Vertebrate herbivory weakens directional selection for earlier emergence in competition

The timing of seedling emergence is strongly linked with fitness because it determines the biotic and abiotic environment experienced by plants in this vulnerable life stage. Experiments and observations consistently find that earlier-emerging plants have a competitive advantage over those emerging later. However, substantial genetic and phenotypic variation in emergence timing is harbored within and among plant populations, making it important to characterize the selective agents-including biotic interactions-that contribute to this variation. In seasonal herbaceous communities, we hypothesized that consumption of early-emerging individuals by vertebrates could weaken the strength of directional selection for earlier emergence in competitive environments. To investigate this, we carried out phenotypic selection analyses on emergence timing in two California grass species, the native Stipa pulchra and non-native Bromus diandrus, growing in intraspecific competitive neighborhoods with and without vertebrate herbivore exclusion. Vertebrate herbivores consistently weakened directional selection for earlier emergence. Our results demonstrate that vertebrate herbivores play an underappreciated selective role on phenology in plant populations, with implications for contemporary evolution, such as the potential of species to adapt to global environmental changes.