Biogeographic history determines local scale community structure in a hyperdiverse clade of Neotropical plants

Background/Question/Methods

A long-running debate in ecology concerns whether local species composition is decoupled from evolutionary and geographic processes that play out over much larger scales of space and time. Recently, Donoghue (2008 PNAS 105, pp. 11549-11555) an others have argued that it many instances “it is easier to move than to evolve.”  That is, due to the widespread phenomenon of phylogenetic niche conservatism, lineages often migrate to areas and habitats to which they are already adapted rather than native lineages evolving the requisite adaptations in situ. Here, we test three predictions of this hypothesis: i) that community composition in an environmentally heterogeneous locality is nonrandom with respect to the geographic origins of member taxa—history explains community structure, ii) that phylogenetic lineages with histories in distinct geographic regions differ in their evolutionary history with respect to major climatic axes, and iii) that these broad-scale climatic niches translate into habitat affinities at a local scale. In order to test these predictions, we examined the biogeographic history, climatic niche evolution, and local community assembly of members of two hyper-diverse genera: Psychotria and Palicourea (Rubiaceae). Previous results indicate that species that co-occur within 28 m² plots on Barro Colorado Island (BCI), Panama, are more closely related than by chance relative to the BCI species pool (i.e. are phylogenetically clustered).  This pattern appears to result from habitat filtering of phylogenetically conserved hydraulic traits. Do these local patterns reflect the disparate geographic histories of species that currently co-occur on BCI?

Results/Conclusions

Biogeographic reconstruction indicates that 49% of the variation in soil moisture distribution is explained by the geographic regions of origin of the 21 BCI species. Over 50% of species variation in leaf water potential, an important hydraulic trait, is likewise explained by geographic history.  Furthermore, climatic niche models that reflect major axes of climatic variation among 152 Neotropical species correlate strongly with microhabitat preferences with respect to the BCI species.  A comparison of alternative models of climatic niche evolution finds that an evolution model that accounts for ancestral geographic distributions of species outperforms models that are naïve with respect to geographic history. Our results suggest that ancestral climatic niches continue to influence species habitat distributions, and even local community composition, long after dispersal into new geographic regions.  These findings support a view of ecology in which regional and historical factors have a significant influence on local ecological processes such as community assembly at fine spatial scales.