Effects of flooding and herbivores on variation in recruitment of palms between habitats

1 It is widely recognized that negative effects of anaerobic stress on growth and survival of flooded plants influence the distribution of numerous species. Less explored is the possibility that heterogeneity in abundance of plants between habitats with distinct flooding regimes may also result from variation in rates of herbivory or in the ability of plants to tolerate losses to herbivores. 2 Flooding and herbivores were tested as factors underlying variation in abundance of two tropical forest palms, Socratea exorrhiza (which is associated with lowlands adjacent to streams) and Oenocarpus bacaba (which is more abundant on plateaux and upper edges of slopes). 3 In a bench experiment, seeds of both palms were either completely immersed in water for a period of 101 days or not subject to inundation. Flooding inhibited germination of both species but, as expected, the adverse effects were much stronger on Oenocarpus. 4 In a field experiment, seeds of both palms were planted with increasing levels of protection against herbivores on plateaux and in lowlands. Seeds were either not protected or placed within poultry-netting exclosures, half of which were sprayed with insecticide. 5 After 17 months, only Oenocarpus had experienced differential mortality between habitats, and this was clearly associated with the negative effects of flooding on seed germination in lowlands. In contrast, growth differed between habitats only for Socratea seedlings, where average above-ground biomass was greater in lowlands. 6 Although protection with exclosures and insecticide increased survivorship of both species, herbivores caused similar proportions of mortality on plateaux and in lowlands, and had no significant effect on seedling growth. Therefore at this site, herbivores do not appear to influence variation in abundance of species between habitats.