The response of wetland quality indicators to human disturbance indicators across the United States
We analyzed data from 1138 wetland sites sampled in 2011 across the conterminous United States as part of the National Wetland Condition Assessment (NWCA) to investigate the response of indicators of wetland quality to indicators of human disturbance at a continental scale. Wetland response indicators were a multimetric index of vegetation condition (VMMI), percent relative cover of alien plant species, soil lead and phosphorus, and water column total nitrogen and total phosphorus. Site-level disturbance indices were generated from field observations of numerous disturbance from 13 plots within a circular 140-m radius area around the randomly chosen site. Summary indices were calculated representing disturbances for ditching, damming, filling, hardening, vegetation replacement, and vegetation removal. Landscape-level disturbance associated with agricultural and urban land cover, roads, and human population were based on GIS data layers quantified in 200, 500, and 1000-m circular buffers around each sample site. Among these three buffer sizes, the landscape disturbance indicators were highly correlated and had similar relationships with the response indictors. Consequently, only the 1000-m buffer data were used for subsequent analyses. Disturbance-response models built using only landscape or only site-level disturbance variables generally explained a small portion of the variance in the response variables (R2<0.2), whereas, models using both types of disturbance data were better at predicting wetland responses. The VMMI was the response variable with the strongest relationship to the disturbances assessed in the NWCA (national model R2=0.251). National multiple regression models for the soil and water chemistry, and percent alien cover responses to disturbance indices were not significant. Logistic regression modeling was more informative, resulting in significant national and wetland subpopulation-specific models predicting site presence/absence of alien species and/or the concentration of lead in wetland soils above background.