Abstract :
[en] The distribution of species is the result of past and present processes that shape the
assemblage of their communities. Niche differentiation and competition are the two
main ingredients of a trade-off between selection of traits for a given environment,
enhancing fitness in the latter, and competition among closely related species. Darwin
was one of the first to hypothesize a connection between niche differentiation and
competition and species relatedness, offering an appealing framework to disentangle
community assembly processes based on phylogenetic diversity patterns. This
framework was later formalized by Chesson (2000) who explains species coexistence by
two types of fundamental processes. Equalizing processes minimize fitness differences
between species, so that coexisting species tend to share similar functional traits in a
given habitat. In turn, stabilizing processes stabilize coexistence via negative density
dependent selection, so that coexisting species tend to have dissimilar functional traits
to avoid competition. Equalizing processes are thus expected, if adaptive traits are
phylogenetically heritable, to generate communities with species more phylogenetically
related to each other than expected by chance, a pattern known as phylogenetic
clustering. Stabilizing processes, conversely, lead to the assemblage of communities
with species less phylogenetically related to each other than expected by chance, a
pattern known as phylogenetic overdispersion. The signature that assembly
mechanisms leave in community phylogenetic structure has been used to infer
community assembly mechanisms from patterns of phylogenetic diversity. Community
assembly is, however, the result of a mixture of several processes, including potentially
confounding factors associated with dispersal limitations and spatial effects, casting
doubt about the application of phylogenetic diversity metrics to infer community
assembly processes.
Here, we re-assess the extent to which phylogenetic diversity can indeed be used as a
proxy for mechanisms of community assembly. We implemented a novel, highly
parametrizable and customizable spatially explicit model involving limited dispersal,
drift, trait-based selection, and competition, to simulate community assembly under
competing processes in a landscape with contrasted habitat connectivity. We
subsequently implemented this approach to infer the evolutionary mechanisms
underlaying one of the most pervasive biodiversity patterns, namely the latitudinal
diversity gradient, using liverworts, a group of early land plants comprised of about 7000
species, as a model.
