Original paper

Predators are more important than conspecifics and water temperature in influencing the microdistribution and behavior of a detritivorous stonefly

Wooster, David E.; DeBano, Sandra J.; Madsen, Anne


Biotic interactions influence the distribution and abundance of lotic organisms at multiple spatial scales. At small scales, intra- and interspecific competition for refugia from high flows and predation can affect the micro-distribution of lotic organisms. The abiotic environment, such as water temperature, can have important impacts on lotic organism behavior, potentially influencing competitive and predator-prey dynamics. In the laboratory, we examined activity and refuge use of stonefly nymphs, Pteronarcys californica, in the presence and absence of con-specifics and signal crayfish (Pacifastacus leniusculus) across three temperatures. We predicted that at high temperatures competitive dynamics would decrease in importance. Crayfish captured and consumed stoneflies in ∼25 % of trials indicating that crayfish are effective predators as well as competitors for refuge. Crayfish influenced stonefly activity and refuge use, and their effect varied with time of day. During the day, crayfish hid in refuge; in response, stoneflies reduced refuge use and increased activity. At night, crayfish decreased refuge use and increased crawling resulting in a sharp decline in stonefly activity. These behavioral responses to crayfish may impact P. californica fitness as increased day-time activity increases risk of detection by visual predators and decreased night-time activity might decrease encounters with food resources. In contrast to the strong impacts of crayfish, stonefly density and water temperature did not significantly affect stonefly or crayfish refuge use or behavior. Our results indicate that a large competitor and potential predator has strong impacts on the microdistribution and behavior of P. californica.


inter-intraspecific competitionpredationmicrodistributionstonefliescrayfishrefuge use