CANCELLED - On the razor’s edge: The importance of seasonal drying to the endangered Hine’s emerald dragonfly (Somatochlora hineana)

Background/Question/Methods

The hydroperiod of wetlands and ephemeral streams is a critical determinant of community structure in these ecosystems. Larvae of the endangered Hine’s emerald dragonfly (Somatochlora hineana) occupy shallow seasonally-intermittent flowages and streamlets fed by a combination of surface and groundwater. Larvae are slow-growing, requiring 4-5 years to reach adulthood and survive dry periods by using crayfish burrows as refugia. While burrows provide damp environments, they also expose larvae to predation risk from crayfish and they generally contain few prey items. Seasonal drying appears to be a normal feature of good habitat for S. hineana, perhaps because it removes many competitors and predators. However, extreme drought events may also have negative impacts on this species. Using field data collected over a ten year period, we evaluate the influence of seasonal drying and drought on population size and age structure in S. hineana populations at sites in Wisconsin and Illinois. Populations were sampled using a combination of netting and crayfish burrow pumping methods. Information on drought events was obtained from nearby weather stations. Where possible, the beginning date and extent of the summer dry period was determined through direct observation or by using temperature loggers.

Results/Conclusions

Results show a correlation between early dry dates and lower S hineana larval population density in the following year.  Although the systems are groundwater fed, the rainfall data also correlated with the date the system dried, indicative of systems with shallow aquifers and a short groundwater residence time. Age distribution of populations was also altered following severe drying events, with younger larvae in their first year being more susceptible to drought than older larvae (>Age 2+). The increased mortality and age specific impacts may be linked to extended time spent in burrows, where larvae are exposed to increased predation risk from crayfish, starvation, and increased risk of cannibalism. These finding indicate the sensitivity of this endangered species to changes in rainfall pattern or to modification of groundwater hydrology that significantly alter the timing of drying events. “Groundwater dependent ecosystems” are often considered to be buffered from relatively short-term changes in rainfall patterns. Such generalizations assume extensive aquifers and long residence times for groundwater.   Our findings with S. hineana clearly indicate the need for a better integration of groundwater-surface water dynamics in the conservation of wetland ecosystems.