Diversification strengthens local controls on community structure
Our results reveal a complex signature of macroevolutionary processes on local community structure, supporting a model in which diversification both strengthens local controls on community diversity within biomes, and elevates local diversity at biome transition zones where sub-faunas abut and mix. Critically, our data suggest that factors promoting the evolution of sub-regional ecological specialists (e.g. Zobel et al. 2011; Jetz & Fine 2012) may be key to determining whether communities achieve local diversity controls at the species level.
Surveying lizard communities on neighboring islands that differ in evolutionary richness but otherwise contain similar environments, we uncovered several patterns predicted if macroevolutionary inputs facilitate the emergence of local diversity controls. First, total community abundances strongly suggest local controls, declining similarly with elevation on Jamaica and Hispaniola, despite a two-fold difference in evolutionary diversity between faunas. Total community sizes on both islands thus appear limited by individual-level competition—most likely for dietary resources (i.e., energy). Interestingly, these patterns revealed little support for macroevolution raising local carrying capacities (e.g., by producing more efficient specialists; Cornell 2013; Storch & Okie 2019), although the slightly greater abundance of Hispaniolan anoles at the highest elevations may represent weak evidence for such an effect.
Second, we found that local patterns of species diversity were similar between Jamaica and Hispaniola where both islands provide ample opportunities for the evolution of endemic habitat specialists (the lowlands), but diverged in areas where opportunities differ starkly (the highlands). In the lowlands, average species diversity and average community morphology were nearly identical between islands despite the larger area and greater regional diversity of Hispaniola. Such similarity suggests that local limits on species diversity have been reached in the lowland tropical forest habitats that predominate both islands (Cornell 1999; Ricklefs 2006). However, community structure diverged in the highlands, with local species richness on Hispaniola exceeding that on Jamaica at elevations greater than ~700m (Fig. 2b). Hispaniolan highland communities were also phenotypically distinctive, occupying a region of morphospace that differs from all sub-regions on either island. In contrast, the average morphology of Jamaica’s highland communities is indistinguishable from the lowlands, in part reflecting the fact that the highlands are largely composed of elevational generalist species (Fig. 3c,g; Table 1). This pronounced highland diversity difference suggests that Jamaica’s high-elevation anole communities are undersaturated at the species level (Cornell 1999; Mateo et al. 2017), despite having similar abundances to those on Hispaniola.
Finally, as predicted if macroevolutionary inputs strengthen local controls (Cornell 2013), the greater evolutionary diversity of anole species on Hispaniola was associated with greater compositional turnover among local communities (Table 1). This pattern was linked to the evolution of distinctive elevational sub-faunas on Hispaniola, but not Jamaica (Fig. 3).