Habitat associations, resources, and predators of an assemblage of Brachinus (Coleoptera: Carabidae) from southeastern Arizona

Abstract

Bombardier beetles (Brachinus) at margins of ponds across gradients of elevation and pond permanence were studied to describe habitat associations and resource use and to form hypotheses concerning the ecological factors affecting these beetles. Assemblage composition differed significantly among pond types, with more permanent ponds dominated by Brachinus lateralis, high-elevation temporary ponds dominated by Brachinus mexicanus, and Brachinus javalinopsis and Brachinus lateralis, codominant at a low-elevation temporary pond. Brachinus mexicanus was sometimes found at dry temporary pond sites, but none of the other species was ever found at dry sites. These species shared at least two potentially limiting resources: food for adults (carrion and arthropods) and hosts for ectoparasitoid larvae (water beetle pupae). Reproductive phenology, host use, and diets overlapped extensively. Host abundances and host-assemblage composition varied among sites and most hosts went unparasitized. Predation on adults by vertebrates appeared to be very rare. Although some predatory arthropods ate Brachinus in the laboratory, these predators were either rare or ineffective in natural situations.