Density estimation and time trend analysis of large herbivores in Nagarhole, India

Density estimates for six large herbivore species were obtained through analysis of line transect data from Nagarhole National Park, south-western India, collected between 1989 and 2000. These species were Chital (Axis axis), Sambar (Cervus unicolor), Gaur (Bos gaurus), Wild Pig (Sus scrofa), Muntjac (Muntiacus muntjak) and Asian Elephant (Elephas maximus). Multiple Covariate Distance Sampling (MCDS) models were used to derive these density estimates. The distance histograms showed a relatively large spike at zero, which can lead to problems when fitting MCDS models. The effects of this spike were investigated and remedied by forward truncation. Density estimates from unmodified dataset were 10-15% higher than estimates from the forward truncated data, with this going up to 37% for Muntjac. These could possibly be over estimates. Empirical trend models were then fit to the density estimates. Overall trends were stable, though there were intra-habitat differences in trends for some species. The trends were similar both in cases where forward truncation was done as well as in those where they were not.