Assessing the relative importance of neutral stochasticity in ecological communities

Abstract

A central current debate in community ecology concerns the relative importance of deterministic versus stochastic processes underlying community structure. However, the concept of stochasticity presents several profound philosophical, theoretical and empirical challenges, which we address here. The philosophical argument that nothing in nature is truly stochastic can be met with the following operational concept of neutral stochasticity in community ecology: change in the composition of a community (i.e. community dynamics) is neutrally stochastic to the degree that individual demographic events -birth, death, immigration, emigration -which cause such changes occur at random with respect to species identities. Empirical methods for identifying the stochastic component of community dynamics or structure include null models and multivariate statistics on observational species-by-site data (with or without environmental or trait data), and experimental manipulations of 'stochastic' species colonization order or relative densities and frequencies of competing species. We identify the fundamental limitations of each method with respect to its ability to allow inferences about stochastic community processes. Critical future needs include greater precision in articulating the link between results and ecological inferences, a comprehensive theoretical assessment of the interpretation of statistical analyses of observational data, and experiments focusing on community size and on natural variation in species colonization order.

Effects of dispersal and selection on stochastic assembly in microbial communities

“…Communities under strong selection (high lignin/N) had the fewest species and lowest biomass (Figure 2). The smaller sizes of these communities (number of individuals summed over species) may have made them more susceptible to demographic drift (Vellend et al, 2014). In addition, the low total biomass in these treatments likely lessened the intensity of competitive interactions, essentially weakening density-dependent selection despite (apparently) strong abiotic selection.…”

“…However, future research might address some of our model's omissions. For instance, we treated dispersal as a one-time, yearly event, but assembly patterns can be sensitive to whether an immigration event is sequential or simultaneous (Steiner, 2014), or whether colonizers are primary (the first to arrive in an area) or secondary (colonizing an area with a pre-existing community) (Vellend et al, 2014). We also did not include variation in dispersal ability among taxa.…”

“…A common framework to understand community assembly is the distinction between deterministic, or niche-driven, processes and those that are stochastic or neutral (Hubbel, 2001;Vellend et al, 2014). Because microbes have high dispersal rates, large population sizes, fast growth rates and a propensity for dormancy, communities were traditionally thought to assemble deterministically.…”

“…Here, we define stochastic processes as those that are probabilistic in relation to species identity (Vellend et al, 2014). Some mechanisms of microbial community assembly (Vellend, 2010) map easily onto the stochastic-deterministic framework: demographic drift, or fluctuation in population size due to random birth and death events, is a stochastic process, whereas selection, equivalent to environmental filtering or species sorting, can be considered deterministic.…”

Chance, Variation and the Nature of Causality in Ecological Communities

“…Finally, the plankton example shows the reverse, where predictable species replacements result in chaotic dynamics of a community which is as unpredictable as the weather (Benincà et al 2008). These contrasts feed the uneasy relationship that ecology has with deterministic versus stochastic processes underlying the structure of ecological communities (Bjørnstad 2015;Chase and Myers 2011;Vellend et al 2014). Is any fundamental process really stochastic, or does it always have an underlying deterministic origin?…”