Currently, P. candidus, P. diadema, P. perrieri, and P. tattersalli are classified as "Critically Endangered" by the IUCN (Table 1), and P. edwardsi is classified as "Endangered." Extinction of one or more of these taxa is an imminent possibility, due to their relatively small population sizes, fragmented and discontinuous habitats, and the continuing human threats. The geographic range and population size of P. edwardsi and P. diadema are still relatively large (though further research is necessary to determine the status of the Tsinjoarivo P. diadema); however, P. candidus, P. perrieri, and P. tattersalli all occupy relatively small ranges.
Before comparing the conservation situation of eastern and western sifakas, it is useful to contrast some key aspects of their ecology. On a typical distribution map, the two groups look relatively balanced: four taxa more-or-less evenly spaced along the west and southwest, five more-or-less evenly spaced along the east. However, important differences in ecology may make the eastern sifakas (and particularly the rainforest taxa) much more seriously threatened.
First, eastern rainforest sifakas (P. edwardsi, P. diadema, and P. candidus) as well as P. perrieri live at low population densities (2-10 individuals/km2; Wright,
1995; Irwin et al., 2005; Banks et al., in press). P. verreauxi is on the order of 6 to 100 times more densely packed (Richard, 2003) and P. tattersalli is intermediate at 17-28 individuals/ km2 (Vargas et al., 2002). Second, rainforest sifakas appear much less able to live in human-dominated landscapes. It is common, where they are not hunted, to find P. verreauxi in small forest patches near villages and water sources. This is likely due to small home range requirements, and tolerance of human-favored tree species (e.g., mango, Mangifera indica). The same is not true of rainforest sifakas; P. diadema at Tsinjoarivo do not range in human-dominated forest patches (usually dominated by Eucalyptus and Pinus) but require endemic forest trees and a minimum patch size of around 25 ha (Irwin, unpublished data). Third, the dry forest sifakas' predisposition to cross open areas gives them a demographic resiliency in fragmented habitat which rainforest taxa may not share.
Thus, it is not possible to directly compare eastern and western sifakas based on geographic range, or perhaps even population size. These ecological "disadvantages" of eastern sifakas should be considered when developing conservation priorities and action plans.
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