Previous studies have shown the predatory effects of fish in habitat partitioning of aquatic insects such as Notonecta species. However, the mechanism behind the heterogeneous distribution of these insects amongst the fishless ponds, where they are considered to be the top predators remain unknown. To find out the relationship between notonectid spatio-temporal distribution and the local and regional processes, the two most common species of Michigan ponds, Notonecta undulata and irrorata,were selected. During the two years of pond survey, notonectid species as well as the pond environmental and biotic variables were collected. Moreover, in multiple pond insitu trials, notonectids were introduced to the ponds lacking these species to examine the possibility of dispersal limitation against species-sorting as an explanation for their absence from these habitats.
The results from our survey showed that the level of pond occupancy by N. undulata and irorata stayed consistent within the fishless ponds during the two sampling seasons. Some of the ponds were occupied by only one of the two species while the other ponds were occupied by both the species. Pond isolation seems an unlikely cause for such an observation since the studied ponds are fairly connected and the mean distance amongst them (= 1 km) is smaller than the least distance that the notonectids are able to fly (1.6 km). Among the local factors studied, Total phosphorus (TP) levels of pond water played the most significant role in habitat segregation of notonectid species. While N. irrorata were abundant in ponds with varying levels of TP, N. undulata only colonized their population in ponds with lower levels of TP (<100 µg/L). The results from the pond insitu experiments demonstrate that adults of both the species survived well and comparably in all the ponds irrespective of the TP level. In conclusion, our findings indicate that TP level has a major role in habitat partitioning of the two notonectid species, however this impact does not seem to be implemented through the pond water TP, phytoplankton, and zooplankton. The outcomes of this study lead us to propose a hypothesis where the TP level affects the notonectid species adult’s spatial segregation indirectly by influencing the pond vegetation composition and/or interspecific interactions. Another possibility is that TP included in the water and planktonic resources could cause notonectid habitat segregation by affecting the adult’s oviposition and/or juvenile’s survival and growth.