Assessing the feasibility of density estimation methodologies for African forest elephant at large spatial scales

Abstract

Effective wildlife management requires information on population status and distribution. Survey methods that provide estimates of these population parameters can vary greatly in effort required, area covered, precision of estimates, and cost. Trade-offs are required, because increasing precision and area coverage generally requires increasing field effort and incurs a higher cost. We compare DNA- and camera trap based-spatial capture-recapture approaches (DNA-SCR and CT-SCR) to the widely-used, dung-based line transect distance sampling (LTDS) method to assess their performance when applied to three relatively large populations of forest elephant Loxodonta cyclotis (>500 individuals), in order to evaluate their feasibility for future use at national and regional scales. Six of the nine surveys had a coefficient of variation below 20%; area coverage via DNA-SCR and LTDS was comparable and greatly exceeded that of the CT-SCR as applied; overall cost was highest for the LTDS surveys compared to the other two methods. We designed a new metric with which to compare survey methods: an integrated feasibility index (IFI). This combines three typical survey components: total area covered, level of precision achieved, and cost. The IFI suggests that DNA-SCR and LTDS are equally acceptable in terms of the combination of the three survey components, and that either survey method is suitable for large (national or regional) spatial scales for forest elephant density estimation. CT-SCR provides more precise estimates, but has double the IFI, due to the high cost per km2. DNA-SCR in particular, given the improvements highlighted in this study, is now being used at a national scale in Gabon. In conclusion, we recommend that the use of these spatial capture-recapture (SCR) methods, and their development, continue. Future findings and improvements should be compiled across studies to ensure their robust evolution as an option for monitoring the African forest elephant across its range and inform strategies and action for its conservation.