The effects of beaver inhabitation and anthropogenic activity on freshwater wetland plant community dynamics on Mount Desert Island, Maine, United States America

I defined hierarchical relationships between environmental gradients and wetland plant community structure in a system of small, isolated wetlands, and then investigated how beaver and humans influenced these gradients at multiple scales. At the wetland scale, the principle gradients that organized plant communities were (1) wetland size, (2) Sphagnum cover and water chemistry, and (3) quadrat-scale microtopography indicative of small-scale hydrology. Vegetation within large wetlands was more predictable based upon climatic and geologic factors than that of small wetlands, which was more affected by historical accidents and intervening levels of local control associated with microtopography.; Wetland plant community structure reflected differences in time since beaver inhabitation; older wetlands had vegetation more typical of forested wetlands. Beaver impacts consisted of hydrologic modification and disruption of antecedent plant communities, but did not extend to water chemistry or wetland size modification (which affected type of emergent wetland community) in the time frame of study. Because hydrologic alterations alone did not change emergent wetland type, wetlands that have in the past been considered part of a hydrosere may in fact be located along different successional trajectories associated with wetland size and water chemistry. The number of ponds, duration of flooding, and types of wetland vegetation were closely tied to beaver population levels. When populations were large, beaver colonized wetlands that they inhabited for only a few years, resulting in vegetation more typical of forested wetland plant communities.; Both beaver and human activities disrupted forested wetland plant communities. By modifying wetland water chemistry and inhibiting Sphagnum colonization, human activity more extensively impacted the wetland environment than did beaver activity. The plant communities of human-impacted (degraded) wetlands were limited to either sedge meadow or marsh vegetation, whereas non-degraded wetlands tended to have fen communities. Two types of disturbance were necessary to shift wetland plant community trajectories into a degraded state: (1) water quality degradation due to humans and (2) vegetation disorganization due to intense human or beaver activity. The antecedent wetland plant community had to be sufficiently disorganized for water quality factors to move it into the degraded state of sedge meadow or marsh vegetation.