Genetic diversity and persistence of mayfly populations in disturbed headwater streams

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Movements of individuals shape the spatial structure of populations and play an important role in their persistence. For aquatic insects with winged adult stages, properties of the terrestrial landscape influence in-stream habitat quality and, in naturally patchy habitats such as dendritic stream networks, connectivity among habitat patches. Connectivity here refers to the population dynamics dependent on migration and gene flow among insect populations in semi-isolated stream segments. When populations are spatially connected, effects of local disturbance (e.g., habitat loss or degradation) can have a ripple effect, ultimately altering regional processes that reflect back to the local patch. But since regional and local population dynamics occur at different rates, detrimental effects of local disturbance are often not detected by biomonitoring efforts at the patch level until they have rippled through regional processes, by which time large-scale population extinction risk may have become unacceptably high.

My dissertation examines the effect of local and regional disturbance on the population density, genetic structure, genetic diversity, and persistence of mayfly populations living in forested and deforested headwater streams in the Central Piedmont region of Maryland and Virginia. I sampled populations of the mayfly Ephemerella invaria (Walker) in 24 first-order streams across 9 headwater stream networks. The sampling period (2001-2004) spanned a regional drought during which some of the streams went dry. Thus I was able to look at the interaction of local deforestation and stochastic regional disturbance in my study system. In summary, my results indicate that in these mayfly populations:

  1. Historically, long-range dispersal of Ephemerella occurred at levels sufficient to maintain gene flow across major watersheds, indicating excellent passive or active dispersal capability in these insects.

  2. Deforestation of small watersheds decreases the rate of stream re-colonization and the recovery of prior population densities following a major disturbance.

  3. Deforestation is correlated with loss of population genetic diversity.

  4. Highly differentiated migrants represent a disproportionate share of the diversity in some mayfly populations.

  5. Stochastic regional disturbance (e.g., drought) interacting with local disturbance (e.g., small scale watershed deforestation) can increase population extinction risk.