Habitat segregation is one mechanism by which functionally similar species may reduce conflict with each other and coexist. Assessments of habitat use and interspecific relationships are frequently based on daytime observations, but for many animals, interactions need not cease at night and may in fact change substantially. Here I investigated the diel habitat use and feeding ecology of the intraguild predators Notonecta and Buenoa, aquatic hemipterans, to determine whether spatial segregation could sufficiently account for their common coexistence.

Fishless pond surveys revealed that habitat use and activity of many pond animals changed dramatically from day to night (Chapters 4 and 5). Animals were more abundant in the near-surface waters at night than during the day; additionally, many animals found primarily in vegetation during the day were also found in open water at night. These results in fishless ponds were surprising, since such extreme diel behavioral changes in aquatic animals (e.g. diel vertical and horizontal migrations) are overwhelmingly attributed to the presence of visually foraging fish. Here it was clear that strictly daytime assessments of habitat use would have poorly characterized the strength and variety of biotic interactions possible for community members.

To further investigate the potential for diel changes in interaction strength of conflicting community members, I focused on the notonectid genera Notonecta and Buenoa. These predatory insects prefer similar food and are well known for voraciousness. Notonecta is the dominant in conflicts, and readily preys on Buenoa in the field and laboratory. Casual observation suggested that Notonecta is associated with vegetation while Buenoa uses the open water, where the potential prey were taxa less preferred by both notonectids.

I hypothesized that Buenoa was excluded from the comparatively food-rich littoral zone by the larger Notonecta. However, field enclosures (Chapter 1) and lab experiments (Chapter 2) subsequently showed that Buenoa feeds extensively on the largest members of the small-bodied (<0.5 mm) open water community and thus may not perceive the open water as poor habitat. Several small rotifer taxa benefited dramatically from Buenoa’s predation on dominant species.

With food resources in the open water and its enemy Notonecta in the vegetation, it was puzzling that Buenoa showed no distinct preference for open water in outdoor mesocosms with and without Notonecta (Chapter 3). Conversely, Notonecta exhibited clear spatial use patterns; Notonecta was primarily in vegetation during the day but evenly distributed across open and vegetated water at night. Laboratory experiments confirmed that Notonecta can feed effectively in all combinations of light and dark with open and complex environments, such that Buenoa’s risk to Notonecta predation increased at night when spatial overlap was complete. Notonecta’s high predatory versatility and relatively frequent habitat shifts may prevent Buenoa from perceiving the open water during the day as a spatial refuge from predation.

My work highlights the importance of understanding the response of ecological systems to abiotic factors that vary over the temporal scale of interest. In this case, nighttime observation was necessary to characterize biotic interactions, even in fishless lakes where diel changes may not be expected.

please contact: Stephanie.E.Hampton.Adv02@Alum.Dartmouth.ORG