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.