Mechanisms and Consequences of Intraspecific Competition in a Carabid Cave Beetle

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We have been able to clearly study the mechanisms and consequences of intraspecific competition, in contrast to most studies, because of the cave beetle (Neaphaenops tellkampfi, Carabidae) has no competitors and only a single available prey item, cricket eggs (Hadenoecus subterraneus, Rhaphidophoridae), in our study area in Mammoth Cave National Park. The sexes do not differ in size or external morphology and we have never seen interference with copulating pairs and so the likelihood of competition for mates or female choice seems low or nonexistent compared to competition for food. Both in the field and laboratory we assessed: (1) foraging effort by numbers of holes dug: (2) potential foraging success by whether the holes were deep enough to reach a buried cricket egg; and (3) time since foraging success by abdominal distension and body mass. We assessed both exploitation competition, the indirect effect of beetles on each other via cricket egg prey, and interference competition, the direct effect of beetles on each other. To demonstrate exploitation competition we showed that (1) beetles in the field reduced the density of egg prey, (2) lowered egg density in the laboratory resulted in decreased per capita harvest rate, and (3) in the field reduced harvest rates resulted in reduced fecundity at times of year when cricket numbers, fecundity, and egg—laying were lowest and beetle densities were highest. To demonstrate interference competition we showed that beetles reduced each other's foraging effectiveness. In laboratory arenas per capita foraging effectiveness decreased when egg density was held at 10 and beetle density increased from 1, to 2, to 4 per arena. With increasing beetle density there were significant declines in holes dug per beetle, depths of holes, and eggs eaten per beetle. In addition beetles spent less time foraging. In the field interference competition, as assayed by decreasing hole depths, was significantly greater in areas of high beetle density than in areas with low beetle density. Fights for holes occurred only in areas of high beetle density. Preempting holes or stealing eggs could be advantageous since bioenergetic calculations showed that the cost of digging for eggs is as high as two—thirds of a beetle's energy budget. In the field in areas of high beetle density there was no defense of holes <3 mm deep and defense of all holes >7 mm deep. Beetles that had recently eaten eggs were found significantly more often under rocks and on the ceiling where we infer that chances of eggs being stolen are low.