perspective: Individual variation and weak neutrality as determinants of forest diversity

Abstract

Niche‐based and neutral processes are alternative mechanisms proposed to maintain the di‐ versity 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 varia‐ tion 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 tempo‐ ral 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 en‐ vironmental 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 individ‐ ual 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. Keywords: diversity, forest dynamics, neutrality, niches, species richness, trees Introduction Delineating the mechanisms that maintain the diversity of forest trees has been a central but elu‐ sive problem in forest ecology. One longstanding explanation has been that tree species partition environmental variation into niches; tree species can co‐exist in a forest if they differ sufficiently in traits important to the utilization of limited re‐ sources (e.g., Grubb 1977, Wright 2002, Silver‐ town 2004). Tradeoffs in fitness traits are pre‐ sumed to result in a species having a competitive advantage under some set of environmental con‐ ditions (e.g., micro‐environments), which together define that species’ niche. As a result, determinis‐ tic selection of species relative to the suite of envi‐ ronmental conditions is expected to drive commu‐ nity dynamics and patterns of species diversity (e.g., Vellend 2010). The diversity of forest trees would then depend on the breadth and overlap of species’ niches as well as the spatial and temporal distribution of micro‐environments (e.g., Comins and Noble 1985, Pacala and Tilman 1994). Niche partitioning in forest trees has been closely associated with variability in understory light levels that is, in turn, largely driven by can‐ opy gap dynamics. Sunlight is a limiting resource in forest understories, and low light levels be‐ neath the canopy mean that even the most shade‐ The photograph shows (from the left) William Platt, Louis Gross and Brian Beckage at Beckage’s lab on October 2011. Personal webpages: LG—http://nimbios.org/personnel/gross; WP—http://www.biology.lsu.edu/faculty_listings/fac_pages/ bplatt.html; WG—http://www.biol.canterbury.