Regional contributions to diversity at biological suture zones
Even if communities are effectively saturated with individual organisms, species richness may still be augmented by regional diversity where processes that remove species with redundant niches (e.g. competitive exclusion) are slow relative to those that add them (e.g. immigration from nearby areas) (Cornell & Harrison 2014). We see no evidence for such regional signatures in the lowlands on either island, or above >1200m on Hispaniola. However, the evolution of distinct elevational sub-faunas on Hispaniola is accompanied by a mid-elevation diversity peak where low-elevation and high-elevation faunas meet, consistent with an ecotone-associated dynamic equilibrium (Storch & Okie 2019). Middle elevations may be especially susceptible to mixing if fitness differences equilibrate between lowland and highland-adapted species with redundant dietary or structural niches, preventing clear competitive dominance (Chesson 2000). Although hypothesized to be common (Lomolino 2001), such mid-elevation richness peaks appear infrequently in reptiles (McCain 2010), and maximum diversity only rarely occurs at transition zones between reptile faunas (McCain & Beck 2016).
Our results suggest two potential reasons. First, faunal mixing can only occur where evolutionary opportunities allow faunas at either end of an environmental gradient to have sufficient species to result in richness increases where faunas meet. On Jamaica, a highland sub-fauna of one is insufficient to engender a middle-elevation diversity peak, given that several lowland species have already been lost. Our data thus point to a key role for evolutionary opportunity in determining whether an elevational gradient is likely to exhibit a middle-elevation peak in alpha-diversity (e.g., Hispaniola) or not (Jamaica). Indeed, deletion of highland endemics, which are much more diverse on opportunity-rich Hispaniola, obliterates the middle-elevation diversity peak on that island, and creates a middle-elevation abundance deficit, just as expected if highland and lowland species co-occurring at the ecotone are competing for limited resources in communities saturated at the individual level (MacArthur et al. 1972).
Second, the emergence of local diversity peaks at sub-region boundaries will depend on the dispersal abilities of community members. Anoles are relatively poor dispersers (Williams 1969), so mass and rescue effects that facilitate persistence in unfavorable conditions should be minimal. As such, the mid-elevation diversity peak on Hispaniola is fairly narrow. Ecotone peaks would be wider for stronger dispersers, and in the extreme case could encompass an entire environmental gradient, supplementing alpha-diversity island-wide.