Spatiotemporal variation in deer browse and tolerance in a woodland herb

Abstract

Herbivory can shape the dynamics of plant populations, including effects on survival and reproduction, and is in turn affected by environmental factors that vary in space and time. White-tailed deer are significant herbivores in North America that have been broadly documented to affect plant reproductive success. If variation in the frequency and impact of herbivory by deer correlates with a broad-scale latitudinal gradient, climactic effects may be important for shaping plant-herbivore interactions, Alternatively, a lack of broad-scale gradients would suggest local factors such as plant community composition and deer densities are affecting herbivory. To investigate broad-scale patterns of deer herbivory, we examined the frequency and reproductive consequences of deer browse over three years in 17 populations of Campanulastrum americanum spanning the latitudinal extent of its range. 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 plant reproductive success. The four southernmost populations experienced high levels of herbivory, and were responsible for generating a negative relationship between latitude and herbivory. In general, patterns of variation in the frequency and impact of herbivory across the entire latitudinal gradient pointed to the importance of local rather than broad-scale factors. Within a population, deer consumed larger plants. Across many populations and years, average fitnesses of browsed and uneaten plants were similar, suggesting that plants are tolerant to browse. However, since large plants have greater reproductive success and are more likely to be browsed, tolerance may be influenced 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.

Latitudinal patterns of herbivore pressure in a temperate herb support the biotic interactions hypothesis

“…Variation in deer herbivory likely is explained more by idiosyncratic features of the immediate patch surroundings than climatic factors correlated with latitude (Prendeville et al . ), so it was not quantified for the purpose of testing our hypotheses.…”

“…P. americana experiences occasional deer herbivory (less than 5% of branches at our sites), which is easily differentiated from insect herbivory because deer strip leaves. Variation in deer herbivory likely is explained more by idiosyncratic features of the immediate patch surroundings than climatic factors correlated with latitude (Prendeville et al 2015), so it was not quantified for the purpose of testing our hypotheses.…”

Multiple glacial refugia lead to genetic structuring and the potential for reproductive isolation in a herbaceous plant

“…Th ese variables were selected to encompass those likely to be most biologically important to C. americanum given its growing season and susceptibility to drought ( Galloway, 2002 ;Prendeville et al, 2015 ), followed by removal of variables with a correlation coefficient >0.8 ( Sheppard, 2013 ).…”

Lineage and latitudinal variation in Phragmites australis tolerance to herbivory: implications for invasion success

“…To date, most investigations of the LHDH have focused solely on resistance mechanisms, largely ignoring other defense strategies such as plant tolerance (Anstett et al ). To our knowledge, only five studies have examined tolerance to herbivory along a latitudinal cline, with two finding support of the LHDH for resistance and the opposite pattern for tolerance (Lehndal and Ågren , b), and others finding no relationship between latitude and tolerance (Więski and Pennings , Prendeville et al , Sakata et al ). However, no studies have considered how tolerance in invasive species and co‐occurring native species might differ across a broad geographic range and contribute to invasion success.…”