Summary 1The difference in gap size requirements observed for neotropical pioneer species is a classic example of habitat partitioning among tropical tree species. Previous research has shown that the three species Miconia argentea , Cecropia insignis and Trema micrantha differ in the minimum gap size that they occupy as adult plants in semi-deciduous tropical forest in Panamá. Here we test whether these differences result from variation in seed germination and seedling establishment requirements, or from differences in susceptibility to herbivores. 2 We planted seeds of Miconia argentea and Trema micrantha into artificially created small (25 m 2 ), medium (64 m 2 ) and large (225 m 2 ) gaps in secondary forest, and transplanted seedlings of all three species into small and large gaps inside and outside mesh cages that excluded invertebrate herbivores. 3 Miconia seedlings emerged and established successfully across a broad range of gap sizes, and transplanted seedlings grew slowly but survived well in all gap sizes. By contrast, emergence and survival of Trema seedlings occurred only in the large gaps and transplants died in small gaps. Survival of Cecropia seedlings was intermediate between Trema and Miconia in the small gaps and growth was greater than either species in the large gaps. 4 Herbivore exclosures reduced foliar herbivory on all three species. The median lifespan of Trema seedlings in small gaps was reduced when the seedlings were exposed to herbivores, but survival of the other species was not affected by the exclosure treatment in either gap size. The growth of Miconia seedlings was lower outside exclosures in both gap sizes. 5 Differences in germination and establishment could generate the contrasting gap size distributions observed between Miconia and Trema , but they are partially offset by measured differences in seed bank density. However, patterns of seedling growth and survival reinforce differences among species at the establishment phase and provide a mechanism for the observed differences in gap size distribution at the sapling stage. 'Cross-overs' in species growth rates result in specialization to different environments and reduce their survival in unfavourable conditions. We found no evidence for a definitive influence of herbivory as a mechanism driving differences in gap size distribution among species.

Adaptive trade-offs underlie the specialisation that permits habitat partitioning in species rich plant communities. We investigated the influence of the trade-offs that determine differences in growth and survival among six species of neotropical pioneer trees in gaps in semideciduous forest in Panama. Seedlings of Miconia argentea, Cecropia insignis, Luehea seemannii, Trema micrantha, Ochroma pyramidale and Croton bilbergianus were planted into artificial small (25 m(2)), medium (64 m(2)) and large (225 m(2)) gaps in secondary forest in the Barro Colorado Nature Monument. Trema and Ochroma suffered >/=50% mortality across all gap sizes, while Cecropia had high mortality only during the dry season and in the small gaps, and Miconia and Croton suffered low to zero mortality across all environments. The highest growth rates in large gaps were attained by Cecropia seedlings and in the smaller gaps by Miconia seedlings, although there were indications that Trema and Ochroma required gaps that were larger than any used in this study. Variation in growth and mortality could not be attributed to differences in foliar herbivore damage. Instead, there was strong evidence of a trade-off between maximum growth in the wet season and the ability to survive seasonal drought, particularly in small gaps. We conclude that variation in allocation in response to multiple limiting resources may be as important as allocation to growth and defence in determining the habitat preferences of neotropical pioneers.

The view of terrestrial arthropod communities as potentially exhibiting a mixture of three-and four-trophic-level dynamics creates a new challenge for researchers: the indeterminacy of predator function. Here we evaluate two predictions linking a predator's foraging behavior with its likely ecological role: (1) widely foraging predators may act as effective regulators of sedentary herbivore populations, and (2) sit-and-wait predators are unlikely to regulate sedentary herbivore populations but may act as top predators, consuming widely foraging intermediate predators and allowing herbivore populations to escape from control. We tested these predictions by manipulating the predator community associated with a sedentary herbivore, the spider mite Tetranychus cinnabarinus, feeding on papaya, Carica papayae. The predators included a widely foraging specialist consumer of spider mites, the ladybird beetle Stethorus siphonulus, and a sit-and-wait generalist predator, the tangle-web spider Nesticodes rufipes. The experiments provided support for both predictions. The widely foraging predator Stethorus when tested alone was capable of effective suppression of spider mite populations. In contrast, the sit-and-wait predator, Nesticodes, never suppressed spider mites. Instead, Nesticodes consistently increased the population growth rates of spider mites. This effect was most likely due to Nesticodes suppressing populations of the intermediate predator Stethorus. Our results underscore the presence of both three-and four-trophic-level dynamics within this arthropod community and begin to address the challenging problem of the indeterminacy of predator function.

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