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Impacts of habitat fragmentation on the terrestrial small mammal communities in the tropical rain forest of the Anaimalai hills in the Western Ghats, south India

Impacts of habitat fragmentation on the terrestrial small mammal communities in the tropical rain forest of the Anaimalai hills in the Western Ghats, south India
A. Prabhakar

1998

Department of Zoology, Bharathiar University, Coimbatore, Tamil Nadu, INDIA.

ABSTRACT

The loss and fragmentation of the species-rich tropical rain forests is the most serious conservation crisis currently facing us. In the Western Ghats in south India as much as 40% of the forests was lost between 1920s and 1990s, with a 17-fold increase in forest patches. Such fragmentation of forests leads in the long run to loss of species due to the problems faced by small isolated populations, the cascading effects of the changes in the micro and macro climate, and the effects of invading species. The goal of this study was to make an assessment of the changes in the small mammal community due to the fragmentation of their rain forest habitat in the Western Ghats. The study animals included rodents of the Families Muridae and Muscardinidae, and shrews of the Order Insectivora. The specific objectives were: (a) to evaluate the differences among rain forest fragments in the community structure of small mammals, in relation to macrohabitat features of the forest fragments; (b) to examine the synchrony among forest fragments in the seasonal variation of several demographic parameters; and (c) to identify species differences in microhabitat preferences and examine their relevance to the changes in community structure due to habitat fragmentation.

This study was carried out in the Indira Gandhi Wildlife Sanctuary (Tamil Nadu State) in the Anaimalais Hills of the Western Ghats. During a preliminary survey I identified 25 rain forest fragments. For this study, I selected seven forest fragments which varied in area (from less than 10 ha to more than 2,000 ha) and other habitat features. Small mammals were sampled using Sherman live traps, each trapping session lasting for five days. The number of traps per session varied from 50 to 200 depending on the area of the fragment. Seventeen microhabitat parameters were measured from 1m2 quadrats centred on all sprung trap stations, and from every alternative trap station. The sampling was replicated in four seasons.

Live trapping covered 10,595 trap nights with an overall trapping success of 5.40 animals per 100 trapnights. I captured six species from the Family Muridae, one endemic dormouse of the Family Muscardinidae (the Malabar spiny dormouse, Platacanthomys lasiurus) and one endemic shrew (Sorex niger) from the Order Insectivora. The most abundant species R. r. wroughtoni formed nearly 50% of the total captures, followed by S. niger (30%), M. booduga (15%) and M. musculus (1.76%). The other four species formed less than 4% of the captures.

A principal component analysis identified two factors that accounted for 77% of the total variance in macrohabitat parameters and their Coefficient of Variations (CVs). Factor 1 represented the status of the fragment in terms of area, tree density etc., while Factor 2 represented spatial heterogeneity. Factor 1 was linearly related to area of the fragment while Factor 2 had a quadratic relation. Seasonal species richness showed a nonlinear relationship with some of the habitat parameters and habitat status (Factor 1), and a linear relationship with habitat heterogeneity (Factor 2). The mean seasonal species richness was highest in medium sized fragments, which were moderately disturbed and had the highest spatial heterogeneity. This was due to the greater abundance of invaders which are commensal to man.

The total abundance of rodents and shrews did not show any clear relationship with habitat parameters, even though individual species did. Thus, the abundance of R. r. wroughtoni increased with habitat status and decreased with habitat heterogeneity, while M. booduga and M. musculus showed an opposite relation. S. niger showed no clear relationship. Species overlap between fragment pairs was generally high and was not related to similarity in habitat parameters. Fragments with similar spatial heterogeneity had similar species composition. The proximity of human settlements seemed to affect the similarity in species occurrence among fragments.

The subadult to adult ratio over all the fragments indicated that the breeding period for all three species was in the late summer and southwest monsoon. This might be correlated with peak fruit and foliage production in the habitat. There was no difference between forest fragments of four size classes in the age ratio in the case of R. r. wroughtoni. The differences in the other two species did not show any consistent pattern with fragment area. There was some synchrony among fragments in the seasonal variation of age ratio only in the case of S. niger.

The adult sex ratio did not show any consistent pattern with fragment area in R. r. wroughtoni and S. niger, while in M. booduga it seemed to decrease with fragment area. Synchrony was evident only in the case of S. niger. When data from fragments were pooled, the body weights of all species were greatest during the southwest monsoon. The body weight of R. r. wroughtoni increased with increasing tree density, decreasing CV of tree density, and increasing Factor 1 score. M. booduga showed an opposite relation with the same parameters while S. niger did not show significant relationship. The body weight of all species was greater in the wet seasons in most fragments.

The lack of demographic synchrony among forest fragments in the case of R. r. wroughtoni and M. booduga might be related to the differences among fragments in plant species composition and the lack of dispersal between fragments. Invertebrates, the major food of S. niger, may show less variation among fragments. Moreover, this species might be also able to disperse between fragments. These two factors might promote greater demographic synchrony among forest fragments in the case of S. niger.

Bivariate analysis showed considerable overlap in the responses of R. r. wroughtoni and S. niger to variation in almost all microhabitat variables. M. booduga also showed a similar response, but captures were greatest at medium values of some variables such as shrub density and herb cover. In contrast, captures of M. musculus were greater at low values of most of these variables, and at mid values of the others. Logistic regression segregated these species better with reference to their response to the microhabitat variables. R. r. wroughtoni showed significant response to the presence of rattans, buttressed trees and shrub density. S. niger responded to shrub density, rock cover, litter cover and herb cover. M. booduga responded to shrub density, rock cover, herb cover and canopy cover, the last of these negatively. Shrub density was the only variable to which all three species responded. However, nonlinear response of M. booduga to some variables may not be represented adequately in the logistic regression.

Since R. r. wroughtoni and M. musculus are almost totally segregated in their microhabitat selection, competition among them is unlikely to be a factor in the decline of R. r. wroughtoni in forest fragments. Overlap with M booduga in microhabitat selection can lead to competition among them in moderately disturbed fragment, and thus to the decline of R. r. wroughtoni. However, loss of optimal microhabitats of R. r. wroughtoni could also be a reason. The insectivorous S. niger is more generalist than rodents, and is unlikely to be affected by competition with invading or resident rodents, in spite of considerable overlap among them in microhabitat preferences.

The low species richness among the rodents, mice, and shrews in the Western Ghats may reflect the faunal impoverishment in these taxa in India. However, lack of taxonomical expertise and systematic sampling might also be reasons. Human settlements near forest fragments have introduced several species of commensal rodents into forest fragments, while two endemics (P. lasiurus and R. r. wroughtoni) seem to have declined. Changes in the community structure of small mammals could have major ecological implications. Being major seed predators, changes in the abundance of rodents could affect the long term vegetation dynamics of their habitat. Being a major preybase, changes in the abundance of small mammals could affect the communities of avian, reptilian and mammalian predators. Rodents are also major carriers of several pathogens and parasites. The invasion of several generalist species into forest fragments might introduce pathogens and parasites to which the native species might be ill adapted. Thus, changes in the small mammal community due to forest fragmentation can have a cascading effect on the ecosystem.