Response of black wildebeest, a specialised grazer, to grass availability

The black wildebeest Connochaetes gnou, a southern African endemic largely confined to the Grassland biome, was used as a model species to assess the response of a specialised grazer to grass availability, as an indicator of habitat quality. The occurrence of animals in low-quality habitats may have detrimental effects on population performance and persistence. Robust techniques are therefore required to assess the response of animals to varying habitat quality, to ensure sustainability of populations and species. Black wildebeest habitat use was assessed in relation to grass availability and visibility, and faecal analysis was used to estimate variation in diet composition and quality, parasite loads, and faecal pellet dimensions, across a grass gradient. Grass consumption, diet quality, and faecal pellet size and weight of black wildebeest increased with increasing grass availability. While black wildebeest consumed up to 55% browse at grass-poor sites, they consistently preferred grasses, potentially resulting in an increased intake of low-quality grasses at grass-poor sites. This, combined with the increased intake of indigestible fibres in browse, may explain the observed trends in diet quality, which in turn may explain variation in faecal pellet size and weight (indicators of body size and weight). Furthermore, black wildebeest population trends within the study area suggest that populations in grass-poor habitats, outside their natural distribution range, may exhibit reduced population performance. These findings suggest that grass availability may be a useful indicator of habitat quality for specialised grazers. The study also provided an opportunity to explore the utility of various diet assessment techniques for estimating herbivore diet composition and quality. Caution should be applied when interpreting and comparing estimates of diet composition and quality that were estimated using different techniques, because different techniques may yield different diet estimates. The study also suggests that black wildebeest may switch to C3 grasses in the winter (as indicated by isotope analysis)