Within‐Habitat Spatial Separation of Two Notonecta Species: Interactive vs. Noninteractive Resource Partitioning

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Two aquatic insect predators, Notonecta undulata and N. insulata, partitioned space in a small (< 0.25 ha) Connecticut pond. Spatial separation was established during recruitment and persisted throughout the breeding season. Over 70% of the N. undulata population developed in shallow (< 0.3 m) water near the pond margin, while > 70% of the N. insulata population occurred in deep (> 0.5 m) water in the center of the habitat. Mutual predation did not appear to play an important role in separating the populations. Although species size relationships in the pond favored predation by N. insulata on N. insulata showed little inclination to prey on N. undulata in laboratory aquaria. Cage experiments and temperature tolerance tests suggested that spatial separation reflects fundamental niche differences between the species. The survival rate of N. insulata nymphs in shallow—water cages was significantly poorer than in deep—water cages and was not significantly affected by interaction with N. undulata. N. insulata is intolerant of high water temperatures and appears to be poorly adapted for survival in shallow water exposed to solar heating. N. undulata, on the other hand, is tolerant of high temperature and, except during the fifth instar, survived as well or better in shallow—water cages than in deep—water cages. Second— and third—instar N. undulata survival rates were depressed by interaction with N. insulata in population cages, but fourth— and fifth—instar survival rates were not. Although the cage results indicated that N. insulata can depress early instar N. undulata survival rates, the hypothesis that N. insulata was responsible for the low N. undulata densities recorded in deep water is weakened by evidence that N. undulata prefers densely vegetated microhabitats during oviposition and early nymphal development. Aquatic vegetation was abundant in shallow water but sparse in deeper parts of the pond. This study reveals how the coexistence of potential competitors in a particular habitat can depend upon the interplay between the unique features of that habitat and physiological and behavioral traits of the species involved.