Ecological impacts of forest fragmentation on diademed sifakas (Propithecus diadema) at Tsinjoarivo, eastern Madagascar: Implications for conservation in fragmented landscapes.

摘要

Forest fragmentation is known to seriously threaten primate populations, yet the ways in which fragmentation causes local extinction remain relatively unknown. Primate species vary widely in their susceptibility to fragmentation, but the causes of this variation are poorly understood. Because of long generation times, some species may persist in fragments for many years despite ecological stress, yet their extinction over the longer term may be inevitable without intervention. An increased understanding of the ecological mechanisms affecting primate persistence in fragmented habitats will allow for more effective conservation, by targeting species that are most at risk. In this dissertation, I examine the effects of forest fragmentation and disturbance on the ecology, behavior and demography of the diademed sifaka (Propithecus diadema) at Tsinjoarivo, eastern central Madagascar. Data are presented on four study groups (two in continuous forest and two in forest fragments) over an annual cycle, with demographic surveys over a three-year period. Fragmentation has severely altered the forest's physical structure, species composition and phenology. Sifakas in continuous forest eat more fruit, specifically large fleshy fruits of canopy trees which are absent in fragments; the mistletoe Bakerella clavata is an important keystone resource during the dry season. Sifakas in fragments, in contrast, rely on B. clavata throughout the year, and spend more time feeding per day. Home range is severely reduced in fragments, but daily path length is relatively unchanged. Sifakas in continuous forest tend to forage near forest edges, suggesting that low-level disturbance may increase resource abundance; in contrast, sifakas in fragments avoid their heavily-disturbed forests' edges. Sifakas in fragments also exhibit reduced group cohesion, resulting from the small patch size of their staple resource, B. clavata. This has led to reduced levels of grooming and aggression, but group composition and dominance relationships are largely unchanged. Preliminary data indicate that birth rate and infant survival are not adversely affected but mortality of immatures is increased (largely due to predation), and some evidence suggests constrained dispersal. In summary, although diademed sifakas have survived in forest fragments over the short term, the considerable ecological trade-offs documented here place their long-term survival in jeopardy.