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:
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.
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.
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.
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