although some regions, such as the central highlands, lost more primate species (possibly because they lost more wooded habitat) than others. There are still some lemurs living in central Madagascar, but they persist only in isolated forest patches. Still-extant taxa were far more widespread in the past than they are today, and they coexisted with now-extinct taxa in regions that are today entirely devoid of lemurs (Godfrey et al., 1999). There is no reason to believe Madagascar's extinction "event" is over. The very factors that apparently killed the giant lemurs continue to threaten their smaller-bodied relatives.

Clearly and without exception, large body size increased the risk of extermination. There are a number of reasons why large-bodied taxa may be most vulnerable to extinction. Certainly, they are easy targets of human hunting. Second, larger-bodied species often require more suitable habitat area to maintain minimum viable populations, and are therefore more vulnerable to habitat loss and fragmentation. Finally, large-bodied indriids are surprisingly slow reproducers (despite their rapid dental development; see Godfrey et al., 2004b; Richard et al., 2002) and we suspect that the same was true of many of the subfossil species.

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