Understanding the role of stochasticity in metacommunity assembly under global changes

Abstract

Understanding the organization of biodiversity on Earth has puzzled ecologists and biogeographers for centuries. For a long time, only part of these patterns around the globe was explained by deterministic processes related to environmental filters related to functional traits and biotic interactions, while much of the variation that disobeyed these rules were termed to be rather stochastic (i.e. unpredictable, unknown cause). Improving our understanding of this stochastic variation in community taxonomic, functional and trophic composition is crucial to forecast how nature will respond to global changes and how this will affect ecosystem services for society. In the last decades we discovered that stochasticity emerges from demographic processes (birth, reproduction, dispersal); biological events that are inherently variable at the individual level and therefore unpredictable at the community level. Recently, however, ecologists invested a large effort to describe the factors and processes that could enhance or undermine the effect of demographic processes in biodiversity patterns. For example, population size is important as the dynamics of small populations are relatively more impacted by stochastic demographic events. Also, communities under higher temperatures with enhanced metabolism are thought to be more stochastic as damages accumulate faster (more oxidative reactions, telomere shortening, and mutations) in all organisms increasing death rates independently of species identities and reducing their ecological differences. In riverine ecosystems, stochasticity should come from multiple sources and vary at multiple spatial scales. At the riverine scale, we predict that headwaters and mainstems should sustain populations with different sizes and dynamics owing to their differences in heterogeneity and habitat size, impacting how demographic events, such as individual dispersal and recolonization scale up to patterns of community composition. At the global scale, differences in climate and seasonality should drive variations in individual metabolism and population life cycles with enhanced stochasticity in warmer and more unpredictable environments. This multiscale variation in the preponderance of stochastic processes should impact the functioning of ecosystems through changes in trophic interactions mediated by species traits and organisms' body size. In order to test these predictions, we will conduct a novel coordinated in-situ spatio-temporal study involving tropical and temperate riverine ecosystems. Our aim is to quantify and understand the relative importance of different sources of stochasticity during community assembly and how this would impact functional aspects of trophic interactions and energetic flows. We will generalize our findings by parameterizing a Beverton-Holt model with empirical data from the field. By analyzing the results of both the field and models we expect to advance our understanding of the factors and processes behind stochasticity during metacommunity assembly that ultimately entails our global patterns of biodiversity. (AU)