Rodent seed predators and a dominant grass competitor affect coexistence of co-occurring forb species that vary in seed size

Propagule size and number often vary by several orders of magnitude among co-occurring plant species. Explaining the maintenance of this variation and understanding how propagule size contributes to coexistence remain a central challenge for community ecologists. The dominant paradigm is that a competition–colonization trade-off maintains interspecific variation in seed size, but empirical support is limited and other coexistence mechanisms, such as size-dependent seed predation, have not been examined. We examined how seed size, fecundity, and other functional traits of 18 co-occurring perennial forbs trade-off with both vulnerability to rodent seed predation and competitive response to a community-dominant perennial bunchgrass. We added seeds of these species to 1 m² plots at 10 sites where we factorially manipulated rodent seed predation and competition from the community dominant, Festuca campestris. For a given plot, seeds of each species were added at densities that reflected the fecundity (i.e., per capita seed production) for each species. Moreover, we varied total seed density among plots by adding seeds at one of five relative densities (0, 0.25. 0.5, 1.0, and 1.5 times each species’ fecundity) to examine how fecundity affected overall recruitment for each species. There was a trade-off between seed size and fecundity, as expected, but larger seed size was also associated with greater plant height, lower C/N ratios, and lower water use efficiency, suggesting that seed size represented a “resource acquisitive” trait syndrome (as quantified by trait principal component scores). In the field experiment, the suppressive effects of seed predation on seed recruitment rate increased with increasing seed size. In contrast, smaller seeded species with less resource-acquisitive traits were more negatively affected by competition from F. campestris than were species with more resource acquisitive traits. Synthesis. While the competition–colonization trade-off has been the predominant mechanism thought to maintain coexistence among species that vary in fecundity and seed size, our work suggests that susceptibility to rodent seed predation and competitive response to community dominants represent alternative mechanisms that can differentially influence plant recruitment of species based on their seed size and associated traits.