Geographic scaling of diversity
Species richness across the entire landscape increased with the
cumulative amount of habitat (as all patches were sequentially summed;
Fig. 5A). However, small patches contributed disproportionately to this
increase because they typically added distinct sets of species to the
regional pool (high beta-diversity; steep increase in the left region of
Fig. 5A and 5B). Landscape diversity resulted mostly from heterogeneity
in species composition among patches, rather than from the local
diversity observed within individual patches (Fig. 5B-D; compare blue
and green circles in Fig. 5D). Although local diversity in small
fragments was lower than that in larger fragments (Fig. 5B), small
patches greatly differed in composition and added new species to the
regional pool (Fig. 5C). Beta-diversity rapidly increased with
additional habitat at low values of overall habitat amount in the
landscape (Fig. 5C). When the number of species added by beta-diversity
reached an asymptote, additional species could still be added to the
landscape by adding large fragments with high local-diversity (Fig. 5A).
However, this increase was relatively small compared to gains due to the
beta-diversity among small fragments. Beta-diversity was therefore the
main component of regional diversity in virtually all possible scenarios
(Fig. 5D).
When comparing the SAR with the expectation based on an unitary
community, we estimated an R value of 3.44, indicating that smaller
patches contributed a significantly higher number of species to the
regional diversity than what would be expected based on area alone.