Archive for the ‘Grasslands’ Category

The principle of habitat substitutability

Monday, November 14th, 2011

Biodiversity offsets, whether they focus on species (and their habitat requirements), habitat types, ecosystem properties or ecosystem services, are all based on the idea that the elements they target are – to a degree – substitutable: e.g. the breeding habitat of a particular bird species here can be substituted by an “equivalent” habitat somewhere else.

In an interesting article*, recently in-press in Biodiversity & Conservation, Kate Sherren and her former colleagues at ANU present survey results on how land-holders in rural Australia view the substitutability of different arrangements of trees and woodlands on their properties. This can be very important for aligning conservation policy such as offset schemes with the values and experience of the people they target.

The rationale for the survey is that at the farm level, substitutions between these elements are made daily, albeit at a small scale: a patch is planted, scattered trees are cut-down etc. These decisions could reveal farmer’s views on their value and their substitutability. The survey found that farmers could be divided into three groups:

  • Farmers, mainly older and less educated, who valued a “tidy” farm but did not care for the specific arrangements of trees and woodlands
  • Farmers who strongly supported the need for a diversity of tree cover arrangements on their land. Because of limited financial or time resources, these views were only rarely translated into concrete action.
  • Farmers who preferred woodlands and connective strips over scattered trees. This group included those that also crop their land using machinery.
  • What can be done with this knowledge? Well, the authors argue that the main risk with widespread offsetting schemes is that tree cover arrangements will homogenize, towards wooded paddocks that are easier to create, maintain, monitor etc. This could have unintended consequences in terms of landscape-level heterogeneity in habitat for species or ecosystem services, especially those related to scattered trees.

    Scattered trees in Australia

    To avoid this homogenization, specific policies could be devised that target the first two types of land-holders, to get them to increase heterogeneity on their land.

    This could be done by allowing land-holders to actively suggest measures in favour of tree cover (and bid for funding) such as “crash grazing”, adding coarse woody debris, weed control or planting of under-storey species… These different measures are conducive to improving the “quality” of existing woodland rather than focusing on area-based measures such as grazing exclusion which could tend to homogenize the landscape and have the major caveat of taking land out of production which could be called into question in the long-term.

    Measures targeting the management of existing trees and woodlands also have drawbacks. The main one is how long-lasting they can be made, and thus how long the binding contracts have to be made. The USDA’s Conservation Reserve Program is one long-lasting program that can provide inspiration for such renewable management-based contracts with land-holders.

    The Conservation Reserve Program - a long-lasting contract-based PES scheme

    Another difficulty with management based measures such as those outlined above is measuring the actual “gain” they generate, so that they can be sized adequately to offset impacts elsewhere. This probably requires a conservative approach – i.e. over-sizing of offsets – as well as further research on baseline trends and short- and long-term effects of these management changes.

    * Sherren K., Yoon H-J., Clayton H. & Schirmer J. (2011): Do Australian graziers have an offset mindset about their farm trees? Biodiversity & Conservation, in press.

    Long-term floodplain meadows cannot realistically be recreated!

    Friday, October 28th, 2011

    Biodiversity offsets are making headlines as a new instrument or tool for biodiversity conservation in the UK. DEFRA defines offsets as actions in favour of biodiversity that are carried-out in compensation for planned impacts (e.g. from development) and provide a measurable outcome. Whenever possible, offsets should target the same biodiversity components (species, habitat types etc.) as those that will be impacted. This raises the question of their “restorability”.

    In a recent paper published in the Journal of Applied Ecology, Ben Woodcock, Alison McDonald and Richard Pywell of CEH investigate the restorability of long-term floodplain meadows on agricultural land in South-Eastern England. Using an 22 years old restoration experiment, they show that given the current restoration trajectory, it would take over 150 years for the former arable land to have a species composition close to that of long-term floodplain grasslands. Even when being less restrictive in terms of restoration goals, i.e. focusing on the “types” of species (described using their morphological and reproductive characteristics or “traits”), it would take over 70 years. This is slightly more realistic but still a very long term prospect.

    Ecosystems or habitat types for which restoration is a (very) long-term endeavour might fall outside the scope of offset schemes. As the authors say:

    any compensation scheme proclaiming they can replace floodplain meadows lost to development (i.e. gravel extraction) is being wholly unrealistic.

    As well as actually avoiding the destruction of hard or impossible to replace ecosystems and habitat types, these findings raise two issues:

  • can the destruction of these habitats be offset by restoring or enhancing degraded habitats (of the same type). This amounts to exchanging area (for which there will be a net loss) by quality (for which there would be no net loss). Is this acceptable? Another option considered by DEFRA is to use out-of-kind offsets.
  • how can the delays associated with restoration or enhancement of habitat types be taken into account in the design and sizing of biodiversity offsets. DEFRA proposes to use “multipliers” for this but these will probably be very hard to justify, ecologically, as ensuring that offsets lead to no-net-loss of the particular target habitat type.
  • Hopefully, the pilot scheme launched by the UK government will give the opportunity to test these solutions…

    On time-lags and location selection in offsets

    Monday, October 10th, 2011

    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.