Convex hulls illustrate habitat filtration in the forests of French Guiana

Background/Question/Methods The degree to which habitat filtering shapes the distribution of functional trait values in ecological communities has been little studied, in part because of a dearth of statistical measures of habitat filtration. In a community, as individuals (and eventually, species) intolerant of its prevailing conditions are eliminated, those that remain should share ecological tolerances. Habitat filtration thus effectively reduces the spread of trait values. Convex hulls are a geometric construct that have been recently been applied to the evaluation of habitat filtration in ecological communities. A convex hull is a multi-dimensional shape in trait space with minimum volume that encloses all species (or individuals) in a community. We employ convex hulls to explore trait value distributions in tropical rain forests of French Guiana. In seven 1-ha plots arrayed across a rainfall gradient, we analyzed almost 4000 individual trees for a range of functional traits, including specific leaf area (SLA), leaf toughness, chlorophyll content, wood and twig density, and diameter at breast height (DBH). Results/Conclusions We show that habitat filtration reduces the volume of trait space occupied by species in observed plots by up to 52%, relative to the mean volume occupied by species in randomly generated plots of equivalent species richness. Significant reductions in trait space volume were observed in the three plots that receive the least annual rainfall. The volume of trait space occupied by individuals was almost four times that occupied by species mean values, indicating substantial intra-specific variation in trait values. Even so, results based upon species mean trait values were largely concordant with those based on individual trait values. Our results suggest that habitat filtration, perhaps imposed by water availability, may significantly shape the distribution of trait values in tropical tree communities.