Testing compensatory habitat mitigation for biodiversity

Cotton grass by Anne Carrington-Cotton

Management “prescriptions” to mitigate effects of habitat damage and loss caused by development are often used as conservation tools. However, the effectiveness of such prescriptions is not routinely assessed. An area of protected Scottish moorland was managed to mitigate effects of habitat damage by mining development in this and an adjacent area. In a 10 year study, BTO researchers monitored the responses of breeding bird populations to the chosen management prescriptions. Most species failed to respond positively, probably due to insufficient control of bird and mammal predators. The effectiveness of mitigation management prescriptions must be assessed to ensure the successful delivery of their objectives.

Testing the effectiveness of mitigation for coal extraction activities on moorland

Mitigation of environmental damage is often a condition for the approval of proposed developments.  This can take the form of rehabilitation, enhancement, or creation of habitat in or near to the area being developed.  Such actions may be legally binding and cost the UK taxpayer millions of pounds. However, the efficacy of these measures is rarely assessed.

From 2002-11 the BTO took part in a study to examine the effectiveness of a suite of management prescriptions designed to increase the numbers of breeding birds in a discrete area of moorland, in order to mitigate the effects of habitat loss to surface coal extraction at an adjacent moorland site.

Several types of management were undertaken to improve the value of the area for moorland birds. These included changes in grazing management, drain blocking and other interventions to improve habitat condition for moorland-breeding birds, and the legal control of generalist predators such as foxes and crows.  Data from the BTO/RSPB/JNCC Breeding Bird Survey collected in comparable moorland areas enabled BTO researchers to compare bird population trends on the mitigation area relative to trends on reference sites elsewhere.

Management had little impact on bird populations

Habitat surveys revealed that the condition of the moorland habitat was maintained or improved during the course of the study. Even so, most bird populations either declined or failed to respond positively to this carefully devised regime of management.  Evidence from the study suggested that ineffective predator control was an important causal factor. This was probably due, in large part, to reduced levels of control on land adjacent to the study site from 2004 onwards.

The importance of predation

Predation is a natural process and is integral to ecosystem functioning although, in some contexts, increasing generalist predator populations may threaten species of conservation concern. The conservation of some moorland bird populations may therefore be enhanced by the control of those predators. A better understanding of how and where predation affects bird populations could help to improve the efficacy of conservation measures. This study represents a rare example of industry-sponsored monitoring to inform conservation management, and should be replicated elsewhere to help our understanding of how to mitigate any negative environmental impacts of development most effectively.

Assessment of outcomes is vital

To ensure the delivery of proposed conservation objectives, the outcomes of management prescriptions must be examined critically.  This enables policy-makers, practitioners and regulators to learn from failures, as well as from successes. In the long term, studies such as this one, which also relied upon the availability of long-term BTO monitoring data, will help to make conservation management more robust.

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