Evaluating the non-consumptive, positive effects of a predator in the persistence of an endangered species

Failure to consider both the consumptive and non-consumptive effects of predators on prey can lead to erroneous conclusions about the net effect of the relationship. The predatory devil crayfish, Cambarus diogenes Girard functions as an ecosystem engineer constructing extensive burrow systems through aquatic habitats. Despite crayfish posing a serious predation threat, preliminary data indicate that the federally endangered Hines Emerald dragonfly larvae, Somatochlora hineana Williamson regularly inhabit crayfish burrows. During late summer, S. hineana larval habitat dries up; leaving crayfish burrows as some of the only wetted habitats. Thus, C. diogenes can affect S. hineana through both direct, negative and indirect positive effects. We examined the positive role of crayfish burrows as drought refuges, and the threat of predation by C. diogenes on S. hineana larvae. Monthly field sampling indicated that S. hineana use open channel areas in spring and early summer moving into burrow systems in mid summer when channel areas normally dry. Laboratory experiments and field observations confirmed that crayfish prey on S. hineana larvae. Adult crayfish were a larger predation threat than juvenile crayfish. Despite their negative predatory impact, removal of crayfish from burrows in the field did not enhance densities of S. hineana larvae. Although S. hineana may face the threat of predation in burrows, they face a greater risk of desiccation if they remain in the open channel. These results lead to the counterintuitive conclusion that the maintenance of a predator is important for conserving an endangered prey species. (c) 2006 Elsevier Ltd. All rights reserved.