Scenario-based modelling of local land-use policy choices to achieve No Net Loss of wetland functions in the face of cumulative impacts

Abstract

Going beyond project by project approaches to biodiversity offsetting is challenging and local governments are struggling to find ways to achieve the no net loss (NNL) goal at the landscape level. In the Grenoble area (French Alps), land-use change was modelled, up to 2040, based on recent dynamics of urban expansion. We added offsetting of the impacts of urbanization on wetlands to the model, to investigate the consequences of various types of offsetting. In particular, we studied the effects of a recent shift towards "functional" loss-gain metrics to demonstrate ecological equivalence and NNL, rather than area-based calculations, and towards offsetting within the sub-catchments where impacts occur. Simulations were based on spatially-explicit modelling and take into account existing planning rules and recommendations to guide the location of offsets.

Two mechanisms were tested for offset implementation: (a) case-by-case compensation where each developer compensates its impacts, resulting in many restored wetlands of various sizes distributed across available land, and (b) an aggregated approach where larger sets of adjacent parcels of land are used to compensate for several projects at once, generating larger wetland units. In addition, two different methods for sizing offsets were compared: (i) an area based method whereby a coefficient is applied to the impacted area to determine offset size, and (ii) a method where losses and gains of ecological function are calculated and offsets sized so as to generate enough gains to achieve functional NNL. Wetland function was assessed using detailed information on agricultural practices (rotations) and expert opinion from agronomists and ecologists. Mechanisms and methods were combined into four biodiversity offsetting scenarios, and 5000 simulations were run for each. A sensitivity analysis of several methodological choices was carried out.

Results show that a focus on wetland area leads to net losses of wetland function from development. Current regulations for wetlands, mixing a focus on area and functions, lead to better results than the area-based approach with a net gain of ecological function. With a function based approach the NNL goal can be met on a smaller offset area. Finally, aggregated offsets also achieve NNL but need more area, given the constraints of restoring neighboring parcels. Aggregated offsets are increasingly recommended and used worldwide to ensure longer-lasting biodiversity gains from more ambitious and larger-scale restoration projects, and to enable stronger governance to be put in place, as monitoring and enforcement is easier on a single offset site. Striking the right balance between case-by-case and aggregated function-based biodiversity offsets should lead to improved outcomes for maintaining or restoring wetland functions within watersheds, so as to enable them to provide necessary ecosystem services.