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Although the evolution and diversification of flowers is often attributed to pollinator-mediated selection, interactions between co-occurring plant species can alter patterns of selection mediated by pollinators and other agents. The extent to which both floral density and congeneric species richness affect patterns of net and pollinator-mediated selection on multiple co-occurring species in a community is unknown and is likely to depend on whether co-occurring plants experience competition or facilitation for reproduction. We conducted an observational study of selection on four species of Clarkia (Onagraceae) and tested for pollinator-mediated selection on two Clarkia species in communities differing in congeneric species richness and local floral density. When selection varied with community context, selection was generally stronger in communities with fewer species, where local conspecific floral density was higher, and where local heterospecific floral density was lower. These patterns suggest that intraspecific competition at high densities and interspecific competition at low densities may affect the evolution of floral traits. However, selection on floral traits was not pollinator mediated in Clarkia cylindrica or Clarkia xantiana, despite variation in pollinator visitation and the extent of pollen limitation across communities for C. cylindrica. As such, interactions between co-occurring species may alter patterns of selection mediated by abiotic agents of selection.
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This content will become publicly available on June 2, 2027
Trait-mediated interactions drive local diversity
Identifying overarching processes that maintain biodiversity in natural communities remains a challenge in ecology. Although functional traits help explain regional species distributions, they often fall short at the local community level. We investigate whether traits can offer mechanism-based insights into local diversity maintenance due to associations with the sign and strength of plant interactions. We examine the effect of 12 plant functional traits on the sign and strength of pairwise species interactions across two Mediterranean annual plant communities. Results show that traits mediate a spectrum from facilitative to competitive interactions and are influenced by neighbor density and identity. At low densities, species with conservative resource-use traits are consistently stronger facilitators than acquisitive species. The traits of the focal species, however, explain facilitation at low densities better than the traits of neighbors or the pairwise differences in traits. When neighbor density increases, facilitation switches to competition, a pattern we hypothesize reflects a density-mediated reduction in how much neighbors’ traits drive interactions. Moreover, species frequently receiving heterospecific facilitation also display traits associated with self-competition and low intrinsic population growth rate. This study of community-level trait sorting provides insights into the persistence of co-occurring populations, showing trait-based generalizable insights into the local context dependency of species interactions.
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- Award ID(s):
- 2047239
- PAR ID:
- 10696901
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 123
- Issue:
- 22
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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