Cascading effects of elephant-human interactions in a savanna ecosystem and the implications for ecology and conservation

Abstract

Our study monitored the changes in elephant numbers, distribution and ecological impact over a fifty-year period as the free-ranging intermingled movements of wildlife and traditional subsistence pastoralists across the Amboseli ecosystem were disrupted by a national park, livestock ranches, farms, settlements and changing lifestyles and economies. Elephants compressed into the national park by poaching and settlement turned woodlands to grassland and shrublands and swamps into short grazing lawns, causing a reduction of plant and herbivore diversity and resilience to extreme events. The results echo the ecological findings of high-density elephant populations in protected areas across eastern and southern Africa. The impact has led to the view of elephants in parks being incompatible with biodiversity and to population control measures. In contrast to Amboseli National Park, we found woody vegetation grew and plant diversity fell in areas abandoned by elephants. We therefore used naturalist and exclosure experiments to determine the density-dependent response of vegetation to elephants. We found plant richness to peak at the park boundary where elephants and livestock jostled spatially, setting up a creative browsing-grazing tension and a patchwork of habitats explaining the plant richness. A review of prehistorical and historical literature lends support to the Amboseli findings that elephants and people, the two dominant keystone species in the savannas, have been intimately entangled prior to the global ivory trade and colonialism. The findings point to the inadequacy of parks for conserving mega herbivores and as ecological baselines. The Amboseli study underlines the significance of space and mobility in expressing the keystone role of elephants, and to community-based conservation as a way to win space and mobility for elephants, alleviate the ecological disruption of compressed populations and minimize population management.