The proposed book will be composed of 16 chapters, each written by one lead author and 2-7 additional authors/contributors. The book draws on the results of five years research on the dynamics of an ecosystem where elephant and antelope populations were severely decimated more than a century ago, and then recovered. We show that both the decline and return of elephants caused profound ecosystem perturbations. A heterogeneity framework (Pickett et al. 2003) provides the conceptual structure of the book. Following Pickett et al. (2003) we consider the effects of elephants (agents) feeding on vegetation (substrate) and thereby being responsible for the vegetation changing between states. Controllers affect the action by the agent on the substrate or on the resultant transition between states of the substrate. Responders include other herbivores, predators, decomposers, and abiotic variables. In the first section of the book the sub-humid, dystrophic, broad-leafed savannas of the Chobe area in northern Botswana are presented and the importance of the elephants for the resource heterogeneity of the system and, hence, for the structure of biotic communities is discussed. We then describe the dynamics of the Chobe elephant population, which was hunted virtually to extinction in the late 1800’s and subsequently recovered. By foraging on nutrient poor woody vegetation elephants are essential for nutrient cycling in these systems, and by regularly moving between the woodlands and the Chobe riverfront (to drink), they are major (re)distributors of nutrients. We claim that the elephants are the prime agents of change in this ecosystem. The substrate affected by the elephants is the vegetation, and we discuss the historical changes in the riverfront vegetation, described as open flats in the 1870’s, as tall woodland in the mid 1900’s and currently consisting of shrublands. We show that the woodlands on the alluvial soils were established when artificially low populations of elephants and mesobrowsers provided a “window of opportunity” for tree establishment, and that the trees subsequently disappeared when herbivore populations recovered. The present vegetation in a landscape scale is characterized by the differences in plant strategies and functionality between the relatively nutrient rich alluvial soil close to the river and on the nutrient-poor Kalahari sand dominating most of the system. The difference in resource availability between soil types is a main controller of the effect of elephants on plants and on the vegetation. Other important controllers are smaller herbivores, which modify the effect of elephants on woody vegetation for example by preventing tree regeneration through seedling predation. Mesograzers may also control elephant foraging by scramble competition. Thus, we show that elephants in interaction with other herbivores and with soil resources are important determinants of vegetation composition and physiognomy.
Responders to the elephant induced states of vegetation include nutrient cycling and dynamics. Also interactions between plants and mesoherbivores are largely a function of ecosystem heterogeneity induced by elephant activities. We further show that elephants may interact differently with the grazing and the browsing guild of mesoherbivores. Also a large carnivore, the lion, seems to respond to elephant induced changes in physiognomy of the habitat offering a favorable combination of open land and dense shrub. In the 6th section o f the book we briefly discuss the interactions between humans and elephants, which have co-existed in the African savanna ecosystem throughout the existence of the species. Compared to many other protected areas in southern Africa with high elephant densities the Chobe National Park is large, unfenced and situated in a relatively sparsely populated area. Still, the increasing elephant population leads to increased contact between elephants and local societies creating problems and, perhaps, opportunities. We discuss the “Chobe elephant problem” versus the “Chobe elephant opportunity”. Finally, in a synthesis chapter, we contrast the broad-lefed, dystrophic savannas of Chobe, characterized by an elephant-dominated browser community, with the savannas and grasslands of Serengeti and other fine-leafed eutrophic savannas with mesoherbivore dominated grazer communities. We use evidence from the Chobe research project to explore general mechanisms by which very large herbivores interact with ecosystem heterogeneity and functioning, and how this differs from the impact by mesoherbivores. We discuss the functional composition of wildlife communities with and without megaherbivores across gradients in moisture and in soil resources, and we compare these extant communities with records of fossil ones on other continents. This discussion briefly includes the debate in North America on introducing surrogate megaherbivores to restore lost ecosystem processes, and explores the contribution of science to the emotive issue of elephant control through culling.
Keywords: Conservation Science