Resilience To Habitat Change And Fragmentation

As mentioned earlier, eastern sifakas were studied much later than western sifakas; later still was the development of research programs (in either region) investigating sifakas' responses to habitat fragmentation and alteration. Early studies followed the general trends of primatology, studying groups within habitat which was as pristine as possible. The (valid) reasons for doing so were academic (understand a species' true behavior) as well as practical (increase the chances your study groups would last at least as long as your study period). However, the rate of habitat conversion in Madagascar (and elsewhere) has reached crisis proportions: Green and Sussman (1990) estimated that the eastern rainforest cover in 1985 had already diminished to 34% of its original extent, and an extrapolation of the observed disappearance rate predicts a complete loss of this ecosystem ca. 2020. What forest remains is increasingly fragmented and impacted by human activities. The impact of these changes on lemur populations can no longer be ignored.

So what are the prospects for eastern sifakas? Understanding the nature of the threat is the first step. One can conceptualize the threats fragmentation poses to sifaka populations as three sequential challenges. First, direct anthropogenic effects (e.g., hunting) threaten most proximately. Second, fragmentation-related habitat changes may affect the ecological compatibility between sifakas and their habitat—and even if compatibility is maintained, it may be through compromises which affect other aspects of behavior. Finally, on the longest time scale, there is the demographic threat of population subdivision and constrained dispersal.

The immediate anthropogenic effects are hard to estimate and notoriously variable among regions. In many areas, sifakas are protected from hunting by fady (taboo); these often apply preferentially to sifakas and indri because of their large size and orthograde posture (resembling humans or human ancestors). However, this protection is by no means universal. P. edwardsi is hunted throughout much of its range, especially the northern part (Irwin et al., 2005). Other eastern sifakas seem to be protected in some, but not all, regions. This threat is controllable through human activities (unlike the purely ecological pressures); it is important that education and enforcement of applicable laws (which deem hunting of lemurs illegal) continue to be applied and extended in rural areas where sifakas live.

The second threat, loss of ecological compatibility, is only beginning to be investigated. My dissertation research (Irwin, 2005a,b, 2006) compared the ecology and behavior of two P. diadema groups resident in forest fragments and two in continuous forest at Tsinjoarivo. I found that continuous forest groups relied on various tree species to provide fruit during the rainy season, but relied heavily on a small hemiparasitic mistletoe (Bakerella cf. clavata) during the middle of the dry season (devoting 45-70% of monthly feeding time to this one species). For these groups, mistletoe is a fallback resource. Fragment groups ate fewer fruits, and the loss of preferred fruit trees forced them to consume mistletoe at high levels throughout the year; for them, mistletoe is best described as a staple. Arrigo-Nelson (2005) similarly found reduced frugivory and loss of preferred fruit resources in disturbed areas for P. edwardsi at Ranomafana, suggesting that this pattern may be consistent across sites.

The fact that groups in fragmented or disturbed habitats can sustain themselves, however, is not in itself reason to discount the threat of habitat change; one must consider the effects of behavioral shifts. One direct line of research involves the nutritional composition of foods; this is currently being studied at Tsinjoarivo. If the altered diet is less nutritious, long-term effects on body condition and reproduction would be apparent. Indeed, reduced body mass of adult sifakas in fragments has already been documented, for P. edwardsi at Ranomafana (Dehgan, 2003), and P. diadema at Tsinjoarivo (Glander and Irwin, unpublished data). Other effects are also apparent: for example, fragment groups have greatly reduced group cohesion and an increased rate of feeding alone (Irwin, 2005b). The mistletoe on which they rely has an extremely small crown diameter (<2m); animals are forced to spread out because sharing food patches is impractical (and subordinates are unlikely to be tolerated by dominant individuals). These altered resource distributions and decreased group cohesion could lead to altered food competition regimes and changes in social behavior which could affect group structure and reproduction.

The third major threat is the demographic consequence of population subdivision. Even when populations can emerge unscathed from the first two threats, they may be threatened by the longer-term effects of inbreeding and reduced dispersal opportunity. The severity of this threat depends on how reluctant individuals are to cross the nonforested areas between fragments. Most eastern sifakas, in contrast with western congeners, seem extremely reluctant to do so. Dehgan (2003) found that a P. edwardsi group in a forest fragment did not leave the forest fragment in which they lived, except to cross distances less than 30 m to smaller satellite patches. Among two P. diadema groups in forest fragments at Tsinjoarivo, no crossing between patches was observed during a 1-year study (Irwin, 2006). One adult male did later disperse secondarily across open areas, but only after the rest of his group was decimated by predation. In contrast, Mayor and Lehman (1999) noted that P. perrieri regularly crosses open areas, in one instance traversing 600 m. These results suggest that sifakas in drier forest may be predisposed to crossing between fragments, possibly because they have historically lived in more open (possibly mosaic) habitats. Rainforest sifakas, in contrast, may be more suspicious of open areas due to their long evolutionary history in dense forest with little need to come to the ground.

The long time scale of demographic threat means that long-term study is necessary before assessing whether fragmented populations are population sinks, or whether they can be a viable part of the larger population. Given the increasing rarity of pristine forests in Madagascar, the ability to include fragmented populations in effective population sizes would definitely paint a better picture for conservation; only time will tell us for which species this approach is justified.

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