Evolutionary opportunity and the assembly of faunas and
communities
Community abundance patterns suggest at least broadly comparable
resources for anoles in tropical montane forests on Hispaniola and
Jamaica. So why are Jamaica’s montane communities less diverse?
Ecological explanations such as colonization limitation are
unlikely—all Jamaican anole species not found in highland communities
are otherwise distributed islandwide, and no obvious barriers prevent
colonization. Instead, diversity differences are best explained by the
relative paucity of high-elevation specialists in the species pool of
Jamaica – indeed, computationally “deleting” the endemic highland
subfaunas of each island reconciles their divergent relationships
between local species richness and elevation (Fig. 4). Thus, to
understand Jamaica and Hispaniola’s highland community differences, we
must turn to the evolutionary buildup of their species pools (Jetz &
Fine 2012; Cornell 2013; Mittelbach & Schemske 2015).
Speciation-driven contributions to highland diversity could arise either
by evolutionary colonization, in which lowland-derived lineages evolve
to tolerate or specialize on cool upland conditions, or by in
situ radiation from a highland ancestor (Ricklefs 2006; Rosindell &
Phillimore 2011). The first mechanism has occurred on both islands, but
appears difficult, with only one instance on Jamaica and a just a
handful in northern Hispaniola. Indeed, evolutionary transitions of this
sort appear to be rare, with one study estimating that speciation across
biomes is ~25 times less likely than within them (Crispet al. 2009). The second mechanism, in situ highland
radiation, occurred exclusively on Hispaniola, where a clade of
highland-endemic anoles diversified into a variety of ecological types
(Figure 3e), raising the taxonomic, functional, and phylogenetic
uniqueness of the Hispaniolan highlands. The smaller cumulative area of
Jamaica’s highlands likely precluded enrichment of its montane fauna via
this mechanism. Allopatric separation of populations is likely a
pre-requisite for speciation in anoles (Glor et al. 2004, 2005),
and anoles likely need at least 3000km2 to undergoin situ speciation (Losos & Schluter 2000). The highland “sky
island” in northern Hispaniola above 700m clears this threshold with
4000km2 (Fig. 1), yet Jamaica’s equivalent zone
occupies less than 400km2. Thus the rarity of
cross-biome speciation events, plus insufficient space for subsequentin situ radiation, appear to have deprived highland Jamaica from
the 2-3 additional species supported locally at similar elevations on
Hispaniola.
In contrast to the highlands, the lowlands on both islands are large,
well over the 3000km2 threshold. Even though the
lowlands of northern Hispaniola are nearly 5X larger than in Jamaica
(18,500 versus 3,900km2), alpha-diversity in the
lowlands is identical, suggesting that local diversity caps predominate,
and have been reached. Northern Hispaniola’s greater total diversity
thus appears to come about not because it is larger per se , but
because its high-elevation biome is large enough to generate an
additional set of distinct species, boosting diversity from 6 species
(lowland only) to 11 (total).
A growing body of recent work has revealed the importance of
geographical and environmental determinants of evolutionary radiation in
setting regional diversity patterns (Fine & Ree 2006; Jetz & Fine
2012; Quintero & Jetz 2018). Our work shows that such “evolutionary
opportunities” can also structure communities by producing species
capable of maintaining local diversity equilibria. Such equilibria are
consistent with evidence from temporal studies of local-scale diversity,
which provide evidence for stable community-level richness limits over
timescales ranging from decades to tens of millions of years (Bambach
1977; Brown et al. 2001; Gotelli et al. 2017; Closeet al. 2019). However, they contrast with local diversity
patterns in Hawaiian trees, one of very few other groups to be
investigated for signatures of macroevolution on local community
structure (Craven et al. 2019). In that system, greater
island-level evolutionary diversity was linked to increased local-scale
diversity, suggesting that local diversity caps in that system either
have not yet been reached, or are more easily overridden. Understanding
this difference will require further research, and we suggest that the
likelihood that evolution will strengthen local controls will depend on
potentially system-specific mechanisms of dispersal, competition, and
coexistence. For example, compared to tropical trees, anoles are poorer
dispersers (Williams 1969), more likely to engage in interference
competition (Schoener 1977; Culbertson & Herrmann 2019), and, lacking
dormancy, less likely to benefit from temporal storage effects (Chesson
& Warner 1981), all of which could favor the primacy of local controls
in anole communities.