The utility of normalized difference vegetation index for predicting African buffalo forage quality

Many studies of mammalian herbivores have employed remotely sensed vegetation greenness, in the form of Normalized Difference Vegetation Index (NDVI) as a proxy for forage quality. The assumption that reflected greenness represents forage quality often goes untested, and limited data exist on the relationships between remotely sensed and traditional forage nutrient indicators. We provide the first study connecting NDVI and forage nutrient indicators within a free‐ranging African herbivore ecosystem. We examined the relationships between fecal nutrient levels (nitrogen and phosphorus), forage nutrient levels, body condition, and NDVI for African buffalo ( Syncerus caffer ) in a South African savanna ecosystem over a 2‐year period (2001 and 2002). We used an information‐theoretic approach to rank models of fecal nitrogen (N f ) and phosphorus (P f ) as functions of geology, season, and NDVI in each year separately. For each year, the highest ranked models for N f accounted for 61% and 65% of the observed variance, and these models included geology, season, and NDVI. The top‐ranked model for P f in 2001, although capturing 54% of the variability, did not include NDVI. In 2002, we could not identify a top ranking model for phosphorus (i.e., all models were within 2 AIC c of each other). Body condition was most highly correlated ( Radj² = 0.75 ; P ≤ 0.001) with NDVI at a 1 month time lag and with N f at a 3 months time lag ( Radj² = 0.65 ; P ≤ 0.001), but was not significantly correlated with P f . Our findings suggest that NDVI can be used to index nitrogen content of forage and is correlated with improved body condition in African buffalo. Thus, NDVI provides a useful means to assess forage quality where crude protein is a limiting resource. We found that NDVI accounted for more than a seasonal effect, and in a system where standing biomass may be high but of low quality, understanding available nutrients is useful for management.