Plant communities in wetland mitigation banks surpass the quality of those in the most degraded, naturally occurring wetlands, but fall short of high-quality wetlands

Abstract

The U.S. Clean Water Act requires that development projects causing negative impacts to wetlands must provide compensation for wetland losses through the wetland mitigation process. Compensation can be achieved through the purchase of credits from wetland mitigation banks, which are large wetland restoration projects constructed by third-party bank sponsors. To evaluate how effectively wetland mitigation banks have achieved the goal of “no net loss” of wetland resources, we compared mitigation banks to natural wetlands in the Chicago (Illinois, USA) region. We surveyed vegetation plots in 20 mitigation banks to compare vegetation metrics and composition between banks and 114 natural wetlands, representing a gradient of ecological quality, in northern Illinois. Based on metrics of species richness and floristic quality, mitigation banks possessed wetland plant communities of greater conservation value than the lowest quality, degraded, natural wetlands, but banks were not close to reaching equivalence with high-quality, reference, natural wetlands. Overall, the plant communities in banks were distinct from those of natural wetlands, a condition that appears to be driven by the abundance of the non-native species Typha angustifolia and Phragmites australis in mitigation banks. We found some evidence that dominance by native species may be lower in older banks, but otherwise did not find evidence for a relationship between vegetation metrics and bank age. These results will help those involved with wetland mitigation and similar offsetting programs assess whether compensation sites meet no-net-loss goals, informing goal setting, monitoring, and offsetting policies.