Neutrality, niches, and dispersal in a temperate forest understory

Abstract

A fundamental goal of ecology is to understand what controls the distribution and abundance of species. Both environmental niches and trade-offs among species in dispersal and competitive ability have traditionally been cited as determinants of plant community composition. More recently, neutral models have shown that communities of species with identical life-history characteristics and no adaptation to environmental niches can form spatial distribution patterns similar to those found in nature, so long as the species have a limited dispersal distance. If there is a strong correlation between geographic distance and change in environmental conditions, however, such spatial patterns can arise through either neutral or niche-based processes. To test these competing theories, we developed a sampling design that decoupled distance and environment in the understory plant communities of an old-growth, temperate forest. We found strong evidence of niche-structuring but almost no support for neutral predictions. Dispersal limitation acted in conjunction with environmental gradients to determine species' distributions, and both functional and phylogenetic constraints appear to contribute to the niche differentiation that structures community assembly. Our results indicate that testing a neutral hypothesis without accounting for environmental gradients will at best cause unexplained variation in plant distributions and may well provide misleading support for neutrality because of a correlation between geographic distance and environment. N eutral theory (1-3) demonstrates that the compounded effects of dispersal limitation, speciation, and the role of chance through time can cause ecologically identical species to form patterns of distribution and abundance similar to those found in nature. The assumption of ecological equivalence among species in neutral theory challenges contemporary views on the importance of evolution and ecological adaptation in determining patterns of distribution and abundance, and thus current approaches to species conservation (2, 4). Although some assumptions of neutral theory have been questioned (5, 6), the value of the theory lies in the degree to which it can predict or refute the importance of species-specific traits in determining distribution and abundance. Aspects of neutral theory can be tested by model fitting (6), but a stronger test lies in directly assessing the degree to which species distributions are explained by environmental heterogeneity versus neutral processes operating on natural landscapes (4, 6, 7).Because of the spatial effects of dispersal limitation, neutral theory predicts that the compositional similarity between plant communities will decrease as the distance between two points increases (3,8). In contrast, niche theory predicts that community composition will change as a result of species-specific differences in evolved adaptive responses along environmental gradients (9, 10). These hypotheses are hard to distinguish in natural ecosystems, because a change in envi...

The distance decay of similarity in ecological communities

“…The study extent played also a major role in determining the initial similarity, as the extent and the initial similarity exhibited a significant negative relationship. The large residual variation in the relationships between the study extent and both the halving distance and the initial similarity implies, however, that the distance decay is not only dictated by spatial scale, but is also affected by the organism properties and the strength of environmental gradients (Nekola and White 1999, Tuomisto et al 2003, Gilbert and Lechowicz 2004, Horner-Devine et al 2004, Thompson and Townsend 2006. Stronger environmental gradients should generally result in shorter halving distances and lower initial similarities, yet these relationships Table 1.…”

“…Most of the studies to date have been conducted in terrestrial ecosystems and have examined the distribution patterns of vascular plants either in tropics or in boreal region (Nekola and White 1999, Condit et al 2002, Tuomisto et al 2003, Gilbert and Lechowicz 2004, Qian et al 2005. Studies have shown that the relationship between plant community similarity and distance can be typically approximated by a logarithmic model suggesting non-random propagule rain, and have generally lent only a weak support for neutrality (Tuomisto et al 2003, Gilbert and Lechowicz 2004, Dornelas et al 2006.…”

“…Most of the studies to date have been conducted in terrestrial ecosystems and have examined the distribution patterns of vascular plants either in tropics or in boreal region (Nekola and White 1999, Condit et al 2002, Tuomisto et al 2003, Gilbert and Lechowicz 2004, Qian et al 2005. Studies have shown that the relationship between plant community similarity and distance can be typically approximated by a logarithmic model suggesting non-random propagule rain, and have generally lent only a weak support for neutrality (Tuomisto et al 2003, Gilbert and Lechowicz 2004, Dornelas et al 2006. A number of studies have also examined the distance decay of marine fish parasite communities (Poulin 2003, Oliva andGonzález 2005), or distance decay among insects and mammals (Harrison et al 1992, Wiersma and Urban 2005, Lloyd et al 2005, McDonald et al 2005, Thompson and Townsend 2006.…”

“…First, similarity decays with distance because of decreasing similarity in environmental features. Following Nekola and White (1999), this could be attributed to niche-based community processes, with species differing in their ability to perform under different environmental conditions (Tuomisto et al 2003, Gilbert andLechowicz 2004). In metacommunity ecology, this process has been referred to species-sorting (Leibold et al 2004).…”

Taxonomic groups with lower movement capacity may present higher beta diversity

“…Accordingly, several studies have demonstrated a host of different factors that may influence variation of species composition through space: (1) spatial and environmental gradients (e.g. Clark et al, 1999;Nekola & White, 1999;Oliveira-Filho & Fontes, 2000;Carneiro & Valeriano, 2003;Steinitz et al, 2006Steinitz et al, , 2007bLegendre et al, 2009); (2) differences in the physiology, in the degree of biological interactions and in the dispersal ability of the species (Nekola & White, 1999;Tuomisto et al, 2003;Gilbert & Lechowicz, 2004); (3) barriers imposed by the configuration of the landscape and the influence of weather on species' dispersion (Nekola & White, 1999;Hubbel, 2001); (4) stochastic processes generated randomly and independently of environmental dissimilarities (Neutral Theory sensu Hubbel, 2001;Soininen et al, 2007b;Steinbauer et al, 2012); (5) species' tolerance to fragmentation (Arroyo-Rodríguez et al, 2013); (6) spatial scale (extension, resolution; Nekola & White, 1999;Steinitz et al, 2006;Soininen et al, 2007b;Arroyo-Rodríguez et al, 2013). Therefore, more accurate analysis involving other variables could yield further explanations regarding the spatial distribution observed in this study.…”