Local abundance and richness across islands
We first assessed whether total community size depended on
macroevolutionary diversity or was instead set by local environmental
conditions. Abundances within local communities varied substantially,
but community size declined monotonically on both islands as elevation
increased (Fig. 2a; best model: Elevation only, Elevation term: P
< 0.001; marginal R2 = 0.57). Despite the
nearly 2-fold difference in gamma diversity between faunas (11 in
northern Hispaniola versus 6 in Jamaica), local communities at a given
elevation possessed a statistically indistinguishable number ofAnolis individuals (Island term: P = 0.70), and local
abundance declined with elevation at similar rates
(Island*Elevation interaction P = 0.11).
But while abundance patterns are similar across islands, the way that
diversity is partitioned over the elevational gradient is not equivalent
(Island*Elevation interaction P < 0.001,Elevation2 term P < 0.001, marginal
R2 = 0.86). As predicted by the evolutionary
opportunity model, Jamaica and Hispaniola have equivalent species
richness in lowland communities, where opportunity is substantial. But
above the lowlands the trajectory diverges: Jamaica displays a steady
loss of species with elevation, while Hispaniola contains a
mid-elevation hump, followed by a slow decline. Notably, above the
lowlands (>500m) communities are consistently richer on
Hispaniola than Jamaica (Fig. 2b).