Source-Sink Dynamics: a Neglected Problem for Landscape-Scale Biodiversity Conservation in the Tropics

Abstract

Ecologists have long recognised that populations can persist in poor-quality habitats with below-replacement population growth rates (‘sinks’) provided there is continual immigration from areas with positive intrinsic growth (‘sources’). Source-sink dynamics complicate the assessment of species-environment relationships, because species’ presence or density can be poorly correlated with underlying population productivity. Yet, applied conservation research often uses presence or abundance data to assess species responses to environmental change, particularly in the tropics where few long-term ecological studies are established. This approach assumes that abundance data reliably indicate habitat quality, but in sinks, this assumption can be violated. We review the recent literature and identify a regional bias in reporting of source-sink phenomena, with 71% of the 210 studies considered coming from temperate regions, particularly Eurasia and North America. Very few studies come from tropical and subtropical biomes, where human-driven biodiversity loss is occurring most rapidly, with over 80% not providing strong evidence in the form of demographic, dispersal or molecular data. Source-sink studies in tropical regions have predominantly investigated populations exposed to hunting/exploitation, with few examining land-use change. We review policy-relevant arenas where better treatment of source-sink dynamics is a priority: spatial conservation planning, assessments of land-sparing versus land-sharing, the conservation value of selectively logged forests, and species distribution modelling. Finally, we discuss ways to improve understanding of source-sink dynamics, particularly in tropical regions. Failure to detect source-sink patterns across the hyperdiverse tropics could limit the efficacy of conservation practice, leading us to underestimate the severity of human impacts on biodiversity.