Abstract
The relative contributions of primary and secondary seed dispersal to plant demography have received little investigation. Evidence on these seed dispersal types, on seed fate and seedling recruitment of the tropical rain forest tree Dipteryx oleifera, is presented. The study was conducted in a 6.37ha permanent plot where seeds and seedlings were located and tagged for the 2007 cohort. A total of 2 814 seeds were threaded and their fate was followed one year after germination. Primary seed dispersal by bats protected seeds from insect larval predation below the adult tree. Bats congregated seeds in bat seed piles located at a mean distance of 40.94 +/- 1.48m from the nearest adult individual of D. oleifera. Terrestrial vertebrates congregated seeds in caches located 41.90 +/- 2.43m from the nearest adult individual of D. oleifera. The results of the fitted proportional hazard model suggested that primary seed dispersal decreased seed hazard probability by 1.12% for each meter from the adult conspecific (p<0.001) and that secondary seed dispersal decreased it by 23.97% (p<0.001). Besides, the odds ratio regression models results showed that the overall effect of unviable seeds was a reduction in viable seed predation rate. For each unviable seed deposited by bats into the seed piles, the rate of seed predation by terrestrial vertebrates decreased 6% (p<0.001). For each damaged seed by terrestrial vertebrates in the seed piles, the rate of germination decreased 4% (p<0.001). For each germinated seed in the seed piles, the rate of recruitment increased 16% (p=0.001). Seedling survival of seeds that emerged after secondary seed dispersal events, showed no statistically significant difference in arthropod herbivory, in relation to seedlings that came from seeds that were dispersed only primarily by bats (F=0.153, p=0.697, df=1.98). Thus both primary and secondary dispersal contributed to higher seedling survival away from the nearest adult D. oleifera (r2=0.713, n=578, p=0.004). The distribution of D. oleifera seedlings is consistent with the Janzen-Connell Hypothesis and depends on primary dispersal by bats, secondary dispersal by terrestrial vertebrates, a seed masking effect and, the constant threat of insect herbivores on seedlings.
Highlights
Seed dispersal away from the seed source can be fundamental for plant demography (Howe & Miriti 2004)
It is fundamentally important to understand the effect of density and distance on survival, an aspect that has received little investigation is the measurement of the relative contributions of primary versus secondary seed dispersal to plant demography (Vander Wall et al 2005), the focus of this study
Secondary seed dispersal was frequent away from the nearest adult D. oleifera tree (Fig. 3), mean secondary seed movement is approximately 1.5m
Summary
Seed dispersal away from the seed source can be fundamental for plant demography (Howe & Miriti 2004). It is fundamentally important to understand the effect of density and distance on survival, an aspect that has received little investigation is the measurement of the relative contributions of primary versus secondary seed dispersal to plant demography (Vander Wall et al 2005), the focus of this study. After removing viable seeds from seed piles throughout the forest floor, those terrestrial vertebrates may forget the location of removed viable seeds (Boucher 1981), resulting in higher recruitment rate away from the seed piles This seed “masking” effect would largely determine the degree of plant recruitment, arthropod herbivory and seedling distribution with respect to the nearest conspecific adult tree
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