Incorporating secondary metabolites, tannin-binding proteins, and diet breadth into carrying-capacity models for African elephants

Elephant populations have been historically difficult to manage and have sometimes been subjected to drastic management procedures. One way to monitor elephant populations to estimate a sustainable carrying capacity is through the use of standing-crop carrying-capacity models. Previous carrying-capacity models designed for grazers have posed issues when applied to browsers such as elephants, predicting much higher carrying-capacity estimates than are realized in nature. This might be attributed to the level of plant secondary metabolites, which limit the available nitrogen in browse material that browsers and mixed feeders encounter while foraging. Many browsers and mixed-feeders, including elephants, have a physiological mechanism to tolerate a portion of encountered tannins in the form of salivary tannin-binding proteins. We constructed an optimal-foraging model for carrying capacity for elephants that incorporates the negative effects of plant secondary metabolites and the partial neutralization of these chemicals by the tannin-binding affinity of elephant saliva. In addition, our model includes diet breadth and dietary contribution of browsed species, browse quality, and the available standing crop of browsed species. Ultimately, our model produced more plausible estimates of elephant carrying capacity when compared with estimates that use 100% nitrogen availability and use. Moreover, as the key variables needed for the model can be obtained easily, our model is not site-specific or limited to elephants, but rather can be applied to a wide range of browsing herbivores across a number of reserves of different sizes.