Leaf Bites to Landscapes: Scaling the Effects of Ecologically Significant Herbivores
Open Access DepositedHerbivores are keystone fixtures in any ecosystem as they alter the abundance, diversity, and community composition of vegetation communities through preferential herbivory. Although the direct impact of an herbivore occurs on individual plants at a fine-scale and these accumulate over time and spaces, herbivores also respond to spatial heterogeneity in the landscape. What an herbivore chooses to eat is not only influenced by spatial heterogeneity in the landscape, but it can also directly impact the vegetation dynamics on the landscape that produce that spatial heterogeneity. My dissertation work focuses on the direct impacts of herbivory across the landscape focusing on three large-bodied, numerous, and ecologically-influential herbivores: white-tailed deer (Odocileus virginianus), sika deer (Cervus nippon), and resident Canada goose (Branta canadensis). In Chapter 2, I examined the impact resident Canada geese have on altering the landscape of tidal freshwater wetlands and the overall success of tidal freshwater wetland restoration. I found that plant communities exposed to herbivory by resident Canada geese are vastly different than communities protected from herbivory pressure. When communities were protected in excloures, total percent cover, percent cover of annuals, species richness, and diversity increased compared to paired unfenced plots. Not only did plant cover and diversity increase once herbivory was excluded but the overall community composition shifted in response to fencing treatment becoming distinct from the plant communities left unprotected. In Chapter 3, I examined the effectiveness of culling resident Canada geese in recovering tidal freshwater wetlands heavily impacted by resident geese herbivory. I found that unprotected plots were able to recover after the goose population control measures were enacted with total cover, cover comprising annual species, species richness and Shannon-Wiener Diversity Index increasing in response. The overall trajectory of recovery was influenced by initial vegetative state, recovery of vegetative cover occurred earlier in initially vegetated plots and recovery of diversity metrics occurred earlier in initially unvegetated plots. While the community assemblages in unfenced plots responded to population reductions in resident geese, they still remain distinct from the community assemblages in fenced plots. In Chapter 4, I looked at how sika deer shape the plant communities of coastal forests adjacent to migrating marshes and test whether sika deer reduce biotic resistance of the native understory plant community to the invasive common reed (Phragmites australis). I conducted a cage exclosure experiment of the forest understory plant community directly adjacent to migrating marshes and transplant study of Phragmites australis. Over the 5-year study, I found plots exposed to sika deer herbivory decreased in overall plant cover, species richness and diversity. Plant community composition was also significantly different in plots exposed to the top-down pressure compared to plant communities protected in exclosures. However, sika deer herbivory on Phragmites had a greater impact on Phragmites survival than the biotic resistance provided by the recovered plant community, highlighting the possibility of apparent competition between the native understory plant community and Phragmites. Lastly in Chapter 5, I tested the role that adjacent habitats play in crop damage inflicted by white-tailed deer. Through combining observational surveys of crop damage in agricultural fields and landscape analysis of the area surrounding each field, I was able to test how distance to edge, edge composition, landscape and class metrics influence predicted herbivory. I found that field edges adjacent to Conservation Reserve Program (CRP), marsh and forest had higher rates of herbivory than other edge types and herbivory declined as distance to the edge in increased. The mean area and cohesion index of CRP and forest in the landscape as well as core area index of the landscape altered the herbivory experienced by the field. Overall, my dissertation represents a broad approach to understanding how herbivores are not only influenced by the landscape but impact the future landscape through their pronounced impact on vegetation dynamics.
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