Spatiotemporal variation in deer browse throughout the range of a forest herb

Background/Question/Methods

Herbivory can shape the dynamics of plant populations, including effects on survival and reproduction, and act as a selection pressure that results in evolutionary change. White-tailed deer are a significant herbivore in North America that have been broadly documented to affect plants. Because deer directly affect fitness components, plant-deer interactions also have the potential to affect patterns of natural selection in populations. To investigate broad-scale patterns of deer herbivory, we surveyed the frequency and reproductive consequences of deer browse over three years in 17 populations of Campanulastrum americanum spanning the latitudinal extent of its range. To determine if patterns of browse are driven by plant attributes, we measured plant size and reproductive success within each population. Also, we compared reproductive success of uneaten and eaten individual plants to assess the ability of a plant to tolerate herbivory. 

Results/Conclusions

Even though deer are overabundant throughout the range of C. americanum, we found spatiotemporal variation in deer browse frequency (0-0.96, mean 0.46) and its effects on the reproductive success. The four southernmost populations experienced high levels of herbivory, and were responsible for generating a negative relationship between latitude and herbivory. Within a population, deer consumed larger plants. Among many populations and years, average fitness of browsed and uneaten plants was similar, suggesting that plants are tolerant to browse. However, since large plants have greater reproductive success and are more likely to be browsed, tolerance is likely affected by plant size. When plant size was accounted for, most populations did not fully compensate for browsing. There was no relationship between browsing intensity and tolerance, suggesting that browsing may be too variable to consistently select for tolerance, or that increases in deer density are too recent for increased tolerance to evolve.