Ecological correlates of single-seededness in a woody tropical flora.

A reduction in seed number per fruit is recognized as a common evolutionary trend among flowering plants. In order to evaluate the ecological role of single-seededness and its possible adaptive significance, we used van Roosmalen's (1985) descriptions of fruits for woody species in the Guianan flora to identify fruit and seed characters and dispersal syndromes associated with the single-seeded trait. We coded the following characters: seed number (one seed or more than one seed), fleshiness (dry or fleshy), dehiscence (dehiscent or indehiscent), dispersal syndrome, seed size (length), and fruit size (length). We ordered the data in a multi-dimensional contingency table and used maximum likelihood logistic regression to identify traits significantly correlated with single-seededness. Seed size and fruit size were treated as quantitative variables. Indehiscence and endozoochory are positively associated with single-seededness, with indehiscence contributing most to the best-fit model. Fruit size and seed size are also important with the probability of single-seededness generally increasing with seed size and decreasing with fruit size, although a (fruit size) x (seed size) interaction term is significant. Dry fruits are positively associated with single-seededness and dispersal by synzoochory or myrmecochory negatively associated when the full data set is examined, but neither parameter is significant in two models constructed to remove effects of phylogeny. A nested ANOVA revealed that most variation occurs below the family level for almost all of the traits considered, with the exception of the dry vs. fleshy trait for which there is no variation within genera. We argue that the strong association between indehiscence and single-seededness suggests selective advantages for single-seeded dispersal units but acknowledge that energetic trade-offs between seed number and seed size probably also occur. We suggest that the post-dispersal fates of seeds - especially those deposited in clumps by endozoochory - should be examined with the idea of identifying selective pressures on seed number per fruit.