The internal structure of metacommunities
Abstract
Current analyses of metacommunity data largely focus on global attributes across the entire metacommunity, such as mean alpha, beta, and gamma diversity, as well as the partitioning of compositional variation into single estimates of contributions of space and environmental effects and, more recently, possible contributions of species interactions. However, this view neglects the fact that different species and sites in the landscape can vary widely in how they contribute to these metacommunity‐wide attributes. We argue for a new conceptual framework with matched analytics with the goals of studying the complex and interactive relations between process and pattern in metacommunities that is focused on the variation among species and among sites which we call the ‘internal structure' of the metacommunity. To demonstrate how the internal structure could be studied, we create synthetic data using a process‐based colonization–extinction metacommunity model. We then use joint species distribution models to estimate how the contributions of space, environment, and biotic interactions driving metacommunity assembly differ among species and sites. We find that this approach to the internal structure of metacommunities provides useful information about the distinct ways that different species and different sites contribute to metacommunity structure. Although it has limitations, our work points at a more general approach to understand how other possible complexities might affect internal structure and might thus be incorporated into a more cohesive metacommunity theory.
What the paper shows and why it matters
(AI-generated)
Metacommunity ecology asks how communities linked by dispersal are assembled by environment, movement between sites, chance and, more recently, species interactions. Analyses mostly describe the whole system with a few numbers: mean diversity, or one split of the variation in species composition into the contributions of space and environment, and sometimes of interactions. This goes with the assumption, common in metacommunity theory, that these processes act much alike on all species and sites. The problem the paper takes on is that species and sites may differ widely in how the processes act on them, and a single whole-system value would hide this.
The paper argues that metacommunity analyses should also examine the “internal structure” of a metacommunity: how individual species and individual sites differ in the contributions of environment, space and co-distribution (the tendency of species to occur together or apart beyond what environment and space explain). To show this can be done, the study simulates metacommunities with a process-based colonisation–extinction model, in which the true processes are known, and analyses them with a joint species distribution model, which fits the occurrences of all species together and splits the explained variation into these three parts. Broken down by species and by site, the split differs with a species’ dispersal and environmental optimum, and with how distinct a site’s environment is from the average across sites. The co-distribution part does not simply recover the simulated interactions. The paper judges it likely to absorb any process that environment and space capture poorly, such as unmeasured environmental factors, and names separating these from true interactions a major task for future work.
The paper reads the results as showing that a species-level and site-level view yields more nuanced information than whole-system summaries. It argues that including co-distribution avoids lumping interactions and other processes into residual variation, which may have exaggerated the apparent role of chance. It presents the work as pointing to a more general approach that might be incorporated into a more cohesive metacommunity theory, and as possibly useful in conservation, where particular species or sites are often the focus. In its view, joint species distribution models are one way to estimate internal structure; other methods might serve as well.