Large herbivore population performance and climate in a South African semi-arid savanna

Abstract

Long-term population performance trends of eight large herbivore species belonging to groups of disparate foraging styles were studied in the semi-arid savanna of the Kruger National Park, South Africa. Over the past century the number of bulk feeders (buffalo, waterbuck, blue wildebeest and plains zebra) had increased towards comparatively high population densities, whereas population numbers of selectively feeding antelope species (sable antelope, roan antelope, tsessebe and eland) declined progressively. Detailed analyses revealed that population numbers of buffalo and waterbuck fluctuated in association with food quantity determined by rainfall. Population performance ratings (1944–2003) of the species for which forage quality was important (blue wildebeest, zebra and selective grazers) were correlated negatively with minimum temperature and positively with dry-season rainfall.

Interpretation according to a climate–vegetation response model suggested that acclimation of forage plants to increasing temperature had resulted in temperature-enhanced plant productivity, initially increasing food availability and supporting transient synchronous increases in population abundance of both blue wildebeest and zebra, and selective grazers. As acclimation of plants to concurrently rising minimum (nocturnal) temperature (T_min) took effect, adjustments in metabolic functionality occurred involving accelerated growth activity at the cost of storage-based metabolism. Growth-linked nitrogen dilution and reduced carbon-nutrient quality of forage then resulted in phases of subsequently declining herbivore populations. Over the long term (1910–2010), progressive plant functionality shifts towards accelerated metabolic growth rather than storage priority occurred in response to T_min rising faster than maximum temperature (T_max), thereby cumulatively compromising the carbon-nutrient quality of forage, a key resource for selective grazers.

The results of analyses thus revealed consistency between herbivore population trends and levels of forage quantity and quality congruent with expected plant metabolic responses to climate effects. Thus, according to the climate-vegetation response model, climate effects were implicated as the ultimate cause of large herbivore population performance in space and over time.

Conservation implications: In its broadest sense, the objective of this study was to contribute towards the enhanced understanding of landscape-scale functioning of savanna systems with regard to the interplay between climate, vegetation and herbivore population dynamics.