Seed Predation in Relation to Tree Dominance and Distribution in Mangrove Forests

The ecology of seed predation was studied in mangrove forests. Controlled field experiments were used to assess the magnitude of predation on mangrove propagules, to test for differences in predation among five species of mangroves, and to examine how differential amounts of predation on seeds are correlated with conspecific dominance and may contribute to observed patterns of tree distribution across the intertidal region. Averaged over all species and forests, 75.7 ± 28.7% (° ± 1 sd) of the seeds used in the experiments were consumed by predators (small crabs belonging to the family Grapsidae). Significant differences existed in the amounts of predation among mangrove species. Avicennia marina was most heavily preyed upon, followed in decreasing order by Bruguiera exaristata, Ceriops tagal, Bruguiera gymnorrhiza, and Rhizophora stylosa. Differences in chemical composition of the propagules accounted fro ≥97% of the differences in amounts of predation among the five species. Nutritive quality of the propagules, expressed as a function of protein, simple sugar, fiber, and tannin content, was more important in explaining the differences than was propagule size. Significant differences also existed among three forest types, which differed in species dominance. Lowest amounts of predation for C. tagal were in the forest where adults of this species were absent, and highest in the forest where the species was most dominant. For A. marina, the two Bruguieras, and R. stylosa, however, amounts of predation were significantly higher in forests where conspecifics were rare or absent and lowest where conspecifics were dominant. For these latter four species, conspecific dominance in the canopy explained ≥63% of the variation in predation among forest types. The pattern of differences in seed predation among forests across the intertidal region indicates that seed predators may play a significant role in determining the intertidal distribution of the mangrove species studied.