POSTDISPERSAL PREDATION ON ISOLATED SEEDS - A COMPARATIVE-STUDY OF 40TREE SPECIES IN A SOUTHEAST-ASIAN RAIN-FOREST

Many studies of post-dispersal seed predation have focused on density and distance dependent mortality, while relatively few have examined t he 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 m ost widely dispersed individuals may be critical for recruitment. We c ompared predation rates on single, isolated seeds, among 40 species of trees in lowland tropical rain forest at Gunung Palung, West Kalimant an, Indonesia. Seeds were placed along four replicate transects and mo nitored for damage by predators, removal and germination in four trial s, each lasting at least 30 days. Tethering of seeds did not affect re moval rates, indicating that removals were attributable to seed predat ors and not merely to physical disturbance by animals or abiotic facto rs. After 30 days, mortality due to seed predation, averaged over spec ies, 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 fre sh weight) predation rates were negatively associated with seed size a nd with the thickness and hardness;of the seed coat. Lower predation o n larger seeds is contrary to theoretical predictions and some prior e mpirical finding, and may be partially explained by the scarcity of pr edators capable of penetrating the physical defenses of large seeds wi th hard seed coats. Large, soft seeds with low predation rates may hav e poor nutrition or may be protected by chemical defenses. Species dif fered 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 a re discussed. Predation rates were not associated with species' natura l dispersal mode (clumped vs scatter-dispersed), contrary to theoretic al predictions. Spiny rats (Maxomys spp.) were the most abundant seed eating rodent. Caged spiny rats avoided large, hard seeds and preferre d soft, medium sized seeds. The substantial rates of post dispersal pr edation on isolated seeds that we measured may be sufficient to influe nce strongly the population dynamics and life history evolution of tre es in this rain forest community.