Habitat fragmentation predicts reduced abundance of a flightless planthopper but does not erode gene flow among fragments

Background/Question/Methods

Biodiversity in coastal salt marsh ecosystems is threatened by the displacement of the flood intolerant high elevation marsh grass, Spartina patens, with the flood tolerant low elevation marsh grass, Spartina alterniflora. Rising seas restrict S. patens, once occupying vast fields as large as twenty hectares, to increasingly small patches, some as small as a few square meters. We asked how Tumidagena minuta, a flightless, brachypterous planthopper and specialist herbivore of S. patens, will respond to this habitat fragmentation. Because T. minuta does not feed on the S. alterniflora matrix and because it is expected to have low dispersal ability, we expected to find a negative population response to decreasing patch size, lower genetic diversity in smaller patches, and high genetic differentiation among patches. We sampled T. minuta populations from 13 S. patens patches varying in size from ~17 m2 to ~8,500 m2 and with varied geographic dispersion (66 m to 2,005 m to nearest the neighboring patch). We estimated the census size of each patch by counting the number of collected insects and adjusting for sampling effort. We used 9 polymorphic microsatellite markers to measure allelic diversity and estimate genetic differentiation using global and pairwise RST.

Results/Conclusions

We found that patch size strongly predicted T. minuta abundance, with larger patches having more individuals. The microsatellite loci that were used exhibited between 5 and 17 alleles per locus. The nine locus estimate of RST for all patches was 0.0038, indicating very little population structure. When we tested the effect of distance between patches on pairwise RST using a Mantel test, we found that distance had no effect on genetic differentiation among patches. We also found no differences in allelic richness among patches. These results suggest that there is widespread gene flow among patches, having a homogenizing effect on allelic diversity and composition. This is surprising considering that we found that >99% of the individuals in our samples were brachypterous and because some patches were over 2 km apart. This pattern could suggest an alternative dispersal mechanism for T. minuta. These results indicate that T. minuta will decline with its host plant, but that immediate declines may be offset by migration from other patches.