The processes that generate species diversity within individual habitat patches are a fundamental area of interest in ecology. Traditionally, work focused on post-colonization effects such as predation, with little consideration of pre-colonization processes, such as habitat selection. Both stages often are equally important determinants of community structure, yet many studies focus on one stage or the other, not both. In small, ephemeral habitats, insects are often top predators, yet their effects have been poorly studied at both the colonization and post-colonization stages. We conducted three experiments to assess how a predaceous aquatic insect (Notonecta irrorata) affected colonization of aquatic beetles, oviposition by Culex mosquitoes, and the vulnerabilities of these colonists to predation by N. irrorata.
Results/Conclusions
We had 11 abundant species of aquatic beetles representing three families (Dytiscidae, Haliplidae, Hydrophilidae) in the predation experiment. Four of these species had <2% mortality in the presence of N. irrorata (Berosus infuscatus, Laccophilus fasciatus, Peltodytes sexmaculatus, Tropisternus lateralis), and two species had >92% mortality (Copelatus glyphicus, Cymbiodyta chamberlaini). The other five species (Enochrus ochraceus, Hydrocolus oblitus, Hydroporus rufilabris, Neoporus blanchardi, Paracymus) had mortality rates ranging from 20% to 60%. Of the 11 species above, 6 were abundant in our colonization experiment. Berosus infuscatus, E. ochraceus, and T. lateralis all avoided patches containing N. irrorata. Tropisternus lateralis did not respond to N. irrorata. Copelatus glyphicus and L. fasciatus both colonized patches containing N. irrorata at higher rates than controls. Culex restuans did not selectively oviposit in patches based on N. irrorata presence, but their larvae had ~0% survival in the presence of N. irrorata.
Our results illustrate that the pre-colonization habitat selection decisions of some prey species match vulnerabilities to predation, likely maximizing expected fitness. However, among several taxa, habitat choices do not match vulnerabilities to predation, and in C. glyphicus, the two patterns are contrasting. The avoidance of N. irrorata among species with adults that are not vulnerable to predation may be due to higher vulnerability of offspring to predation. However, why vulnerable species like C. glyphicus are attracted to a predator they have no defense against is potentially an ecological trap. Overall, we report that a predaceous aquatic insect has unique effects on prey species at both the pre-colonization habitat selection stage and post-colonization predation stage. These lethal and non-lethal processes have effects that vary in magnitude and direction, but ultimately work together to generate the communities observed in freshwater systems.