Modification of the Savanna Functioning by Herbivores

Abstract

Beyond consumption of a given amount of the net primary production (NPP), herbivores may have major effects on ecosystem structure, functioning and dynamics (e.g., [35]). Many authors have represented plant-herbivore interactions by predation-like relationships [8] assuming that herbivory has a purely negative impact on plant growth. It is now recognized that grazing may be not detrimental, and even favorable for plants. In particular, herbivory can promote grassland soil nitrogen cycling [43, 35, 29] which strongly influences plant responses to grazing [9]. Furthermore, herbivores can largely influence the temporal changes in tree/grass balance, directly through the reduction in competition intensity or indirectly through the reduction in fire frequency and intensity (e.g., [25, 10, 38]). In Lamto, the large herbivore biomass has often been regarded as low as for other West African savannas [2, 11]. The scarcity of large herbivores in Lamto was assumed to be the result of man’s actions that have, for instance, virtually exterminated elephants and hippopotami. This historical pattern is consistent with the results of several comparative studies showing that apparent low herbivore biomass in West African savannas were due to the loss of large mammal species, i.e., truncated herbivore communities, primarily in relation with human demography and development in the course of the last century [15, 7]. Studies on herbivory in Lamto savannas have mainly addressed two issues to date: (1) quantifying large and small herbivore densities, biomasses and green grass consumption rate and (2) testing the effect of grazing on the functioning and primary production of the grass layer.