Density as an explanatory variable of movements and calf survival in savanna elephants across southern Africa.

1. Southern Africa's 'elephant problem' is often attributed to an overabundance of elephants (Loxodonta africana) in conservation areas. Paradoxically, the African elephant is listed as 'vulnerable' (IUCN Redlist) despite occupying a large geographical range and numbering about 600 000. How densities influence elephant populations is therefore important for conservation management decisions, particularly because a move towards non-equilibrium management of savannas implies a need for elephant populations to fluctuate in response to variation in intrinsic (demographic) and extrinsic (resource) factors. 2. A study on one of the world's largest elephant populations demonstrated that population regulation is driven by a spatial response to water availability, environmental stochasticity and density. The challenge remains to identify the demographic and behavioural variables that drive density dependence. 3. We evaluated whether the movements of elephant family groups from 13 populations across a wide resource gradient were explained by variability in primary productivity, rainfall and population density. We then assessed whether density-related movements explained variability in juvenile survival, hence inferring a spatially driven behavioural mechanism that may explain density-dependent population growth. We also analysed whether management actions modified this mechanism. 4. In the dry season, daily-displacement distances (DDDs) increased non-linearly with density, and declined with increased vegetation productivity and previous wet season rainfall. In the wet season, DDDs were primarily explained by vegetation productivity. 5. The survival of weaned calves (4-7 years) decreased with increasing dry season DDDs, but this did not hold for suckling calves (1-3 years) or sub-adults (8-11 years). 6. Fences and supplementary water modified the shape and strength of relationships between DDDs and densities, vegetation productivity and rainfall and negated the relationships between DDDs and weaned calf survival. 7. We suggest that density dependence in weaned calf survival is driven by the response of dry season roaming activities of family groups to variations in density, rainfall and the distribution of food. Fences and supplementary water that alter this mechanism may contribute to the relatively high population growth rates of some populations.