A dimmer shade of pale: revealing the faint signature of local assembly processes on the structure of strongly filtered plant communities

Abstract

Trait-based ecology suggests that abiotic filtering is the main mechanism structuring the regional species pool in different subsets of habitat-specific species. At more local spatial scales, other ecological processes may add on giving rise to complex patterns of functional diversity (FD). Understanding how assembly processes operating on the habitat-specific species pools produce the locally observed plant assemblages is an ongoing challenge. Here, we evaluated the importance of different processes to community assembly in an alpine fellfield, assessing its effects on local plant trait FD. Using classical randomization tests and linear mixed models, we compared the observed FD with expectations from three null models that hierarchically incorporate additional assembly constraints: stochastic null models (random assembly), independence null models (each species responding individual and independently to abiotic environment), and co-occurrence null models (species responding to environmental variation and to the presence of other species). We sampled species composition in 115 quadrats across 24 locations in the central Pyrenees (Spain) that differed in soil conditions, solar radiation and elevation. Overall, the classical randomization tests were unable to find differences between the observed and expected functional patterns, suggesting that the strong abiotic filters that sort out the flora of extreme regional environments blur any signal of other local processes. However, our approach based on linear mixed models revealed the signature of different ecological processes. In the case of seed mass and leaf thickness, observed FD significantly deviated from the expectations of the stochastic model, suggesting that fine-scale abiotic filtering and facilitation can be behind these patterns. Our study highlights how the hierarchical incorporation of ecological additional constraints may shed light on the dim signal left by local assembly processes in alpine environments.

Spatial Scale Dependence of Ecological Factors That Regulate Functional and Phylogenetic Assembly in a Mediterranean High Mountain Grassland

“…In addition, phylogeny seems to be involved in assembly processes because it summarizes the evolutionary history of the local species pool (Losos, 1996;Webb et al, 2002;Cavender-Bares et al, 2009;Kraft and Ackerly, 2010;Mouquet et al, 2012;Gerhold et al, 2015), and it is often used as a proxy of current functional information as it is assumed that evolutionary diversification has generated predictable trait diversification (Flynn et al, 2011). Thus, comparisons of the functional (and/or phylogenetic) diversity of species assemblages and those expected from null assemblages have been performed routinely during the last two decades to identify the assembly processes that structure plant communities at different spatial scales (Díaz et al, 1998;Cavender-Bares et al, 2006;Kraft and Ackerly, 2010;Chalmandrier et al, 2017;López-Angulo et al, 2021).…”

Aridity drives the loss of dung beetle taxonomic and functional diversity in three contrasting deserts

“…Community assembly could be driven by three main processes at the scale of our study: (1) environmental filtering along the strong aridity gradients of the three deserts; (2) biotic interactions; and (3) stochastic processes (Götzenberger et al, 2012). In our null model analysis, the random model shows stronger trait convergence than the other two suggesting that dung beetle community assembly along aridity gradients is mainly driven by environmental conditions (see López-Angulo et al, 2020). This finding provides support for the idea that limitations in water availability underlie trait convergence in communities inhabiting arid environments (Conenna et al, 2021;Stomeo et al, 2013).…”

“…This method ensured that observed and simulated communities had identical taxonomic richness and abundance distributions. For each desert, the randomization of the species pool was performed by considering hierarchical assembly constraints (de Bello et al, 2012; López‐Angulo et al, 2020). Thus, we used: (1) random null models in which all plots are equally accessible for any species (classical randomization test); (2) an environmental null model for which the accessibility of a plot depends on the response of each species to aridity; and (3) a co‐occurrence null model in which species occurrences depend on the presence of other species in the assembly.…”

“… Co‐occurrence null model . Probabilities of species occurrence in a plot were estimated using Beals smoothing (Beals, 1984; De Cáceres & Legendre, 2008; López‐Angulo et al, 2020). The ‘Beals’ function in the ‘vegan’ package (Oksanen et al, 2010) estimated the probability of occurrence of each species accounting for the known occurrences of the rest of the species.…”

“…This is, certainly, the case for aridity gradients where responses to both the extreme abiotic environment and biotic interactions could promote either trait convergence (e.g., Conenna et al, 2021), even distribution of traits (see Maestre et al, 2021) or opposing patterns that neutralize each other, leading to practical and methodological limitations for separating factors driving different changes in traits (de Bello et al, 2012; Götzenberger et al, 2012). Disentangling effects may be assisted by new analytical approaches comprising novel frameworks, based on null models, which aim to incorporate hierarchical constraints to test if trait convergence/divergence is caused by abiotic, biotic, or stochastic processes (de Bello et al, 2012; López‐Angulo et al, 2020). This approach provides a useful tool to study the relative importance of differences in assembly rules along any environmental gradient and, thus, assist in understanding the drivers of diversity patterns.…”