Critical Habitat Determination for Linear Infrastructure: Approaches to Delineating Discrete Management Units under IFC Performance Standard 6
Authors: Bennun Leon , Pollard Edward , Starkey Malcolm , Temple Helen 
Published: March 2016

Abstract
Increasingly, development projects are managing biodiversity and ecosystem service risk through the application of IFC's Performance Standard 6. Central to PS6 is the identification of Critical Habitat, which in turn requires the definition of one or more Discrete Management Units. Guidance for this is limited, and DMU delineation can be especially challenging for linear infrastructure, which may have a small direct footprint but can run for long distances across many ecological zones. Options for DMU definition for linear infrastructure include A single large DMU including contiguous habitat or the administrative unit fully including the pipeline. This simple approach may be appropriate when infrastructure runs across a single landscape containing migratory or nomadic wildlife. Elsewhere, the resulting DMU may be unfeasibly large (including an entire country/province or several ecoregions and thus not helpful for mitigation planning.Defining a buffer of fixed or variable width around linear infrastructure. This is also a simple approach but usually lacks the ecological and/or management rationale required by PS6. However, this may be the only feasible approach in some circumstances, e.g. the open ocean. Buffer size should consider the infrastructure's likely area of influence, including indirect and cumulative impacts. This will be larger than the simple footprint.Defining multiple DMUs, where a buffer around the linear infrastructure intersects with coherent ecological or management units of recognised high biodiversity value. This approach allows some degree of spatial differentiation of areas that are or are not Critical Habitat, assuming that ecological or management units at a suitable spatial scale can be identified. Multiple DMUs can be combined with a ‘simple buffer’ approach for a more comprehensive area of assessment.
DMUs specific to one or more biodiversity features are a cross-cutting approach that can be used with any of the above. This may be effective if features can easily be grouped at different spatial scales (e.g. wide-ranging mammals vs. plants confined to a particular geology). Greater complexity and possible difficulty of communication needs to be weighed against the greater specificity that can be achieved.
These approaches are illustrated with an example from Simandou, Guinea, for a 640 km long rail corridor. This uses a simple buffer (20 km each side) to identify multiple intersecting DMUs for Critical Habitat determination.
There is no one right way to define DMUs for linear infrastructure. Methods should apply ‘common sense’ taking into account the project context and bear in mind the overall aim, to allow Critical Habitat determination at an appropriate ecological/management scale. A precautionary approach is preferable, for instance to ensure buffers are wide enough to encompass potential impacts and through collective rather than individual assessment of DMUs against PS6 threshold.