Organization of predator assemblages in Neotropical tree holes: effects of abiotic factors and priority

1. Water-filled tree holes in a lowland forest in Panama harbour an assemblage of large predators consisting of the larvae of five common species of Odonata, the mosquito Toxorhynchites theobaldi, and tadpoles of Dendrobates auratus. Odonate females oviposit in both large and small tree holes. However, the three largest species emerge from larger tree holes, on average, than do the two smallest species. Can assembly rules explain this and other patterns of predator distribution? 2. Past experiments suggested that fast growth of the largest, but later-colonizing odonates enabled them to out-compete the smaller, slower-growing Mecistogaster in large holes. In small holes, however, the first predator, regardless of species, should presumably kill any later arrivals. Priority effects in small holes were tested for their consistency across predator species. Two alternative explanations for differential odonate survivorship were also tested: abiotic conditions and the effect of non-odonate predators. 3. Diurnal fluctuations in oxygen content, pH, and temperature within holes were as great as the variation found between large and small tree holes; abiotic conditions were poor predictors of species occupancy. 4. Exchanging the largest and smallest odonate species from their original holes did not affect survivorship, suggesting that the observed patterns of emergence are unlikely to result from differential tolerance to abiotic factors that were not measured. 5. When larger and smaller predators were paired in 400-ml pots and provided with alternative prey, typically only the larger predator survived, regardless of species. The exception was T. theobaldi, which was often killed by odonates and tadpoles smaller than itself. 6. Between May and July, the occurrence of Mecistogaster in large tree holes declined, before larvae could have emerged. Neither abiotic effects nor differential predation by non-odonates could explain this habitat-specific decrease in survivorship. This temporal pattern is most consistent with the previous conclusion that in large holes, Mecistogaster suffer intraguild predation from later-colonizing, but faster-growing Megaloprepus and aeshnids. Whereas Mecistogaster can pre-empt guild members from small tree holes, they cannot do so in large holes where predators with fast initial growth realize a competitive advantage.