New paper on the evidence-base behind species mitigation measures – by Bronwen Hunter

In the UK, infrastructure expansion, in particular urban development, is a key driver of species declines (Hayhow et al. 2016). However, infrastructure improvement and improved housing affordability are also important for ensuring good standards of living and the post-pandemic economic recovery. Last year, the UK government outlined commitments to investing £640 billion in infrastructure delivery under ‘Project Speed’. At the same time, recently announced amendments to the Environmental Bill include what might be one of the most ambitious pieces of biodiversity legislation in recent years – a legally binding, species populations target aiming to halt losses. Given these seemingly conflicting objectives, we urgently need development approaches that protect, and even benefit, UK species.

Currently, species are protected from development via several legal instruments, such as the Conservation of Habitats and Species Regulations (2019). Hence, developers must mitigate and compensate for unavoidable impacts to protected species, often leading to the application of measures such as such as translocation or artificial roosting sites (e.g. bat boxes). Although these measures are widely applied, evidence for their effectiveness, and the ability of ecological consultants to make evidence-based recommendations, is often lacking (Hill and Arnold 2012).

What evidence is there that mitigation measures work?

In our newly published paper, Evidence shortfalls in the recommendations and guidance underpinning ecological mitigation for infrastructure developments, we aimed to investigate this evidence gap, by identifying mitigation measures recommended for a sample of housing developments and evaluating the scientific evidence for their effectiveness. To do this, we utilised the Conservation Evidence synopses, which provide estimates for the effectiveness of conservation interventions, based on expert assessments of available scientific literature.

We found that, of the 446 species mitigation measures we identified in total (65 different mitigation measures relating to eight taxa), real-world evidence on the effectiveness of those measures had been gathered on less than half of them. Additionally, the Conservation Evidence synopses suggest that only 15 of 65 measures are actually supported by evidence that suggests they work in practice. This paints a worrying picture of the ability of current ecological mitigation practice to compensate for the impacts of development and thus halt species declines in the context of rapid development.

The importance of evidence-based guidance

As well as the measures themselves, we also explored the guidance used to inform their recommendation. The use of guidance publications in conservation practice is widespread, yet their quality, and the extent to which they are evidence-based, has so far been largely unaddressed (Downey et al. 2021). In our review, we found that, although most recommended mitigation measures were justified by citing guidance, only 10% of the texts referenced within this guidance related to empirical studies of measure effectiveness. In fact, the most commonly referenced texts were actually other guidance documents, indicating a lack of transparent evidence use.

In addition to the nature of evidence used to support guidance, we also noted an absence of recently published guidance, with some of the most highly utilised guidance documents being published over 15 years ago. What is more, in some cases, five-year updates to guidance included no additional references, suggesting that either no new evidence was generated in that time, or newly generated evidence is not being integrated into guidance publications.

Combined with the identified evidence gaps, the issues surrounding mitigation guidance point to a situation in which a lack of evidence for the outcomes of conservation measures, along with resource constraints, may have impeded the production of updated, evidence-based guidance. Importantly, if guidance is not updated, ineffective or even harmful measures may continue to be applied. A key example is the practice of destructive search, reported to be harmful in Natural England’s 2011 reptile mitigation guidance, which was retracted shortly after publication due to controversies over its recommendations.

Recommendations from our work

What we need is more documented evidence for the outcomes of ecological mitigation. Although habitat-based biodiversity offsetting has received a lot of academic and policy attention, our study makes clear that we also need to address the issues surrounding these commonly applied, species-based measures.

We should acknowledge that this gap between evidence and practice may be particularly pronounced for ecological mitigation, since ecological consultancies often work to different targets (e.g. adherence to legal requirements) than those of mainstream conservation organisations. Hence, the generation of new evidence will require collaboration between developers, ecological practitioners and academics, to collect and make available post-development monitoring data, that is often unused due to commercial sensitivities.

Our research also highlights the need for regularly updated, evidence-based guidance. Tools such as the Conservation Evidence synopses can also support decision-making (in the absence of good guidance) by allowing practitioners to rapidly assess the evidence for potential mitigation measures.

Finally, to meet ambitious national biodiversity targets, we believe that development policies must bolster requirements to avoid impacts on protected species, rather than defaulting to the application of mitigation measures which are not supported by evidence.


  • Hayhow, D.B. et al. (2016) State of Nature 2016. The State of Nature partnership.
  • Hill, D. and Arnold, R. (2012) Building the evidence base for ecological impact assessment and mitigation. Journal of Applied Ecology. 49 (1), 6–9.
  • Hunter, S.B. et al. (2021) Evidence shortfalls in the recommendations and guidance underpinning ecological mitigation for infrastructure developments. Ecological Solutions and Evidence, 2(3), e12089. doi: 10.1002/2688-8319.12089.


Bronwen Hunter’s PhD research focuses on the application of text analysis tools to the production of global analyses of wildlife exploitation. She’s interested in conducting policy-relevant research and understanding the spatial patterns of biodiversity loss. Prior to her PhD she completed a MSc in Environmental Resource Management at Imperial College London and a BA Biological Sciences at the University of Oxford. Contact Bronwen at:

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