The interactions between mammalian herbivores and plants can be complex. While some plant species rely on zoochoric seed dispersal, others can see their spatial distribution negatively impacted by herbivory pressure. Such complex interactions are particularly present among prairie plant communities, but it is unclear how herbivores impact the dispersal and spread of flowering prairie plant populations. To investigate this, we studied four flowering prairie species: Aster ericoides, Baptisia alba, Eryngium yuccifolium, and Penstemon digitalis to determine how the presence or absence of mammalian herbivores influenced the spatial distribution of each plant species over a seven-year period. This study occurred on a four-acre prairie restoration site in Ames, Iowa with four 32x32m2 plots that were fenced to exclude vertebrate herbivores (specifically deer and voles), and four 32x32m2 plots that were unfenced and open to herbivory. Seeds of these four species were added to the center of each plot in 2012 and the plots were surveyed in 2019 to assess establishment and spread. Every individual of the four target species was recorded in all eight plots using a Trimble Geo7x.
Results/Conclusions
Our results show that mammalian herbivory pressure structures plant communities by impacting abundance and dispersal patterns in a tallgrass prairie. Overall plant abundance of Aster ericoides, Baptisia alba, Eryngium yuccifolium, and Penstemon digitalis was significantly higher in the absence of vertebrate herbivores with an average of ~100 plants per plot compared to the open plots, which had an average of ~60 plants per plot. Penstemon digitalis had the largest variance having over twice as many individuals without herbivores, while the other species did not show a significant difference individually. However, despite lower abundance, establishment for three of the four species was farther from the location of the original seed addition when herbivores were present than when they were excluded. Specifically, Baptisia alba and Eryngium yuccifolium established twice as far in the presence of herbivores. Aster ericoides was found at the same distance in low and high herbivore pressure treatments, but the density was higher at a farther distance with herbivores. Penstemon digitalis dispersed similarly and showed no difference between the treatments. Collectively, this suggests that mammalian herbivores limit abundance but may facilitate spread, highlighting the importance of considering these complex interactions in active prairie management.