Species and genetic diversity are minimally correlated across a highly fragmented landscape and largely independent of habitat patch size
Relative habitat patch size influences both species diversity and genetic diversity, and consequently, these measures of diversity are expected to correlate across environmental landscapes. Small, isolated patches are expected to have reduced species diversity relative to larger expanses of habitat because they are likely to support viable populations of a smaller number of species, to have less resource heterogeneity and fewer available niches, to be more degraded due to increased exposure to external pressures, and as a simple consequence of passive sampling. Small, isolated patches are also expected to have reduced genetic diversity. Because of decreased resource availability, the carrying capacity for individual species will tend to be lower, resulting in smaller populations. With limited inter-patch dispersal and gene flow, genetic diversity will be lost through drift and inbreeding, which can lead to further reductions in population size and additional declines in genetic diversity. We assessed whether species and genetic diversity were correlated across seven Afrotropical forest remnants that ranged from 6 to 5000 hectares. Specifically, the diversity of fruit-feeding butterfly communities (115 total species from 8164 individuals) was compared to genetic diversity (710 bp region of mtDNA COI) in three butterfly species with different relative sensitivities to forest fragmentation.
We found minimal evidence of an association between patch size and measures of diversity, and between species and genetic diversity. Although rarefied species richness tended to be significantly higher in larger fragments, this relationship was only weakly supported. Neither total estimated richness nor Simpson’s diversity were related to fragment size. All measures of genetic diversity for all three species were also independent of patch area. Notably, genetic diversity was highest for the species most impacted by fragmentation (as measured by genetic substructuring) and lowest for the species not affected. Positive correlations between measures of species and genetic diversity were present only for the species showing genetic substructuring. Most of these, however, involved either observed haplotype or observed species richness, which don’t account for sample size differences across sites. All were weak and all disappeared when a single specific site was removed from the analysis. Mantel tests of pairwise community and genetic dissimilarity matrices were all non significant. Our results indicate that diversity of very small habitat patches can rival that found in dramatically larger fragments. They also suggest the processes shaping different measures of diversity across landscapes are complex and one measure may be a poor proxy for another.