NDVI is not reliable as a surrogate of forage abundance for a large herbivore in tropical forest habitat

Abstract

AbstractRemotely sensed vegetation indices are increasingly being used in wildlife studies but field‐based support for their utility as a measure of forage availability comes largely from open‐canopy habitats. We assessed whether normalized difference vegetation index (NDVI) represents forage availability for Asian elephants in a southern Indian tropical forest. We found that the number of food species was a small percentage of all plant species.NDVIwas not a good measure of food abundance in any vegetation category partly because of (a) small to moderate proportional abundances of food species relative to the total abundance of all species in that category (herbs and shrubs), (b) abundant overstory vegetation resulting in low correlations betweenNDVIand food abundance, despite a high proportional abundance of food species and a concordance between total abundance and food species abundance (graminoids), and (c) the relevant variables measured and important as food at the ground level (count andGBH) not being related to primary productivity (trees and recruits).NDVIhad a negative relationship with the total abundance of graminoids, which represent a bulk of elephant and other herbivore diet, because of negative interaction with other vegetation and canopy cover that positively explainedNDVI. Spatially interpolated total graminoid abundance modeled from field data outperformedNDVIin predicting total graminoid abundance, although interpolation models of food graminoid abundance were not satisfactory. Our results reject the utility ofNDVIin mapping elephant forage abundance in tropical forests, a finding that has implications for studies of other herbivores also.Abstract in Kannada is available with online material.