Understanding how climate change will affect ecological communities is an important yet challenging task, as different species and trophic groups respond differently to temperature. While long-term monitoring data can uncover patterns of abundance and diversity change in communities, it remains unclear what mechanisms are driving these patterns. The climate is not only expected to get warmer in the future, but also the increase in frequency and severity of extreme weather events such as heat waves and drought, are expected to become more common. We asked: how does warming plus increased thermal variation affect the diversity and abundance of predatory aquatic insects? In addition, we wanted to isolate the mechanism driving patterns of community change. To answer the first question, we added four species of dragonfly larvae in equal abundances into aquatic mesocosms stocked with phytoplankton and zooplankton. We used larval dragonflies as they serve as important predators in freshwater ponds and lakes and are also intraguild (IG) predators, meaning they can compete and be predators/prey of other dragonflies. We had three treatments, ambient, warmed (+ 3º C above ambient), and warmed plus increased variation (+ 1º and + 5º C above ambient, switching weekly, mean = + 3º C). Ambient treatments received natural diurnal and seasonal thermal variation at our field site. The other treatments tracked the same variation, however with temperatures heated to the specified level above ambient. Following a 6-month developmental period, we removed all remaining larvae and measured final abundance and species diversity. We will also conduct IGP experiments with pairs of larvae to answer the question of what mechanism is driving the observed patterns of community change with predicted climate change.
Temperature had strong effects on species’ relative abundance, with one species dominating in ambient conditions (Pachydiplax longipennis), one species was completely extirpated (Erythemis collocata), and the other two species were found in very low densities (Plathemis lydia and Libellula luctuosa). The warming and warming plus increased thermal variation treatment yielded almost identical results: coexistence between the four species was facilitated in these treatments, with species’ relative abundance nearly matching those of the initial abundances. Next, we aim to isolate the mechanism driving such patterns in these communities. We predict the species-specific differences in behavior, competitive ability and growth will inform the patterns in species’ abundances at different temperatures. Results of these experiments will address important gaps in how communities will be impacted by climate change.