Novel diplochory: Native bats and non-native rats disperse seeds of an island tree

Abstract

Many oceanic islands lacked mammalian seed predators until humans introduced rats (Rattus spp.). Introduced rats are considered major seed predators on the islands where they occur, but their capacity to assist native plant recruitment through secondary dispersal, or diplochory, is poorly known. We monitored fates of >1000 naturally- and artificially-dispersed diaspores of the coastal tree Pandanus tectorius, to assess potential effects of rats on seedling recruitment in Tonga (Polynesia) and to determine if diplochory (phase II dispersal) by invasive rats can enhance primary dispersal (phase I) achieved by native bats. Pandanus diaspores consist of multiple single-seeded fruits (drupes) fused into a “phalange" in which each seed is protected by a stony endocarp, and all endocarps are fused into a single, solid structure. Native bats (Pteropus tonganus) consumed the pulp of the diaspores and dispersed 61% of them away from the crown. Introduced rats (Rattus rattus, R. exulans), consumed pulp and seeds. Rats secondarily dispersed 39% of the phalanges bats dropped below trees, carrying most of them away from trees. The average phalange has 6–7 outer drupes around its circumference, surrounding 2–3 inner drupes. Rats removed seeds from 64% of outer drupes, but opened no inner drupes; therefore, phalanges typically retained at least one uneaten seed. Of these rat-handled phalanges, 69% produced at least one seedling, compared to 96% of unhandled phalanges. Overall, diplochory, the combined effect of phase I dispersal by bats and phase II dispersal by rats, increased the likelihood that a phalange would produce a seedling that was not beneath the parent crown by 34%, when compared to the effect of bats alone. Seedling recruitment is unlikely to be strongly reduced by rodent seed predators, since rats do not consume all seeds in a single diaspore, and losses from predation may be offset by rats assisting the “escape” of the diaspores dropped by bats underneath the fruiting crown.