No abstract

Offset policies are used world‐wide as a means to compensate for destruction of ecosystems by requiring developers to restore or preserve ecosystems elsewhere. 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 replacement 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 quadrupling the offset ratio resulted in modest increases in the percentage of plant species replaced, but restricting trades to within counties did not. Synthesis and application. Many of the plant species present in wetlands that are vulnerable to development were absent from wetland mitigation banks. Policies intended to ensure equivalence between impacted wetlands and banks did not ensure protection of biodiversity. If preservation of taxonomic diversity is to be a goal of offsetting, offset policies will need to ensure that offset sites have similar biophysical conditions to the ecosystems they replace.