Chapter 12 Ecological Impact Assessment

Chapter 12 Ecological Impact Assessment

A formal Ecological Impact Assessment (EcIA) is usually required under an Environmental Impact Assessment (EIA), but can also be usefully employed on any development project to identify significant ecological impacts.
Best practice is that the EcIA is an iterative process. This means that, as ecological information comes to light, the development design should be amended where possible to avoid significant harm to biodiversity.


Scoping is the first step and includes an initial data gathering exercise, which is used to inform the content or scope of the full EcIA. Information is gathered on the proposed development, the ecology of the site and any relevant legislation and policies. Scoping often includes a Preliminary Ecological Appraisal. A desk study draws on published material and databases held by local organisations. An extended Phase 1 habitat survey is used to identify the main habitat types and evaluate the potential of the development site to support important habitats and species. Other interested groups may be consulted at this stage.

Detailed ecology surveys

Detailed ecology surveys may follow if the scoping exercise has shown that the development might affect sensitive or protected habitats or species. Critically, the season for these surveys may be limited, so early planning is essential to avoid delays to the project.

Evaluation of nature conservation value

The information that has been collected is used to inform an evaluation of the importance of the site, its habitats and species. Guidelines, based on those originally developed for the designation of SSSIs, are used to arrive at a level of value for the different components of the site, on a scale ranging from negligible to International importance.

Impact assessment

Interpretation of the survey data enables an assessment of the ecological impact of the development (e.g. loss of habitat); the magnitude of the impact (including positive and negative effects); and the significance of these, depending on the value of what is affected and how much.


If the impacts are considered to be significant, measures to avoid, reduce or compensate for any impacts will be proposed. In some cases, avoiding impacts is a legal requirement, such as not killing protected species. In others, mitigation may be required to satisfy planning policies and best practice guidance. Local planning authorities effectively have a statutory duty to minimise the effects of development on biodiversity and to seek enhancements wherever possible. The aim of the mitigation measures should therefore be to achieve no net loss of biodiversity and if possible a net gain.

Residual impacts

The effect of the development on biodiversity, in the light of the proposed mitigation measures, is reassessed to determine the final (residual) impact.

Environmental statement

The results of an EcIA are often presented as a chapter in the Environmental Statement for the development. This gives decision makers all the relevant information on the environmental impact of the proposed development.

Thomson Handbook Chapter 12 Ecological Impact Assessment