Does intraguild predation influence the distribution of the endangered Hine’s emerald dragonfly (Somatochlora hineana)?

Background/Question/Methods
Predation, especially intraguild predation, is often considered a strong force in determining species distributions. Larvae of the endangered Hine’s emerald dragonfly primarily live in shallow seasonally-intermittent flowages and streamlets fed by a combination of surface and groundwater. They are slow growing, requiring 4-5 years to reach adulthood. In most places where Somatochlora hineana larvae are found there is a virtual absence of other dragonfly species. This is true even when these areas are connected to more permanent water bodies that support numerous other species (e.g. Cordulegaster obliqua and Aeschna spp.). To evaluate whether S. hineana habitat use is limited by competition with, or intraguild predation by, other faster growing and aggressive dragonflies, we used a combination of field sampling and experiments. Populations of S. hineana and other species were sampled over 2 seasons at sites in Illinois and Wisconsin. At 4 sites, dragonfly larvae were placed individually in predator exclusion cages in streamlets and more permanent wetlands, and growth rates were compared between habitat types. A series of laboratory experiments also evaluated the impact of other dragonflies on feeding and activity of S. hineana.
Results/Conclusions
As expected we found S. hineana larvae in the seasonally drying streamlets. However, we also found significant numbers in some of the more permanent stream habitats. Other dragonfly species were found almost exclusively in more permanent habitats. Cage experiments show that S. hineana larvae grow faster and larger in the seasonal streamlets. Most other dragonfly species grew poorly in the streamlets. Preliminary results from the laboratory experiment suggest no strong effects of other dragonflies on amount eaten by S. hineana. The highly endangered S. hineana typically occurs in only a small subset of habitats (mostly those that exhibit seasonal-drying) within larger wetland complexes. Other species do appear to be excluded from significant parts of the habitat occupied by S. hineana, where they forage less effectively and lack the structural or behavioral mechanisms to cope with dewatering. Our study suggests that the limits of S. hineana distribution in more permanently-flooded areas may be only partially shaped by the presence of other dragonfly larvae. Competition or intraguild predation by other dragonflies was an obvious potential determinant of the distribution of S. hineana; however, they are just part of a diverse array of predators and competitors that occupy more permanent streams and wetlands.