Human-mediated species invasions are excellent real-time experiments to assess community assembly. These recent invasions are considered accurate analogues of ancient colonizations, which contribute to today’s natural species communities. Whether this is a correct assumption, however, had until now not been sufficiently tested.
A new paper by Steven Poe available online in the American Naturalist examines this point. His results are very exciting for many people working on anoles, and everyone interested in species invasions and community assembly. Therefore, I present a summary of his findings here.
Previous studies have shown that recently naturalized species are morphologically similar to ancient colonizers, indicating that processes shaping biotic communities could be the same in both situations. However, the unnatural (i.e., human-mediated) mode of dispersal responsible for assembling modern communities and the frequent establishment of animals in urban areas may result in unique species combinations that are not normally present in nature. Poe examined this proposition by comparing natural and nonnative two-species communities of anoles based on morphology and phylogenetic structure.
The results show that the morphological differences among the species in natural communities are not significantly lower or higher than those in naturalized species pairs. Furthermore, the anole species in natural and nonnative communities are morphologically indistinguishable; they have unusually high colonization scores and all morphological trait comparisons of species from naturalized communities versus natural communities are nonsignificant. However, natural species communities show phylogenetic clustering, whereas communities that consist of naturalized species show no clear phylogenetic structure. This is likely due to the size of the colonization pools for both community types (i.e. natural colonizations originate from proximate sources, making them more phylogenetically related).
The phylogenetic differences and morphological similarities among community types clearly indicate morphotype convergence across study areas, and suggest that the formation of natural and nonnative communities may be at least partially governed by the same processes. Similarity among species can be either explained by environmental filtering or competition. The first is supported by the fact that species in both community types show high colonization scores, indicating that the all habitats studied here could be especially suitable for a particular colonization type of anole. Competition is supported by the fact that none of the 35 species pairs include two of the same ecomorph. This pattern could be accounted for by the mechanism of competitive exclusion; i.e., resistance against species that utilize their surroundings in a similar manner.
The morphological similarities among species in the same community don’t reflect the highly significant trend of ecomorph repulsion in those species-pairs. This could indicate that traits not studied here could better reflect ecological differences between ecomorphs, and when included possibly increase intra-community differences in morphology. Furthermore, only two-species communities are studied here, minimizing the diversifying effect of adaptive radiation in anole communities, a mechanism that has repeatedly been denoted as major driver of anole evolution. Therefore, this study indicates that environmental filtering of good colonizers and varying kinds of competition may be particularly important in the initial stages of community assembly. Finally, although human influence is often looked upon as non-natural, destructive and irreversible, Poe’s results imply the existence of general assembly rules apart from human influence that result in species communities that are morphologically similar regardless of when they were formed.
I find this article very insightful as it tests a major assumption in the use of recent invasions for historic community assembly studies. The results are clearly discussed and placed in the current paradigm of community assembly theory. However, I found reading the discussion in some cases counter-intuitive, because the results only feature a table of community differences for every morphological trait, whereas the raw trait values per community are also discussed, but never illustrated. All in all, I think you will enjoy reading this new publication, as I certainly did!
Steven Poe (2014) Comparison of Natural and Nonnative Two-Species Communities of Anolis Lizards. The American Naturalist 184, 1: 000-000.