Community partitioning into sub-faunas
As predicted by the availability of greater highland evolutionary
opportunity in Hispaniola, the two faunas diverge in community turnover
across elevations. As such, total beta-diversity is greater on
Hispaniola than Jamaica, and Hispaniola’s beta-diversity derives almost
entirely from among-site species turnover (Table 1). Jamaica, in
contrast, shows markedly less turnover, with a substantial portion of
its compositional dissimilarity stemming from gradients in richness and
abundance. This difference is a community-level signature from the
evolution of high-elevation specialists on Hispaniola. Specifically, the
Hispaniolan fauna contains 5 species that are broadly distributed across
the lowlands, 1 that is a specialist on dry lowland regions, and 5 that
maximize their abundance above 700m and do not occur at sea level. By
comparison, Jamaica has 5 species that are distributed across the
lowlands, and only 1 high-elevation specialist.
Hispaniola’s species are partitioned into two entirely distinct
sub-faunas: the lowest and highest communities share no species in
common. In contrast, some of the lowland species in Jamaica are among
those that persist at the highest sites (Fig 3a-c). Interestingly the
elevated beta-diversity of Hispaniola contains a strong phylogenetic
signal linked to historical elevational specialization. The Hispaniolan
highland fauna appears to be derived from a combination of in
situ radiation and evolutionary colonization from lowland derived
species. While two highland species each have a lowland species as their
closest relative, the most abundant highland forest understory species
primarily derive from a single clade, which colonized the highlands
reasonably early in the history of the island, seeming to have initiated
an adaptive radiation in miniature (Fig 3d,e). The presence of this
clade drives the formation of the sub-regional fauna on Hispaniola,
resulting in substantial elevational turnover that is absent from
Jamaica.
Evolution of this highland sub-fauna on Hispaniola also results in
community-level differences in functional trait distribution. Average
morphology on Hispaniola diverges between lowland and highland
communities but remains steady on Jamaica across elevations (Fig 3f).
Different environments in the highlands and lowlands should require
different morphological solutions for optimal fitness. Given ample
evolutionary opportunity, communities in similar environments should
reach the same solution. Indeed, the community abundance-weighted mean
morphology in the lowlands of Jamaica and Hispaniola is extremely
similar (Fig. 3g)—more similar than expected if species morphologies
are randomly shuffled (Fig. 4, P = 0.033). In contrast the highlands are
morphologically distinct, without evidence for convergence between
islands (P = 0.44), suggesting that evolutionary opportunity on
Hispaniola has created a community unique in both diversity and
morphology, that has not been reproduced on Jamaica. As a result,
islandwide average morphologies are not more similar across islands than
expected by chance (Fig. S2, P = 0.076)
Finally, the development of the high-elevation fauna on Hispaniola
appears to have granted access to niche space that would otherwise go
unoccupied by elevational generalists. For example, on Hispaniola above
1800m three species were common and species continued to occupy montane
forests up until 2,300m (Fig 2b,c). But while Jamaica reaches 2,250m at
its peak, we detected no anoles in the forests at or above 1800m
(however, even with no detections at the 4 highest plots over six
two-hour surveys, resight models cannot rule out a small number of
individuals, and so model-predicted species richnesses are non-zero; as
in Fig. 1b).