Linking interspecific variation in vertebrate seed predation, insect seed predation, and pathogen attack to fruit traits in tropical woody plants

Background/Question/Methods: Interspecific variation in seed predation and pathogen attack is related to variation in plant life history traits, including fruit morphology and chemical defenses. The objective of this study is to quantify how variation of fruit traits among tree species influences vertebrate and insect seed predation as well as pathogen attack at the pre-dispersal stage. We examine how morphological and chemical defenses vary for several co-existing species within a community, how defenses change throughout fruit development (from the immature to the mature stage), and how defenses vary within the fruit. For immature and mature fruit of nine focal species, we measure physical defense traits (fruit length and width, and dry mass of pulp, diaspore, seed, and protective structures) and estimate chemical defense traits (toxicity) with brine shrimp and fungal bioassays (survival of shrimp and fungi in the presence of pulp and seed extract), which are commonly used and cost-effective. Two summary statistics of fruit traits for each species (physical score determined by principal components analysis and toxicity score) are included as a covariate in an analysis of seed survival in response to removal treatments of vertebrate seed predators, insect seed predators, and fungal pathogens.

Results/Conclusions: We find that morphological traits and toxicity vary widely among species, between immature and mature stages, and within fruit. At the mature (immature) stage, species range in mean diaspore size from 0.9 (0.4) mg to 2196 (185.1) mg. In the large-seeded species, the ratio of physical protection to seed reserve shifts from 5.1 in immature fruits to .59 in mature fruits. In the small-seeded species, the ratio at maturity is 1.8, whereas the seed is undeveloped at the immature stage. Trait data for immature fruits help explain patterns of seed survival among species in response to natural enemy removal treatments. Removal treatments increased germination of larger-seeded species, while having no effect on small-seeded species. Identification of consistent relationships between relatively easily measured fruit traits and the impacts of different natural enemies would greatly facilitate generalization to unstudied species and help identify those species at greatest risk under environmental change. Incorporating plant traits that reflect ecological mechanisms with experimental results aids in generalizing process from pattern and enables prediction. This is particularly important in tropical forests, where high species diversity makes it logistically impossible to study every plant life history stage of every species.