Distance-responsive natural enemies strongly influence seedling establishment patterns of multiple species in an Amazonian rain forest
1. In a faunally intact lowland Amazonian rain forest stand, we conducted a long-term multi-species experiment aimed at determining the primary mechanistic basis of seedling establishment patterns. We deployed a total of 1050 experimental seedlings, representing 11 common tree species in mixed compositions and at fixed, highly elevated densities in shaded understorey sites, representing extremes of distance from large conspecific trees. We used mesh exclosures to isolate the effects of distinct classes of natural enemies, and monitored survival for up to 45 months. 2. Final seedling survival of all species pooled represented a 40% increase at sites located far from ('F' sites) versus close to ('N' sites) large conspecific trees, and median seedling lifetime was 75% longer. These differences between N and F sites were significant for all species pooled, and for five out of 11 (survivorship) and four out of nine (lifetime) individual species examined. Survival analysis based on multiple censuses revealed that a 'distance effect' persisted and intensified over time, with the onset of significant distance-related differential mortality differing amongst species. 3. The use of mesh exclosures (<2 mm mesh size) and a factorial experimental design revealed that host-specific organisms <2 mm in size and/or below-ground soil-borne organisms are more strongly distance-responsive and depress seedling establishment primarily in the vicinity of large conspecific adults whereas above-ground organisms >2 mm in size appear to have a negative impact on seedling establishment at all distances. 4. No evidence was found for the effect of intra-cohort resource competition on seedling establishment even though initial density of experimental seedlings at all sites was elevated to c. 25 times the mean natural density of the mixed-species seedling layer in this forest. 5. Synthesis. Our study provides strong, multi-species support for the influence of host-specific distance-responsive natural enemies on seedling establishment, and suggests that negative density-dependent patterns of tree recruitment in tropical rain forests are at least partly produced at early life stages as an outcome of processes described by the classic Janzen-Connell model. © 2010 The Authors. Journal compilation © 2010 British Ecological Society.
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