Wildlife disturbances as a source of conspecific negative density dependent mortality in tropical trees
Large vertebrates are rarely considered important drivers of conspecific negative density-dependent mortality (CNDD) in plants because they are generalist consumers. However, disturbances like trampling also cause plant mortality, and their impact on plant diversity depends on the spatial overlap between wildlife habitat preferences and plant species composition. We studied the impact of native wildlife on a hyperdiverse tree community in Malaysia where pigs (Sus scrofa) are abnormally abundant due to food subsidies in nearby farmland. Pigs construct birthing nests using hundreds of tree saplings, which accounted for at least 18% of tree sapling mortality at our site. We tagged 34,950 tree saplings in 25-ha plot during an initial census and assessed the likelihood they were later killed by pig nesting disturbances in subsequent years (N = 1,672 pig-induced deaths). At the stand scale, pigs nested in flat dry habitats, and at the local neighborhood scale, they nested within clumps of saplings, both of which are intuitive for safe and efficient building. At the stand scale, flat dry habitats contained higher sapling densities and higher proportions of common species, so pig nesting increased the weighted average species evenness across habitats. At the neighbourhood scale, pig-induced sapling mortality was associated with higher heterospecific and especially conspecific sapling densities. Tree species have clumped distributions due to dispersal limitation and habitat filtering, so pig disturbances in sapling clumps indirectly caused CNDD. As a result, Pielou species evenness in 400-m2 quadrats increased 105% more in areas with pig-induced deaths than areas without disturbances. Wildlife induced CNDD and this supported tree species evenness but they also drove a 62% decline in sapling densities, which is unsustainable. We suspect pig nesting is an important feature shaping tree composition throughout the region.