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Editor's Choice - Habitat offsets in the face of agricultural intensification: why ’business as usual’ banking does not work

April 2010 (Issue 47:2)

Maron, M., Dunn, P.K., McAlpine, C.A. & Apan, A. (2010) Can offsets really compensate for habitat removal? The case of the endangered red-tailed black-cockatoo. Journal of Applied Ecology, 47, 348-355.

Habitat loss and degradation are among the most prominent forms of environmental deterioration in the face of development pressures (e.g. Tilman et al. 1994; Krebs et al. 1999). The responses of wildlife range from adapting to change through behavioural or other mechanisms, to enforced displacement, sometimes leading to detrimental impacts on other species, and in extreme cases resulting in species extinctions. Ecological studies have provided deep insights into these processes, with historical, observational, experimental and modelling studies providing the evidence base. Typically, at some stage during land use change or intensification the loss of habitat causes the dependent species to hit a ‘bottleneck’, beyond which there are insufficient breeding, feeding, roosting or ‘sanctuary’ areas to sustain individuals. Depending on their demography, the rate of decline may be sudden (especially for short-lived and highly philopatric species) or gradual - but punctuated for longer-lived and more dispersive species.

Image courtesy of Bob McPherson, 2009: Endangered south-eastern Red-tailed Black-Cockatoos (Calyptorhynchus banksii graptogyne) feed in old paddock trees, which are rapidly disappearing. With the advent of environmental impact assessments, ecologists have become adept in providing advice on compensation and mitigation measures in the face of development proposals. In the former, the habitat lost or damaged is compensated for by the provision of at least the equivalent alternative resource; and in the latter, measures are put in place to avoid, reduce, offset or otherwise mitigate the losses due to development (e.g. ten Kate et al. 2004).

Applied ecologists carry out important work to address these issues, and typically have to make difficult judgments on the feasibility of compensation or mitigation measures, often against backdrops of the precautionary principle being applied or risks of legal challenge. Consequently, studies such as that by Maron et al. (2010), which critically evaluate the effects of development offsets, are important. In agriculturally developed regions, so-called ‘habitat banking’ is deployed whereby losses of habitats in one area are offset through actions in other areas so that, overall, there are no net adverse impacts (ten Kate et al. 2004; Darbi et al. 2009). Whilst this sounds sustainable, there are concerns over offsetting like with like. These include the effect of time delays, risks of failing to replicate key ecological functions and processes, and the likelihood of overlooking key elements of population ecology (see ten Kate et al. 2004; Quigley & Harper 2006).

For this issue’s Editor’s Choice, Maron et al. (2010) provide an elegant evaluation of the potential consequences of habitat removal in SE Australia for the availability and use of habitats by the endangered red-tailed black-cockatoo Calyptorhynchus banksii graptogyne. These birds rely on scattered large buloke Allocasuarina luehmannii trees, which have declined in range due to agricultural intensification. Indeed, the remaining trees are under so much threat that almost half have been lost over the last 40 years.

The authors developed a range of scenarios depicting how the woodland will respond over 150 years to ‘business as usual ‘ intensification of the land. In particular, they investigated whether habitat offsets could genuinely compensate for the removal of large old trees, which are key feeding habitat for the endangered red-tailed black-cockatoo.

Importantly, in all cases, Maron et al. (2010) predict that offsets will not prevent woodland losses. However, when they allowed for active replanting allied with reduced felling rates as well as protection of more ‘at risk’ mature trees, they predicted reduced deleterious impacts on the trees and dependent cockatoos – so long as overall losses of habitat were avoided.

There is a basic ecological lesson here: you have to adopt a long-term perspective for habitat banking in support of long-lived habitat features. This sounds obvious, but judgments often do not account for this. And of course, once habitat fragments are lost, or bottlenecks reached, species conservation measures move into recovery or reintroduction phases – which are expensive and inherently risky.

Des B. A. Thompson
des.thompson@snh.gov.uk

References

Darbi, M., Ohlenburg, H., Herberg, A., Wende, W., Skambracks, D. & Herbert, M. (2009) International Approaches to Compensation for Impacts on Biological Diversity. Final Report. Dresden & Berlin, Germany.

ten Kate, K., Bishop, J. & Bayon, R. (2004) Biodiversity Offsets: Views, Experience, and the Business Case. IUCN, Gland, Switzerland and Cambridge, UK and Insight Investment, London, UK.

Krebs, J.R., Wilson, J.D., Bradbury, R.B. & Siriwardena, G.M. (1999) The second silent spring? Nature, 400, 611–612.

Quigley, J.T. & Harper, D.J. (2006) Compliance with Canada’s Fisheries Act: a field audit of habitat compensation projects. Environmental Management, 37, 336–350.

Tilman, D., May, R.M., Lehman, C.L. & Nowak, M.A. (1994) Habitat destruction and the extinction debt. Nature, 371, 65–66.