bee-on-a-flowerThis is guest blog contribution from EJ Milner-Gulland, Professor in Conservation Science at Imperial College, London. EJ’s research examines the dynamics of ecological and social systems working at the interface between ecology and human behavior. Her recent publication highlights include a comprehensive review of biodiversity offsetting in theory and practice. Through her collaborative projects, she has established international links on every inhabited continent.

EJ will be bringing her extensive research experience in the field of conservation and biodiversity offsetting to our PolicyLab event on Tuesday 22 October 2013, having kindly agreed to speak on the panel. To set the scene on some the science behind the big questions in biodiversity offsetting, EJ shares her thoughts on some of the big questions.

 What is biodiversity offsetting?

The idea is that, when a development happens in an area, the developer has to go through a series of stages to account for the biodiversity that will be lost because of the development, which is called the “mitigation hierarchy”. First they should avoid causing biodiversity loss in the first place as far as possible, by perhaps siting their development in a less sensitive place. Then, they should minimise the losses that can’t be avoided, for example by taking precautions to dispose of waste appropriately, and after that restore any damage that they have caused at the site. If there is any biodiversity damage left over, after they have done their best to avoid, minimise and restore, then this should be offset – which means compensating for the damage caused in the place where they are working by increasing biodiversity elsewhere or protecting biodiverse areas from otherwise inevitable loss. The idea is that there should be “no net loss” of biodiversity because of the development. This sounds attractive in principle but is tricky to do, and to verify, in practice.

What types of biodiversity can be effectively offset and how can it be measured?

Biodiversity that is precious and irreplacable should not be offset, because by definition there will be uncompensatable loss if it is destroyed. This might mean that threatened species and habitats are not suitable for offsetting, and that it should be confined to biodiversity that is relatively common and resilient. In some senses all biodiversity is unique and irreplaceable, so some people feel that offsetting is never a good idea. Other people might say that development would happen anyway, and so we should make sure that its loss is compensated and not just ignored.

One big issue then, as you say, is how do you measure the losses in biodiversity in the damaged area, and balance them against potential gains elsewhere? This is always going to be imperfect, but one approach is to ensure that the biodiversity gained is as similar as possible to the biodiversity lost (e.g. if an area of a particular habitat is lost e.g. a broadleaf woodland, or a population of a particular species is reduced e.g. a newt, then the gains should also be measured in woodlands or newts, and be as nearby as possible). Biodiversity is a complicated thing though, and it doesn’t have a single simple metric by which to balance gains and losses.

What are the key issues/practical challenges that need to be managed?

Despite being a simple concept in theory, once you actually try to implement an offset all sorts of theoretical and practical challenges emerge. For me one of the biggest challenges is what you compare your development losses to, in order to calculate the amount of biodiversity gain that’s needed. You need to compare the loss from the development against what would have happened if the development hadn’t gone ahead, and you need to evaluate your gains against what would have happened in the absence of the protection or restoration that you’re doing in your offset. These “counterfactuals” could be just what is there at the moment, but in the real world things are always changing, so that’s not a very fair comparison.

Generally, to be precautionary, we might want to think about the losses as the actual loss of current biodiversity from the development (rather than thinking that the area would have been destroyed anyway if this development hadn’t gone ahead), but for the gains which we expect, things might be more complicated. For example in Australia, where offsetting has been very successful, native grasslands are threatened by invasive weeds. Offsetting a development in a native grassland could involve controlling weeds in another area, so that the quality of the grassland is maintained rather than getting worse as it would otherwise.

What can we learn from experience abroad?

Lots! Taking the Australian grassland example, the big success was quite counterintuitive – it was not that lots of native grassland was restored by developers, but instead that the offset regulations meant the number of planning applications for development in native grasslands dropped by 80%. So instead of offsetting, people were choosing to avoid causing damage to the grasslands in the first place. Which is a very good outcome.

Sadly another thing we can learn from experience abroad is how important it is to follow up on offsetting projects; we searched the literature for studies about how offsetting projects had panned out a few years after developments had been completed, and found a pretty dire record. Some had not been done at all, some had not worked in terms of producing the biodiversity gains that were expected, and some were done for a bit but not continued. So although the conceptual issues around how to measure biodiversity losses and gains, for example, are really knotty problems, it’s also very important to ensure that the practical issues are addressed around ensuring that offsets really take place as promised.

How could it work in England?

I think that as long as we learn from experience and ensure that offsets are seen as the last stage of the mitigation hierarchy and only in appropriate circumstances (e.g. not for irreplaceable nature like ancient woodland), then they are a very good addition to the conservation landscape. As we saw in Australia, the requirement to offset, if properly implemented and enforced, can tip the balance towards ensuring that people think hard about the ecological and social costs of each development option and not just the financial benefits.

Really and fully committing to no net loss of biodiversity is a bold step for a company to take (and there are several big companies who have already made that commitment). The ramifications of that commitment are massive. If there is a genuine will to put no net loss into practice then that will be an excellent result for England’s biodiversity.


To follow the event on Twitter check out the live feed on Tuesday 22 October 2013 @RSocLive, #RSbiodiversity.  There will be another blog post to follow next
week and an audio summary of the event will be posted on our PolicyLab page


  • euan

    a very thought provoking blog. I guess as with most things humans try and create it is the ‘looking after’ that we find difficult . doing the offsetting can be good but only if its looked after/maintained for the future!

  • RobYorke

    Looking forward to workshop. Interesting to note counterintuitive stuff: drop in development in biodiversity-rich areas to avoid needing to acheive the tough ‘no net loss’ criteria.
    Rob Yorke @blackgull

  • John Kazer

    I appreciate the principle “only in appropriate circumstances (e.g. not for irreplaceable nature like ancient woodland)” but how to define and regulate “irreplaceable nature”?

  • John Kazer

    I would also be interested in your views on applicability to other countries (e.g. Brazil) and the time period over which a company should be responsible (e.g. historic impacts or maintenance of replacement areas over time)