Density-Dependent Seed Predation and Plant Dispersion of the Tropical Palm Normanbya normanbyi

Abstract

We measured the response of seed predators to variation in seed density and distance between seed sources of palms in North Queensland, Australia. Adult palm trees occurred at a density of 20/ha. Their distribution was highly clumped and more than half the immature plants occurred within 3 m of an adult. Adult palms were also arranged in lines down the slope, apparently reflecting patterns of seed carriage by overland water flow. Seed dispersal is also affected by cassowaries, a ratite which ranges over many hundreds of ha and deposits seeds in dusters. Small mammals which range over a few ha may deposit seeds as single entities (singletons). Many seeds remain undispersed beneath the female parent with heavy predation. In this study seeds were placed beneath adult trees at two densities and were dispersed over varying distances as clusters of ten seeds and as singletons. Pigs and earwigs were the principal predators but they did not destroy all the seeds in any treatment. However, the germination percentage of the remaining seeds was greatly depressed indicating that one or both predators were disproportionately selecting potential germinants. Pigs were responsive to both density of seeds beneath a tree and the distance between seed sources; they did not damage dispersed seeds to a great extent. Earwigs also destroyed seeds more heavily beneath adult trees, but they were more efficient than pigs at locating dispersed seeds. Rodent predation was light and not responsive to seed density or dispersal.