General concept

The mitigation hierarchy is a conceptual framework aimed to mitigate impacts of projects on biodiversity through an iterative process by implementing a series of measures called mitigation measures. This includes avoidance, reduction, and compensation and should be applied to direct, indirect, and cumulative impacts (see Chapter 1 – Ecological effects of infrastructure). In practice, the mitigation hierarchy can lead to No Net Loss (NNL). This means there is no reduction or damage to biodiversity compared to an initial defined state (known as baseline) after the project has ended (Figure 3.1.1). Similarly, Net Gain (NG) of biodiversity associated with a project refers to enhance biodiversity in relation to the baseline (e.g., improve target species or habitat conditions), after the project has ended.

In the first instance, a baseline for biodiversity must be determined, taking into account different aspects of biodiversity according to varying regulations in different countries. At the European level, the mitigation hierarchy is applied within the Strategic Environmental Assessment (SEA), and Environmental Impact Assessment (EIA), and focuses on protected species and habitats in line with the Bird and Habitat directives. Once the baseline is set, expected impacts – positive as well as negative – are assessed, to evaluate net losses associated with a project, plan, or programme that influences biodiversity. Here, we describe the mitigation hierarchy through three steps: Avoidance, Reduction and Compensation. There are many ways in which the mitigation hierarchy is described around the world although all include at least these three steps. Some versions of the mitigation hierarchy include, just after reduction, an additional step called restoration. Here we consider restoration as a reduction and compensation measure and is therefore an activity included within both steps:

1. Avoidance consists of the design of measures aiming to avoid as much impact as is possible. Avoidance is the most efficient part of the mitigation hierarchy and the one where there is more reduction of impacts and costs related to those impacts. Nevertheless, only if a project is cancelled all impacts can be avoided.
2. Reduction (or minimisation) includes measures aiming to reduce the intensity or significance of impacts that cannot be avoided. Reduction can cover multiple actions, including restoration actions, but also physical interventions (e.g., wildlife passes) or operational controls (e.g., reduce operations in certain times of the year).
3. Compensation measures are any subsequent action to compensate from impacts on biodiversity that cannot be avoided or reduced. This includes restoration of degraded habitat. Measures to rectify any significant residual impacts after avoidance and reduction are always the last step and must be undertaken to achieve a NNL or NG target. Conversely, any action that supports the realisation of effective NG in biodiversity is defined as accompaniment measures (also known as additional conservation actions). Accompaniment measures are often designed to maximise the success of compensation measures but are not considered as mitigation hierarchy measures as they do not count for measuring NNL or NG achievement. Nevertheless, they should be considered as ways to improve the efficiency of avoidance and reduction measures as they may improve the integration of biodiversity and create the enabling environment in the application of the mitigation hierarchy to be more efficient (for example, educating and raising awareness of project stakeholders).

Regulation at EU scale

A NNL objective was set by the European Commission as an action to undertake Target 2 of the 2020 Biodiversity Strategy: ‘to maintain and restore ecosystems and their services by including green infrastructure in spatial planning and restoring at least 15% of degraded ecosystems by 2020’. In particular, Action 7 (associated with Target 2), stated the objective of NNL of biodiversity and ecosystem services as: ‘Assess the impact of European Union (EU) funds on biodiversity and, investigate the opportunity of a compensation or offsetting scheme to ensure that there is NNL of biodiversity and ecosystem services’.

At the very least a NNL outcome should be used to regulate and manage trade-offs between economic development and the conservation of biodiversity. Moreover, the NNL objective applies in the EU’s regulation regarding the management of impacts on biodiversity, not only in the Natura 2000 network, but could also be applied to other areas of interest for biodiversity conservation and particularly to those qualified as Green Infrastructure.

The implementation of the NNL objective relies on the correct application of the mitigation hierarchy which should be integrated within both EIA and SEA. According to the EIA and SEA directives, the mitigation hierarchy is applied within the EIA report. For the purposes of the Directive, and in accordance with the precautionary and preventive action principle, a long-term approach should be promoted, and priority should be given to avoiding impacts, the avoidance measures, while reduction and compensation must only be considered as a last resort.

In addition, NG approaches could also contribute to the implementation of the EU Biodiversity Strategy for 2030 particularly in relation to management of habitats and species affected by transport infrastructure (e.g., by reducing the use of pesticides and fertilizers, enhancing habitats for pollinators, applying Nature-based Solutions, restoring degrades habitats or creating new ones, among many other measures).

From theory to implementation

Experience and research associated with the implementation of the mitigation hierarchy has revealed two challenges, which are listed below, along with best practice methods to address these.

What to measure? Currency and biodiversity targeted by the mitigation hierarchy

Measuring biodiversity is the first challenge associated with any kind of ecological assessment and is central to the implementation of the mitigation hierarchy. Moreover, the components that must be considered within the assessment and the design of measures depends on the regulatory context of each member state. Protected habitats, key species, and green infrastructure elements, such as, for example, ecological corridors, must be factored into the assessment within the relevant EU directives. More importantly, this assessment must be done in early stages of the project, before design, and be clear and transparent so it can be assessed by the relevant authorities.

Good practices in relation to the selection of targets and currencies for implementing the mitigation hierarchy are to:

• Follow a target-based approach, in line with EIA and SEA directives. This involves an initial assessment to identify those biodiversity elements that must be included within the ARC sequence, linked with the relevant local regulations, and biodiversity conservation and restoration objectives. Relevant examples include species or habitats included in the Habitat Directive, ecological corridors, sensitive habitats.
• Apply a multi-criterion analysis which integrates multiple factors that do not limit the analysis to a single component of biodiversity, such as the presence of a given species or habitat. Factors that can be incorporated include: genetic diversity, population viability or ecosystem services, as well as habitat function. This assessment explores benefits for different taxonomic groups, habitats, or functions, and looks for synergies in the design of mitigation measures. The integration of multi-criterion analysis is made possible with the use of ecological modelling.
• Choose currencies and metrics to measure impacts on the elements of biodiversity selected for the assessment (i.e., conservation targets). These must be simple but effective. They should be selected through a consultation process to understand and agree on the trade-offs between the use of different metrics.
• Include consultation with local (and relevant) stakeholders and transparency with the regulators. Coordination between the developer, those implementing mitigation measures, and stakeholders, is fundamental to ensure the quality of the assessment. The coordination between the developer, the mitigation sequence operator and these actors will also ensure a successful implementation of the mitigation measures.
• Balance the needs of threatened and non-threatened species. When implementing the mitigation hierarchy or carrying out an EIA, threatened species, which have the highest regulatory requirements, often receive more attention compared to common, non-protected species. This attention to species with higher conservation priority must be considered within the assessment process and, in relation to both EU and country regulations. Species at less risk or with less priority should also be considered in the EIA. Increasing the range of targeted species may improve the conditions of ecosystem and the services they provide. This also increases the quality of the assessment and improves the efficiency of mitigation measures.

The application of the mitigation hierarchy should therefore be appropriate in the sense that any measure considered must balance the importance of biodiversity affected with the expected benefits provided by this measure and that of other measures that could also achieve mitigation.

Measure misclassification

The classification of ARC measures within one of the categories (avoidance, reduction, or compensation), can be a complex task for the developer, consultancies or decision makers. For instance, the definition of the difference between an avoidance or reduction measure depends on the degree of residual impact remaining after its implementation. Misclassification of mitigation measures can have strong impacts on the quality of the EIA and may lead to errors in the equivalence evaluation between NNL and NG.

To solve this problem, clear guidelines regarding how to undertake the mitigation measures classification should be produced in every Member State. They must provide methods that assist developers and consultancies in deciding which measure fits best into the avoidance, reduction or compensation steps, and are then submitted to the relevant authorities. Such guidelines may also contribute to standardising the authorities’ evaluation according to the legal aspects of relevant regulations in each country, which helps ensure legal certainty.