Forest metacommunity structure along an elevational gradient in the Luquillo Mountains, Puerto Rico (LUQ)

Metacommunity theory provides a rich framework for identifying structure along environmental gradients and for evaluating the relative contributions of local and regional processes in molding biological communities. We used a novel suite of analyses to comprehensively evaluate metacommunity structure of forests sampled along two elevational transects (0.1 ha plots at 50 m intervals from ~300 to ~ 1000 m.a.s.l) in the Luquillo Mountains (LM) of Puerto Rico. We quantified three elements of metacommunity structure (coherence, species range turnover, and range boundary clumping); for transects with significantly clumped boundaries, we used the multiplicative model for partitioning diversity to determine the number of compartments (i.e., a group of sites with similar species composition that is distinct from other such groups of sites) in each transect; used cluster analysis to determine the location and extent of compartments in each transect; and identified species that were key indicators of each compartment. Each transect exhibited Clementsian structure with the analyses identifying three or four distinct species compartments along the elevational gradient. Despite the difference in the number of compartments, the elevation of many compartment boundaries were highly consistent between transects. A total of 47 species (40%) were significant indicators of particular compartments for at least one transect; however, the identities of indicator species were not highly consistent between them. High elevation compartments characteristically contained many endemics to the LM or to the island of Puerto Rico, with significant implications for the relative importance of historical processes in molding metacommunity structure.  Sampled transects in the LM differed in species richness and composition, identity of indicator species, history of disturbance, aspect, wind exposure, number of endemic species, and elevational extent.  Nonetheless, their metacommunity structures were quite similar, suggesting that mechanisms affecting the structure of montane vegetation may be resilient to variation in natural processes as well as to anthropogenic disturbances.