Environmental heterogeneity affects the location of modelled communities along the niche–neutrality continuum

Abstract

The continuum hypothesis has been proposed as a means to reconcile the contradiction between the niche and neutral theories. While past research has shown that species richness affects the location of communities along the niche-neutrality continuum, there may be extrinsic forces at play as well. We used a spatially explicit continuum model to quantify the effects of environmental heterogeneity, comprising abundance distribution and spatial configuration of resources, on the degree of community neutrality. We found that both components of heterogeneity affect the degree of community neutrality and that species' dispersal characteristics affect the neutrality-heterogeneity relationship. Narrower resource abundance distributions decrease neutrality, while spatial configuration, which is manifested by spatial aggregation of resources, decreases neutrality at higher aggregation levels. In general, the degree of community neutrality was affected by complex interactions among spatial configuration of resources, their abundance distributions and the dispersal characteristics of species in the community. Our results highlight the important yet overlooked role of the environment in dictating the location of communities along the hypothesized niche-neutrality continuum.

Beyond the continuum: a multi-dimensional phase space for neutral–niche community assembly

“…Meanwhile, neutral theory assumes a fixed species pool in the absence of speciation and invasion, and considers all species to be ecologically equivalent, with stochastic dispersal and ecological drift being the only processes determining community structure. Despite contrasting opinions on the value of neutral theory [1,2], it is now generally accepted that neutral and niche processes interact in natural communities and both contribute towards the structure of species assemblages [3]. The relative roles of neutral and niche processes have been shown to differ across spatio-temporal scales [4,5] and modelling these processes in combination (composite models) better represents biological patterns than neutral or niche models alone [6,7].…”

“…It is based on the variability in the outputs of independent simulation model replicates in a spatially explicit, composite model of plant communities. With this definition, Gravel et al [13] compute a neutrality index using the variability in species abundance, whereas Bar-Massada et al [3] use the variability in the species identity occupying a cell. In these models, strict niche separation leads to a deterministic output with no variability because one habitat can only be occupied by one particular species.…”

“…A different way of defining neutrality along the continuum has also been proposed to account for both distance-limited dispersal and niche separation [3,13]. It is based on the variability in the outputs of independent simulation model replicates in a spatially explicit, composite model of plant communities.…”

“…based on niche selection [13]. Although these composite models are increasingly used to describe community dynamics, the way in which neutral and niche processes interact and their relative importance in structuring communities is still unclear [3].…”

“…The neutral-niche continuum proposes that natural communities lie along a continuum based on the relative importance of these processes, with pure neutral-(no or equal niche selection amongst species) and pure niche-(strict niche separation; each species survives under a single, unique set of environmental conditions) driven communities at either extreme [3,10,13]. For example, Mutshinda & O'Hara [10] locate a community along the continuum using a neutrality index (bounded between 0 and 1), computed as the degree of average niche overlap, i.e.…”