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).