Unraveling the drivers of forage quality variation in the Serengeti

Abstract

Variation in forage quality is a key dimension of herbivore resource partitioning, but the main determinants of such variation across environmental gradients remain poorly understood. It is especially unclear how much variation in plant nutrient contents and stoichiometry is driven by plant species turnover versus by intraspecific variation across sites. We investigated variation in forage quality across nine sites along a key environmental gradient of increasing rainfall and decreasing soil fertility in the Serengeti National Park, Tanzania. We compared leaf elemental contents of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sodium (Na) and three nutrient ratios (N:P, Ca:P, and K:Na) between sites, between species within the same site, and between sites within the same species. Site-average N, P, and K leaf contents decreased with increasing rainfall and decreasing soil fertility. The decline in N and K was primarily associated with species turnover, with their contents remaining relatively stable within species. The decline in P was associated with a combination of species turnover and intraspecific variation, with intraspecific P content decreasing strongly with increasing rainfall (decreasing soil fertility) across sites. Variation in site-average Ca, Mg, and Na leaf contents did not significantly correlate with rainfall or soil fertility and was mainly explained by species turnover between sites. Comparing leaf nutrient content and ratios to literature-derived nutritional requirements for large herbivores suggests that Na is severely limiting in this ecosystem. K seems sufficient everywhere, and the other elements are moderately limiting. If Serengeti herbivores rely on plants for their nutrient intake and are nutrient-limited, these results suggest herbivores with high N, Ca, or Mg requirements should optimize their diet by selecting particular species, relatively independent of sites. Herbivores with a high P requirement can instead best select particular sites, relatively independent of plant species. To obtain sufficient Na, herbivores can target particular species at particular sites. Thus, resource partitioning among Serengeti herbivores may occur at different levels for different elements. Interspecific variation in herbivore nutrient requirements would then drive resource partitioning both across sites (for P and Na) and between plant species (N, Ca, Mg, and Na).