Evaluating the ability of wetland mitigation banks to replace plant species lost from destroyed wetlands

Offset policies are used world-wide as a means to compensate for destruction of ecosystems by requiring developers to restore or preserve ecosystems else-where. A key challenge to effectively implementing offset policies is ensuring ecological equivalence between impacted ecosystems and compensation sites, yet equivalence is rarely tested.  Using data from wetland mitigation banks and naturally occurring wetlands in Illinois, USA, we evaluated the ability of wetland offsetting to replace the plant species composition of wetlands impacted by development. We simulated the effects of three policy strategies used to promote equivalence: disallowing the re-placement of one type of habitat with another, requiring a greater ratio of offset area to impacted area, and spatially restricting wetland trades. Wetland banks replaced an average of 45% of the native plant species present in impacted natural wetlands. Allowing only in- kind habitat replacement and quad-rupling the offset ratio resulted in modest increases in the percentage of plant species replaced, but restricting trades to within counties did not.