The impact of variable precipitation on vernal ponds: Community structure, population size, and density dependence

Background/Question/Methods

Climate change scenarios for the Northeastern U.S. predict an increased water budget in the winter/spring and an increased deficit in the summer / fall. Precipitation events are likely to become more variable regardless of water levels, with longer periods of drought followed by more intense deluges. Vernal ponds are temporary habitats whose hydrology is primarily determined by precipitation events. Rapid climate change induced shifts in precipitation are likely to play an important role in how these communities are structured. To experimentally mimic the effects of these climate changes, I established aquatic invertebrate communities in small artificial ponds in a maple-oak forest in Jericho, Vermont. I orthogonally crossed seven levels of mean water depth with seven levels of variance in depth to simulate different precipitation scenarios that might result from climate change. Each mesocosm experienced a unique time series of experimental filling and drying to represent continuous variability in mean water level and rainfall variance. Results/Conclusions I sampled the invertebrate community once a week from May through August 2007 to measure responses to treatments. Ponds were colonized by 13 families of invertebrate taxa, and the most common taxa were the Dipteran families: Chironomidae, Culicidae, and Chaoboridae. At the community level, I calculated total abundance and species density for each replicate at the end of the experiment. To examine population level impacts, I used time-series analysis of the weekly census data to estimate population growth rates for common species, and an index of density dependence (Ives and Dennis 2003). Total abundance decreased for all taxa in response to increased rainfall variance. Abundance increased for all taxa in response to mean water level except Culicidae which had a decrease in abundance. Both Chironmidae and Chaboridae showed increased intrinsic growth rates as mean water lever increased; however, Culicidae had a decreased rate of growth as water level increased. Density dependence responses also differed by taxa. Chironmidae density dependence increased as water level variability increased. Culicidae density dependence increased in response to increasing mean water level. These results add evidence to the current theory that taxa respond differently to the same change in precipitation and other climate change driven variables.