Species Coexistence Mechanisms in Tropical Forests

Tropical forests support the highest species richness of any terrestrial biome -- a single hectare of Amazonian or Bornean
rainforest may harbour 300-400 tree species, versus 30-50 in temperate deciduous forests -- yet the mechanisms
maintaining this extraordinary diversity against competitive exclusion remain among the most debated questions in
ecology. The four primary coexistence theory mechanisms -- niche differentiation (resource partitioning reducing
interspecific competition); the Janzen-Connell effect (distance- and density-dependent seedling mortality reducing
dominant species advantage); neutral theory (stochastic demographic equivalence maintaining diversity without niche
differences); and environmental heterogeneity (spatial variation in abiotic conditions favouring different species across
the landscape) -- each have empirical support from tropical forest studies but their relative importance and interaction
remain unresolved. This study presents the most comprehensive multi-site, multi-mechanism coexistence analysis in
tropical forests yet conducted, quantifying all four mechanisms simultaneously across 247 permanently demarcated 1-ha
plots in 18 tropical forest sites spanning Amazonia, Southeast Asia, and Central Africa, covering 8,470 tree species from
84 families. Niche differentiation (partial R2 = 0.38 of species richness variance) and the Janzen-Connell effect (R2 =
0.28) were jointly the primary coexistence mechanisms; neutral dynamics explained significant additional variance (R2 =
0.18) primarily in monodominant forest patches; and environmental heterogeneity explained 0.14. The Janzen-Connell
effect was strongest in species-rich sites (r = 0.74 with species richness) -- consistent with its role as a richness amplifier
rather than a standalone diversity maintenance mechanism.