Abstract Detail


Waterton, Joseph [1], Cleland, Elsa [1].

Leveling the playing field: Mammalian herbivores weaken selection for earlier emergence but not larger seeds in two California grasses.

In competitive environments, small advantages in size at the seedling stage can be compounded over time to result in disproportionately large impacts on plant fitness. Emergence timing and seed size are among the earliest traits expressed in plant development that influence initial seedling size, yet both show substantial variation within and among species. Herbivores have sizable impacts on plant fitness, yet their potential to alter patterns of selection on emergence timing and seed size, and thus contribute to variation in each trait, is relatively understudied. In herbaceous plant communities with seasonal growth, generalist mammalian herbivores may have greater impacts on individuals that emerge from larger seeds and emerge earlier in the growing season. Emerging earlier can increase the duration of exposure to herbivores, and the size advantage gained through earlier emergence and larger seeds may result in increased apparency as well as greater relative impacts of grazing. We carried out a field experiment to test the hypothesis that mammalian herbivores weaken selection for earlier emergence and larger seeds. We planted seeds of two California grass species, the native perennial Stipa pulchra and exotic annual Bromus diandrus, in plots in which we either excluded or allowed access to mammalian herbivores. We conducted phenotypic selection analyses, in which we evaluated the relationship between each trait and fitness, measured as aboveground biomass for the perennial S. pulchra and seed production for the annual B. diandrus. In both species, we observed directional selection for earlier emergence and larger seeds across both exclusion treatments. Furthermore, in both species, exposure to herbivores weakened directional selection for earlier emergence, but not larger seeds, independent of differences in mean fitness between exclusion treatments. These results suggest that, in addition to impacting plant fitness, mammalian herbivores may play a cryptic role in influencing selection on traits that drive differences in seedling size and thus the outcomes of plant competition.

1 - University of California San Diego, Ecology, Behavior and Evolution Section, 9500 Gilman Dr., La Jolla, CA, 92103, USA

Seed size
Emergence time

Presentation Type: Oral Paper
Number: 0003
Abstract ID:525
Candidate for Awards:None

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