Post-Dispersal Predation on Isolated Seeds: A Comparative Study of 40 Tree Species in a Southeast Asian Rainforest

Abstract

Many studies of post-dispersal seed predation have focused on density and distance dependent mortality, while relatively few have examined the fates of isolated seeds. Yet, scatter-dispersed seeds (sensu Howe) are commonly deposited singly or in small groups. Furthermore, even in species with highly aggregated seed distributions, the fates of the most widely dispersed individuals may be critical for recruitment. We compared predation rates on single, isolated seeds, among 40 species of trees in lowland tropical rain forest at Gunung Palung, West Kalimantan, Indonesia. Seeds were placed along four replicate transects and monitored for damage by predators, removal and germination in four trials, each lasting at least 30 days. Tethering of seeds did not affect removal rates, indicating that removals were attributable to seed predators and not merely to physical disturbance by animals or abiotic factors. After 30 days, mortality due to seed predation, averaged over species, was more than 50%; among species, predation losses ranged from 0 to 100%. Over the range of seed sizes we examined (0.1 g to 11.6 g fresh weight) predation rates were negatively associated with seed size and with the thickness and hardness of the seed coat. Lower predation on larger seeds is contrary to theoretical predictions and some prior empirical findings, and may be partially explained by the scarcity of predators capable of penetrating the physical defenses of large seeds with hard seed coats. Large, soft seeds with low predation rates may have poor nutrition or may be protected by chemical defenses. Species differed greatly in 30-day germination rates, ranging from 0 to 47%. Some species with low predation rates also had low germination rates; the implications for the overall risk of predation during the seed stage are discussed. Predation rates were not associated with species' natural dispersal mode (clumped vs scatter-dispersed), contrary to theoretical predictions. Spiny rats (Maxomys spp.) were the most abundant seed eating rodent. Caged spiny rats avoided large, hard seeds and preferred soft, medium sized seeds. The substantial rates of post dispersal predation on isolated seeds that we measured may be sufficient to influence strongly the population dynamics and life history evolution of trees in this rain forest community.