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spider communities in rainforest fragments and shade-coffee plantations
In a scenario of constantly changing habitats with continuous human pressures, it becomes important to document habitat alteration and its impacts on wild species, While we have some idea of how processes like deforestation or habitat fragmentation affect taxa such as mammals, birds, and plants, we are still scratching the surface when it comes to understanding the direct and indirect consequences of habitat alteration on invertebrates, despite the fact that they comprise almost 95% of all of life on earth and perform crucial ecological functions in all ecosystems. For example, studies have shown that habitat fragmentation has negative effects on bees, leading to a decline in flower pollination, and on dung and carrion beetles, resulting in a reduction in dung decomposition rates, both of which are crucial ecosystem processes. Such a paucity of knowledge is particularly true in the context of studies on Indian invertebrates.
Spiders form an important conspicuous invertebrate predator group and a single hectare of tropical rainforest may contain up to 800 species. Spiders as a group also seem to possess the attributes that are generally required to promote an organism as a good ecological indicator: they are diverse, abundant, easily sampled, and functionally significant. To assess the role of spiders as potential biological indicators, we need to first document the responses of spider communities to disturbance. This was addressed in our study in the Indira Gandhi Wildlife Sanctuary and private lands of the Valparai plateau in the Anamalai hills.
Ten rainforest fragments representative of varying levels of degradation, and two shade-coffee plantations were sampled for spiders between January and May 2005. More than 4000 individual spiders belonging to 156 species were recorded. Overall, we found that number of individual spiders and number of species of spiders was not influenced by the fragment area. However, the species of spiders comprising the community changed substantially in relation to habitat alteration and altitude. We also attempted to identify species whose abundance varied substantially between undisturbed, disturbed, and coffee plantation sites. One such species found during this study Psechrus torvus of the family Psechridae is a probable indicator of the “good health” of the forest patch it occurs in. This species was found in high abundances in relatively undisturbed rainforest habitats in the study area and was completely absent from the highly disturbed and coffee sites. Conversely, Tylorida culta and Leucauge sp. both belonging to the family Tetragnathidae tend to become more abundant and common in disturbed sites and coffee plantations.
Such changes in spider communities may reflect ecological impacts at the lower levels in the food chain. For restoration efforts aiming to bring back original vegetation and ecological processes to as near a pre-disturbance state as possible, spiders may serve as useful target organisms for long-term monitoring of restored and recovering rainforest habitats.
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