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Impacts of habitat fragmentation on the arboreal mammals in the wet evergreen forests of the Anamalai Hills in the Western Ghats, south India

Impacts of habitat fragmentation on the arboreal mammals in the wet evergreen forests of the Anamalai Hills in the Western Ghats, south India
G. Umapathy


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


Habitat fragmentation is the most severe threat to biological diversity and is the primary cause of the present species extinction crisis. Small populations in fragments are highly prone to extinction due to demographic and environmental stochasticity, disease and catastrophes. Long term degeneration of habitat might also add to this extinction. In south India, wet evergreen (or rain) forest is confined to the Western Ghats mountains. Human activities such as plantations of tea, coffee, teak, etc., and construction of roads, railways, and reservoirs during the last two centuries have led to extensive loss of these forests. More importantly, the remaining forest has been severely fragmented.

The impact of forest fragmentation is expected to be particularly severe among the arboreal mammals due to loss of arboreal connectivity between forest fragments. In this background, the objectives of this study were; 1) To assess the extent to which arboreal mammals disappear from wet evergreen forest fragments with respect to various landscape and habitat parameters associated with forest fragmentation; 2) To identify changes in their activity pattern and feeding ecology; 3) To examine changes in demographic parameters of these species in relation to habitat fragmentation; and 4) To examine species differences in their response to forest fragmentation and to suggest appropriate measures to enhance the survival of arboreal mammals in forest fragments.

Five species of arboreal mammals formed the subjects of this study. Two of these are diurnal primates (Order Primates; Family Cercopithecidae) which are endemic to the Western Ghats; the lion-tailed macaque (Macaca silenus, Subfamily - Cercopithecinae) and the Nilgiri langur (Trachypithecus johnii, Subfamily - Colobinae). Three species were tree squirrels (Order Rodentia, Family Sciuridae), two of them are widely distributed in India, the Malabar giant squirrel (Ratufa indica, Subfamily - Sciurinae) and the large brown flying squirrel (Petaurista philippensis, Subfamily - Petauristinae). Only the small Travancore flying squirrel (Petinomys fuscocapillus, Subfamily - Petauristinae) is endemic to the Western Ghats.

The study was carried out in 1994-1996 in the wet evergreen forest fragments in the Indira Gandhi Wildlife Sanctuary and nearby privately owned forest fragments, in the Anamalai Hills, Tamil Nadu. The field studies included i) a survey of forest fragments to estimate the occurrence and abundance of arboreal mammals, and habitat status; ii) studies on the activity pattern and feeding ecology of the lion-tailed macaque, Nilgiri langur, and giant squirrel; iii) studies on demography of primates; and iv) monitoring of phenology of important plant food species.

Surveys were carried out to estimate the occurrence and abundance of five arboreal mammals in 25 forest fragments in relation to several habitat and landscape parameters. The occurrence and abundance of diurnal arboreal mammals were estimated from transects (400 km). The habitat parameters were estimated from 350 circular plots of 5 m radius. The wet evergreen forest fragments differed among themselves with reference to various landscape and habitat parameters. Fragments which were small in size were most likely to be privately owned and had low tree density, basal area, canopy cover and canopy height. The large fragments were mostly owned by the Forest Department, and had better habitat quality. Among the five arboreal mammals, the lion-tailed macaque was the most affected, being absent from 15 of the 25 forest fragments, where as the Nilgiri langur and the giant squirrel were absent only from six and three fragments, respectively. The best predictor for the occurrence of lion tailed macaque was canopy height, while it was tree density for the Nilgiri langur. The abundance of the lion-tailed macaque and the Nilgiri langur showed a high variability in the smaller fragments, while the density of the giant squirrel and flying squirrel increased with decreasing area and disturbance level. The increase in the densities of giant squirrels and flying squirrel may be due to their wide ecological amplitude and greater tolerance to disturbance. Area may be the best predictor of the occurrence of arboreal mammals only when area of the fragments is very small, the initial occurrence depending on the relative densities at which species occur. Once a species is initially present, its persistence is more likely to be related to other habitat parameters.

Activity pattern and feeding ecology were studied in three arboreal mammals (lion-tailed macaque, Nilgiri langur and giant squirrel) for one year in four forest fragments which varied in their area and level of disturbance. There were no consistent differences in the three species in the time that they spent on various activities between the four forest fragments. A significant change in the feeding ecology of the three species was a reduction in the number of food plant species used per day and a reduction in the use of lianas in the small and degraded fragments. The reduction of food abundance in small fragments was compensated by changes in vegetation. The vegetation that immediately surrounded the small fragments was of considerable food value to the arboreal mammals. Keystone species such as Ficus spp and Cullinea exarillata that have been left behind in small fragments probably have greater productivity due to increased penetration of solar radiation. The giant squirrel was the least affected.

The demographic parameters were estimated for the lion-tailed macaque from 11 groups in eight fragments, and for the Nilgiri langur from eight groups in four fragments. The major demographic effects on the lion tailed macaque were a reduction in birth rate, population growth rate, and immatures survival, and increase in group size and a wide variation adult sex ratio in small fragments. The reduction in birth rate and survival may be due to reduction in the diet quality. The latter may be also due to fall from tree canopy and greater predation. For the Nilgiri langur there was a reduction in birth rate, population growth rate and immature survival and but not in adult sex ratio because they dispersed between fragments. The reduction in the birth rate may be to due to a reduction in diet quality, as in the lion-tailed macaque.

Dispersal between fragments might be limited in the lion-tailed macaque because of their inability to use treeless matrix and plantations (such as teak, eucalyptus, tea and coffee) and the female-bonded social system that prevents female dispersal. The giant squirrels are also unable to disperse between forest fragments in the treeless matrix. In the Nilgiri langur, both male and female disperse between fragments across the landscape. Thus, the fragmented population of the lion-tailed macaque is unlikely to occur as a metapopulation that allows recolonisation, dampening of demographic stochasticity and genetic exchange. The squirrels might exist partially as a metapopulation, while most of the Nilgiri langur populations might form part of a metapopulation. Genetic consequences of population fragmentation and changes in the parasitic and pathogen profiles are two factors that could also influence the continued survival of the study animals.

Among the important steps to be taken to enhance the survival of arboreal mammals in a fragmented habitat are; 1) Retain the man made vegetation immediately around the small fragments which consists of Coffee, Maesopsis eminii and Artocarpus heterophyllus 2) Retention of key stone species such as Ficus spp, Cullinea exarillata and Artocarpus heterophyllus in the forest fragments; 3) Increase in the tree densities in small fragments through assisted regeneration, especially of food plants; 4) Retention and enhancement of canopy contiguity, especially across the roads; 5) Very strict control of even low level of poaching in the case of lion tailed macaque; 6) Periodic monitoring of age/sex composition of small populations, for translocation of individuals if needed; and 7) Research especially of genetic and parasitic consequences.