Impact of Habitat Fragmentation on Diurnal Squirrels in Lowland Tropical Forests of Upper Assam North-East India

Abstract

The effect of forest fragmentation on a set of ecological parameters, namely, species richness, densities, activity patterns, vertical space use for foraging, diet and nesting characteristics of diurnal tropical tree squirrels was investigated in five forest fragments (three in <5 km2 and two in 20-30 km2 size classes) and compared against two large forest tracts (>100 km2) in upper Assam, north-east India. The study focused on the four common diurnal squirrels, Malayan Giant Squirrel, Red-bellied Squirrel, Hoary bellied Squirrel and Himalayan Striped Squirrel. A set of established trails were used to survey the sites. The diversity of squirrels in the fragments was explained by size of the fragments, intactness of the forest and disturbance levels. The fragments were in a state of continuous degradation, habitat variables being influenced more by disturbance levels than by size. Medium-sized fragments continue to retain the diversity of squirrels by the virtue of small pockets of intact habitat in the fragments, while small fragments have been rendered depauperate by synergistic effects of habitat loss, degradation and hunting. Density estimation using trails as transects under distance sampling framework revealed that densities of Malayan Giant Squirrel showed a declining trend in medium fragments and the absence of the species in the small fragments. Densities of Himalayan Striped Squirrel were comparable in large forests and medium fragments and the species was not recorded from the small fragments. Densities of RBS were high across all sites, including the small fragments Time activity budget, vertical space use for foraging and diet were investigated underthe umbrella of changes along basic niche dimensions of time, space and diet. Niche width along time decreased in the fragments for all squirrels suggesting reduced active periods. For the Malayan Giant Squirrel, niche width decreased along the other two dimensions as well in the fragments, suggesting restriction of foraging strata and diet. On the other hand, for the Red-bellied Squirrel, niche width increased along vertical space use and diet axes suggesting expansion of niche in the fragments. These differential responses of species to habitat fragmentation resulted in changes in overlap between species in fragments and a potential alteration of competitive dynamics between species in modified habitat conditions. Comparing trends in density estimates and responses of squirrels along basic niche dimensions revealed that similar inferences can be drawn from the two assessments about the potential sensitivity of a species to habitat fragmentation. Responses along basic niche dimensions can be possibly used to make prior assessments of responses to habitat alteration before differences in densities, which is a more gradual process, start to show up. It is clear from the findings of the study that diversity of squirrels depends on quality of habitat. Medium-sized fragments have the potential to retain the diversity of squirrels. They should be prioritized and accorded enough protection to deter further degradation and hunting in these fragments. Malayan Giant Squirrel and Himalayan Striped Squirrel appear to be sensitive to habitat fragmentation, while the Red-bellied Squirrel appears to be tolerant.