An amphibian-eye-view of environmental offsetting

It ain’t easy being green, especially when your home’s been turned into (other people’s) houses. Andrew Milligan/PA

In its report published last week, the UK Parliament’s green watchdog, the Environmental Audit Committee, was far from convinced by the government’s proposed policy of biodiversity offsetting. The committee’s chair, Joan Walley MP, said it “risked giving developers carte blanche to concrete over important habitats.”

Road building, housing developments and other activities that degrade natural habitats bring with them inevitable, and often severe, costs to natural biodiversity. The idea behind the policy of environmental offsetting is to ensure that, where it is unavoidable, any adverse impact or environmental loss is matched by environmental gains. The overall goal being a neutral outcome of “no net loss”. But the few long-term studies have been mixed.

The father of modern chemistry, Antoine Lavoisier, argued that in any process “nothing is lost, nothing is created, everything is transformed”. While rational, this chemical principal overlooks many aspects that apply when ecosystems are affected. From the tenets of chemical reactions, or more so in an economic model where revenues usually offset the loss associated with the transformation, developers and industrial groups have recently embraced the term of “environmental offsetting” to persuade planning authorities to let them impact natural habitats by offering to create new, “equivalent” habitats as “replacements”. But they such surrogates able to meet the aim of “no net loss”?

Amphibians are suffering what has been called the greatest extinction since the dinosaurs. Changing agricultural practices, loss of habitat and increased road traffic have all contributed to shocking population declines.

The United Nations Millennium Ecosystem Assessment estimates that 10-30% of mammal, bird and amphibian species are currently threatened with extinction due to human activities.

To help understand which features are important for biodiversity when amphibian populations are disturbed by human development, we followed the colonisation success of six frog species across multiple replacement ponds, created to offset the destruction of nearby ponds by highway construction in France.

These species’ population size and distribution was recorded immediately before and for four years after the replacement ponds were constructed. Initially, both the number of different species and their population numbers declined. But, eventually, both returned to pre-construction levels. The results suggest that the species diversity was best maintained in a landscape that contained several types of vegetation in relatively large, open wetland areas. More ponds were colonised over time, and in some cases total population numbers increased.

So this study over several years indicates that the replacement ponds used in a case of biodiversity offsetting can be successfully designed around simple habitat features such as the variety of plants. This provided clear benefits for a range of amphibian species, and their thriving in turn had a positive effect on local biodiversity.

However, not all offsetting stories have the same charming outcome. In many other instances newly manufactured habitats have not met Lavoisier’s principle and wildlife populations, species diversity and even whole ecosystems have been lost. In fact, in this business, and despite being under-reported, failures are as abundant as successes. This is especially the case for wetland habitats, one of the more common ecosystems brought under tentative offsetting. For instance, one study showed that of 31 mitigation sites in Indiana, 71% of forested sites, 87% of wet meadow areas, and 42% of shrub areas failed.

In a society where resources are limited, the costs associated with environmental offsetting and the need to assess its success will always out-compete those steps, often political, that would be required for better environmental safeguards. In the fashion of sustainable development, biodiversity offsetting represents an important tool for maintaining environmental values in those situations where social and economic development take precedence despite its impact on the environment.

But, if the policy is used as a fig leaf by developers as a means to an end, and if failing to achieve the goal of “no net loss” brings no consequences or penalties, then one has to consider who offsetting really benefits. And, more broadly, the question must be asked as to whether such a policy is acceptable at all, at a time when biodiversity is under unprecedented pressure.

So are we to throw out the baby with the bath water? While “no net loss” is plausible, the complexity and variety of ecosystems, and the limited understanding we often have of their role and their functioning, make achieving this a utopian fantasy. For example, as the UK’s Environmental Audit Committee pointed out, it would be hugely destructive to allow – as the policy currently does – offsetting to be applied to ancient woodland and Sites of Special Scientific Interest. No offsetting system could protect or adequately replace such complex, long-established, ecosystems.

The costs to biodiversity will far outweigh the benefits of environmental offsetting and in many cases the main beneficiaries would be perhaps the developers, who could bask in the warm glow of good conscience, good PR, and brownie points from the government for supporting the scheme.