Costs and benefits of capsaicin-mediated control of gut retention in dispersers of wild chilies

A fundamental way in which animal-dispersed plants can influence the viability and distribution of dispersed seeds is through control of retention time in the guts of dispersers. Using two species of wild chilies and their dispersers, we examined how chemical and physical properties of fruits and seeds mediate this interaction. Capsicum chacoense is polymorphic for pungency, occurs in Bolivia, and is dispersed mostly by elaenias. Capsicum annuum is not polymorphic, occurs in Arizona (USA), and is dispersed mostly by thrashers. We first tested whether capsaicin, the substance responsible for the pungency of chilies, affects gut retention time of seeds in primary dispersers. Capsaicin slowed gut passage of seeds but did so in a manner that differed greatly between bird species because the constipative effects of capsaicin occurred only after an 80-minute time lag. Elaenias in Bolivia held only 6% of C. chacoense seeds for >80 minutes, whereas thrashers in Arizona held 78% of C. annuum seeds for >80 minutes. Next we examined the effects of retention time on seed viability and germination. Increased retention resulted in a greater proportion of seeds germinating in C. annuum, had no effects on non-pungent C. chacoense, and had negative effects on pungent C. chacoense. These divergent effects are explained by differences in seed coat morphology: seed coats of pungent C. chacoense are 10-12% thinner than those of the other two types of seeds. Thus, longer retention times damaged seeds with the thinnest seed coats. In C. annuum, seed viability remained high regardless of retention time, but germination increased with retention, suggesting a role for scarification. Thus, in C. annuum, fruit chemistry appears well matched with seed morphology and disperser physiology: capsaicin extends gut retention for most seeds, resulting in greater seed scarification and higher germination rates. Increased retention of pungent C. chacoense seeds is detrimental, but because the primary consumers have short retention times, capsaicin slows only a small proportion of seeds, minimizing negative effects. These results illustrate the importance of context in studies of fruit secondary metabolites. The same chemical can have different impacts on plant fitness depending on its morphological, physiological, and ecological context. © 2008 by the Ecological Society of America.This research was supported by grants from the National Science Foundation (DEB 0129168) and the National Geographic Society (CRE 7190-02)Peer Reviewe

Data and code

Zipped folder containing data from field experiments on seeds and seedlings under fungicide, insecticide, and mammal exclusion treatments and R scripts for analysis

Data from: Linking intra-specific trait variation and plant function: seed size mediates performance trade-offs within species

Substantial intra-specific trait variation exists within plant communities, and in theory this variation could influence community dynamics. Although recent research has focused on intra-specific variation in traits themselves, it is the influence of this variation on plant performance that makes intra-specific trait variation relevant to ecological dynamics within or among species. Understanding the links between trait and performance variation, and the role of traits in mediating relationships among multiple components of performance, is critical for assessing the importance of intra-specific trait variation for community dynamics. Seed size is thought to affect aspects of plant performance including fecundity, seedling growth, dispersal, and tolerance of natural enemies. For two tropical tree species, we assessed how seed size was related to performance variation within each species and determined whether intra-specific trait variation mediates intra-specific performance trade-offs. We used field seed rain collection to characterize size-dependent outcomes of dispersal, sowed seeds of known size in soil collected near or far from conspecifics to characterize susceptibility to soil pathogens, and monitored growth of seedlings from seeds of known size. We found that intra-specific seed size variation caused intra-specific performance variation. The degree of trait-based performance variation was consistently smaller than the degree of trait variation, and seed size influenced different components of performance for each species. One species exhibited a trade-off in which small seeds had a fecundity advantage (more seedlings per unit reproductive mass) but produced smaller seedlings, whereas the other species exhibited a trade-off in which small seeds dispersed to areas of low conspecific density but were less tolerant of density-responsive natural enemies. Our results indicate that a single trait can influence multiple components of performance and can mediate different trade-offs in co-occurring species. Complex and heterogeneous effects of a single trait in multidimensional niche space may favour inter-specific niche differentiation and coexistence

Data from: Multiple natural enemies cause distance-dependent mortality at the seed-to-seedling transition

Specialised natural enemies maintain forest diversity by reducing tree survival in a density- or distance-dependent manner. Fungal pathogens, insects and mammals are the enemy types most commonly hypothesised to cause this phenomenon. Still, their relative importance remains largely unknown, as robust manipulative experiments have generally targeted a single enemy type and life history stage. Here, we use fungicide, insecticide and physical exclosure treatments to isolate the impacts of each enemy type on two life history stages (germination and early seedling survival) in three tropical tree species. Distance dependence was evident for five of six species-stage combinations, with each enemy type causing distance dependence for at least one species stage and their importance varying widely between species and stages. Rather than implicating one enemy type as the primary agent of this phenomenon, our field experiments suggest that multiple agents acting at different life stages collectively contribute to this diversity-promoting mechanism

Data from: Multiple natural enemies cause distance-dependent mortality at the seed-to-seedling transition

Specialised natural enemies maintain forest diversity by reducing tree survival in a density- or distance-dependent manner. Fungal pathogens, insects and mammals are the enemy types most commonly hypothesised to cause this phenomenon. Still, their relative importance remains largely unknown, as robust manipulative experiments have generally targeted a single enemy type and life history stage. Here, we use fungicide, insecticide and physical exclosure treatments to isolate the impacts of each enemy type on two life history stages (germination and early seedling survival) in three tropical tree species. Distance dependence was evident for five of six species-stage combinations, with each enemy type causing distance dependence for at least one species stage and their importance varying widely between species and stages. Rather than implicating one enemy type as the primary agent of this phenomenon, our field experiments suggest that multiple agents acting at different life stages collectively contribute to this diversity-promoting mechanism

Appendix C. Figure showing critical thermal limits of beetles as a function of realized seasonality.

Figure showing critical thermal limits of beetles as a function of realized seasonality

Appendix A. Supplemental tables including a list of focal species used for thermal tolerance experiments and results from statistical models examining tolerance breadth and elevational range of beetles.

Supplemental tables including a list of focal species used for thermal tolerance experiments and results from statistical models examining tolerance breadth and elevational range of beetles