Individual variation and weak neutrality as determinants of forest diversity

Abstract

Niche‐based and neutral processes are alternative mechanisms proposed to maintain the diversity of forest trees. Neutral processes, meaning those that do not invoke fitness differences between species, have been discounted because of their central assumption of species equivalence. We propose weak neutrality as an alternative conceptual basis for the maintenance of diversity that does not require strict species equivalence. Weak neutrality is based on three underlying assertions. (1) Individual variation leads to broad species overlap, reduced rates of competitive exclusion, and forest dynamics that approximate a biased random walk. (2) Environmental variation results in stochastic spatial and temporal fluctuations in the magnitude and direction of the biased random walk, reducing the likelihood of fixation of a species and corresponding exclusion of others. (3) Limited dispersal in conjunction with environmental variation inhibits divergent evolution and increased niche separation. We suggest that the importance of weak neutrality as a determinant of diversity depends on the magnitude of both individual and environmental variability. Niche‐based processes are expected to be more prominent along steep environmental gradients, in landscapes with environmentally heterogeneous patches, and across broad spatial extents along shallow environmental gradients. We distinguish weak neutrality from pure neutrality and other conceptual models of species diversity.