Do assessment thresholds underestimate the mortality impact of wind farms on bird populations?

• Main Authors: Buij, R., Schippers, P., Schotman, A., Verboom, J., van der Jeugd, H., Jongejans, E.
• Format: Report
• Language: English
• Published: Wageningen Environmental Research 2017
• Subjects: Life Science; Common tern
• Online Access: https://research.wur.nl/en/publications/do-assessment-thresholds-underestimate-the-mortality-impact-of-wi; https://doi.org/10.18174/406604

The consequences of additional mortality of birds from collisions with a rapidly increasing number of wind turbines are receiving much attention worldwide. Currently, threshold assessments for an acceptable impact on populations are commonly used to evaluate the expected effect of wind turbines on local bird populations. These provide a seemingly clear-cut method for establishing whether damage to the integrity of a population will or will not occur, but questions have recently been raised as to the validity of their use. In this paper we examine whether two widely used threshold methods for evaluating the impact of extra mortality on bird populations, the 1% mortality norm and Potential Biological Removal PBR, have general applicability, or whether they should be used more cautiously. The 1% mortality norm is based upon the assumption that any additional mortality lower than 1% of the natural mortality has a negligible impact on a population, while the Potential Biological Removal or PBR method is used to estimate the loss of individuals from which a population can still recover. To evaluate the impact of additional mortality resulting from wind turbine collision on bird populations, we model the consequence of an increase in mortality rates on populations assumed to be regulated by logistic growth. We use the logistic growth equation to test how the population persistence of a species may be affected by different levels of additional mortality; and use case studies of existing, declining populations of Common Tern and Marsh Harrier in the Netherlands to determine how the effect of collision mortality operates to influence their population persistence. To examine the impact of additional mortality, we introduce a novel measure, the “Population Persistence Index” or PPI to describe the population persistence and changes therein following increased mortality. The PPI integrates the population growth at various densities - it is determined by the maximum population growth rate at small population size - and the ...