On time-lags and location selection in offsets

Ascelin Gordon and his colleagues from Melbourne recently went through an interesting modelling exercise. They modelled the long-term effects, in both space and time, of different offset policies concerning urban development impacts on native grasslands around the city.

Their modelling explicitly included uncertainties , following the approach described by Langford et al. (2009). These uncertainties were both ecological (edge effects on conservation value, grassland response to management and offsetting, etc.) and political: which offset policy?

They compared five different offset policies:

  • No change
  • Development without offsets
  • Non strategic offsets (wherever, whenever)
  • Strategic offsets (offsets are located together, in designated areas)
  • Strategic immediate offsets (offsets are effective at the start of simulations, a.k.a. habitat banking)
  • Their conclusions are that offset policies that include spatial and temporal constraints on offsets give the best conservation outcomes. They also point out the obvious: the selection of the baseline is central to any assessment of policy outcomes.

    whether (or when) [policies] achieve the objective of a “net gain” completely depends on the choice of baseline.

    It might be obvious but it is certainly tricky when looking at policies that involve long-term ecological dynamics…

    To find out more, check out their paper in Environmental Modelling & Software*. A pdf can be downloaded here.

    * Gordon A., Langford W.T., Todd J.A., White M.D., Mullerworth D.W. & Bekessy S.A. (in press): Assessing the impacts of biodiversity offset policies. Environmental Modelling & Software, in press.

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