Beaver and human activity affect Sphagnum communities in isolated wetlands in coastal Maine, USA

Background/Question/Methods

Sphagnum (peat moss) is an important ecosystem engineer that substantially influences ecosystem stability and plant community organization in wetlands.  Although the tolerance of Sphagnum species to various environmental factors is generally well known, we know less about the interaction between these environmental factors and disturbance on Sphagnum occurrence and community composition. We surveyed Sphagnum communities in 39 isolated wetlands on Mount Desert Island, Maine which varied in the extent of beaver and human disturbance, and environmental variables, such as water chemistry and hydrologic parameters, to better understand the controls on Sphagnum species richness and abundance.

Results/Conclusions

The gamma and beta diversities for Sphagnum communities on Mount Desert Island are moderately high (42 species, β_W = 5.259 and β_H1 = 13.8). Sphagnum abundance was significantly lower in wetlands with either human (F_1,36 = 17.54, P < 0.001) or beaver activity (F_1,36  = 13.65, P < 0.001). The interaction term was also significant (F_1,36 = 4.26, P = 0.046), because beaver activity had little additional effect in human-disturbed wetlands. Sphagnum abundance increased with time since beaver abandonment of a wetland, but then declined as the wetland returned to a forested state. Wetlands with specific conductivity levels higher than 400 µS/cm typically had little or no Sphagnum. Wetlands with deeper water tables were more likely to be acidic and had Sphagnum communities distinctive from wetlands with shallower or surface water tables and higher pH and water ion content. Both beaver (F_1,36  = 3.91, P = 0.056) and human activity (F_1,36  = 4.72, P = 0.036) decreased Sphagnum species richness within wetlands. These results indicate that wetlands with high specific conductivity and active beaver colonies have little chance of providing habitat for Sphagnum. However, beaver wetlands can recover Sphagnum communities over time following wetland abandonment.