Remotely 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. NDVI was 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 between NDVI and 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 and GBH) not being related to primary productivity (trees and recruits). NDVI had 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 explained NDVI. Spatially interpolated total graminoid abundance modeled from field data outperformed NDVI in predicting total graminoid abundance, although interpolation models of food graminoid abundance were not satisfactory. Our results reject the utility of NDVI in 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.

The use of remotely-sensed vegetation indices has increased in wildlife studies but field-based support for their utility as a measure of forage availability largely comes from open-canopy habitats. In this paper, we assessed whether the popular vegetation index, NDVI, actually 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, and the abundance of food species compared to total species abundance varied across different vegetation categories. NDVI was 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 (herb and shrub categories), b) abundant overstorey vegetation resulting in low correlations between NDVI and food abundance despite a high proportional abundance of food species and a concordance between total abundance and food species abundance (graminoid category), and c) the relevant variables measured and important as food at the ground level (count and GBH) not being related to primary productivity (trees and recruits). NDVI had a negative relationship with the total abundance of graminoids, which represent a bulk of elephant and other herbivore diet, because of the presence of other vegetation types and canopy cover that positively explained NDVI. We also found that spatially interpolated total graminoid abundance modelled from field data outperformed NDVI in predicting total graminoid abundance, although interpolation models of food graminoid species abundance were not satisfactory. Our results reject the utility of NDVI as a surrogate of elephant forage abundance in tropical forests with multistorey vegetation, a finding that has implications for studies of other herbivores in such habitats.