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).