Diademed sifaka (Propithecus diadema) ranging and habitat use in continuous and fragmented forest:: Higher density but lower viability in fragments?

The mechanisms through which forest fragmentation threatens the survival of mammal populations remain poorly known, yet knowledge of this process would greatly aid conservation efforts. I investigated ranging behaviors of diademed sifakas (Propithecus diadema) in continuous and fragmented forest at Tsinjoarivo, eastern Madagascar, using focal animal observations to examine home range size, daily path length (DPL), and habitat use relative to forest edges over 12 mo. Sifaka groups in forest fragments had home ranges that were 25-50 percent as large as continuous forest groups, and moderately reduced DPLs. Continuous forest groups foraged more than expected near forest edges while fragment groups avoided edges. Fragments have higher population density than continuous forest; however, several lines of evidence suggest that fragment groups' food resources may be denser, but lower quality. Continuous forest groups appear to be energy-maximizers, maintaining large ranges and preferentially feeding in rare fruiting trees found only in continuous forest interiors, while fragment groups appear to be time-minimizers, using small home ranges and primarily feeding on mistletoe (a fallback food in continuous forest). Therefore, the consequences of fragmentation on long-term viability remain unknown; it is possible that the advantage of increased density is outweighed by longer-term demographic challenges, or other threats (e.g., nutrition, health, social behavior, disease). When animals stranded in forest fragments exhibit complex and potentially unpredictable responses, simple ecological proxies (e.g., incidence patterns and density) are probably inadequate in assessing population health and viability. Ecological study and monitoring is essential in judging the viability of fragmented populations.