The influence of seed predation and plant competition on tree species abundances in two adjacent tropical rain forest communities in Trinidad, West Indies / Judy McKean Rankin.

Tese (Ph.D.) - University of Michigan, 1978

This study assesses the relative roles of seed predation and plant competition in determining tree species richness and abundances in two adjacent tropical rain forest communities. This investigation was conducted during 27 months in the mora forest, numerically and ecologically dominated by Mora excelsa Benth. (LEG.), and the adjacent mixed forest, where the most abundant species is Pentaclethra macroloba (Willd.) Kuntze (LEG.), in Trinidad, W. I. These communities were chosen because of the unique opportunity presented for the study of plant competition by the ongoing invasion of the mixed forest by Mora. Based in the relative distributions and abundance of Mora and Pentaclethra, I hypothesized the (1) these species experience little or no seed predation relative to the other tree species in mixed forest and in mora forest, and that (2) the presence of Mora will have a negative effect on Pentaclethra growth and survival in experimental plantings where the natural distributions of these species overlap and where they are established in either community. Seed predation intensity was assessed for Mora, Pentaclethra, and three other species occurring in the mixed forest and mora forest, Clathrotropis brachypetala (Tul.) Kleinh. (LEG.), Carapa guianensis Aubl. (MELIA), and Brownea latifolia Jacq. (LEG.), by observing attacks on, and removal rates of, seeds placed in many locations in both communities and in the intervening ecotone. Seed attack and removal varied both between tree species and between communities. The bases for these differences were investigated in feeding experiments with native rodent seed predators and laboratory mice plus determination of seed nutrient contents and the presence of certain seed secondary compounds. The presence of toxic alkaloids in Clathrotropis seeds was shown to be correlated with low seed predation intensity while the low level of seed predation for Mora and the community-dependent levels of seed predation for Pentaclethra appear to be the result of a combination of factors, including community differences in predator species, seed secondary compounds, and spatial-temporal variability in fruiting pattern in the case of Mora. The inter- and intraspecific competitive interactions of Mora and Pentaclethra were investigated through experimental seedling plantings in a variety of sites in mixed forest, mora forest, and the ecotone. The nearest neighbor girth-distance relationships for trees greater than 29 cm girth were examined for evidence of the effect of competition on the vegetative growth of larger trees. The presence of Mora had a negative effect on Pentaclethra seedling growth and survival but this was strongly dependent on habitat conditions and the size class composition of Mora at the plot site. This was not due to allelopathic effects of Mora roots and litter. Mora growth and survival were significantly affected by density and plot site in the intraspecific plantings but were not significantly affected by Pentaclethra seedlings. There was no evidence of the effects of interspecific competition based on girths of nearest neighbor pairs, but indirect evidence of the effect of intraspecific competition on reproduction was obtained for Mora. The tentative conclusion of this study is that while a higher Pentaclethra population density could probably be maintained within the mora forest in habitats having high light intensities and low Mora seedling and sapling densities, this is prevented by the grater intensity of seed predation which this species experiences in the mora forest compared to mixed forest. The present distribution of Mora appears to be due to limited seed dispersal and lower seedling growth and survival in sites which are drier and more exposed than those under a Mora canopy, rather than by competition with Pentaclethra or seed predation. Seed predation may also limit the number and abundance of other tree species in mora forest.