Effects of extractive disturbance on bird assemblages, vegetation structure and floristics in tropical scrub forest, Sariska Tiger Reserve, India

Abstract

A study was carried out in a tropical scrub forest in India, to investigate the effects of extractive activities such as fodder and firewood collection, on native bird communities and to study the relative influences of altered vegetation composition and structure. The study was based on comparison between six ‘disturbed’ (extracted) and four ‘undisturbed’ (non-extracted) sites that were delineated using quantitative disturbance indicators. Birds were sampled seven times over two different seasons utilising the fixed radius point count method. Six different variables related to vegetation structure (canopy cover, basal area, average forest height, tree density, tree height diversity and tree species richness) were quantified, as was tree species composition. There was no significant difference in number of recorded species or bird abundance between disturbed and undisturbed sites. However, bird species diversity was significantly lower in disturbed sites in comparison to undisturbed sites. Overall, bird species composition also differed significantly between disturbed and undisturbed sites. Nine of 38 locally abundant bird species (23.6%) showed significant selection for either disturbed or undisturbed habitats. Of these nine species, seven chose undisturbed habitats and two chose disturbed. All seven species adversely affected by disturbance are primarily insectivorous. Canopy cover, tree basal area and height of trees were significantly lower in disturbed sites in comparison to undisturbed sites. In both habitats, bird species composition was significantly dependent on these components of altered vegetation structure. Tree species composition was also significantly altered by disturbance in scrub forest. However, altered tree species composition did not significantly affect bird species composition. Partial Mantel's tests confirmed that there were no significant residual effects of tree species composition on bird composition after the effects of vegetation structure were accounted for. Our study indicates that rural biomass extraction can have significant effects upon bird species composition of tropical scrub forest which is caused principally by alteration of vegetation structure, rather than by changes in forest tree composition.