Consequences of coexistence dynamics in the structure of communities across spatial scales

Abstract

Community ecology has long been chasing the opportunity to untangle the mystery that allows several species to live in a shared environment. Most traditional frameworks follow a sequential path: abiotic filters (such as climate, soil chemistry, and disturbance regimes) govern at broad geographical scales, deciding which species can persist in a region, and generating a “pool” of potential colonizers. After this environmental screening, biotic interactions (i.e., competition, facilitation, predation, or mutualism) can steer the wheel, molding local community composition. This sequential path divides the abiotic and biotic drivers of coexistence into their respective scales, offering what it seems to be a seemingly straightforward explanation for species assembly. However, as we gather empirical evidence from ecological studies, this rigid dichotomy does not seem to be how nature leads the way. More precisely, neither abiotic nor biotic drivers act isolated; they rather operate through dynamic, interdependent processes that interact across scales. For instance, heterogeneity at finer scales (caused by variations in microtopography, canopy structure, or soil moisture gradients) generates considerable abiotic contrasts within small areas, which can alter competitive hierarchies or shift facilitation networks. Indeed, a depression in a dune present in a coastal habitat is able to trap moisture, which in turn enables drought-sensitive species to persist alongside arid-adapted neighbors. Similarly, a nurse shrub’s shade could buffer seedlings from extreme temperatures, rewriting the limits imposed by abiotic factors at a local level. Conversely, biotic interactions can cascade their effects upwards altering the pattern created by abiotic filters at larger spatial scales. For instance, species can be excluded from entire landscapes by more dominant competitors, obligate mutualism can constrain species’ geographic ranges, or trophic cascades can reshape vegetation structure over extents of kilometers. The present manuscript challenges the traditional compartmentalization of abiotic and biotic processes by deconstructing their influence in order to highlight their interplay across spatial and temporal scales. Drawing on Hutchinson’s conception of n-dimensional niche, we integrate Grinnellian (abiotic) and Eltonian (biotic) perspectives to reveal how these forces are able to shape natural communities. As we bridge macroecological and local assembly processes, this thesis advances to a more nuanced understanding of coexistence, one where abiotic and biotic drivers are indistinguishably linked, and their relative importance shifts across contexts.