Density-dependence and marine bird populations: Are wind farm assessments precautionary?

In this post, Catharine Horswill discusses her paper ‘Density-dependence and marine bird populations: Are wind farm assessments precautionary?

“Just one thing would be enough to halt climate change, if clean energy became cheaper than coal, gas or oil, fossil fuel would simply stay in the ground”. Sir David Attenborough made this statement in support of the Global Apollo Program, an international initiative to increase the amount of funding allocated to the research and development of renewable energy technologies. Renewable energy is set to play a central role in the global commitment to reducing carbon emissions, and the leading technologies, wind and solar, are anticipated to expand by several-fold within the next 10 years.

Understanding the local effects of renewable energy developments is an immediate priority facing this industry. For example, there is a growing body of evidence linking wind farms to increased rates of mortality and displacement in birds. Subsequently, many countries require a full assessment of any potential impacts to bird populations before providing wind farms with consent for development. These assessments typically exclude density-dependent processes, such as negative density-dependence that offsets the loss of individuals from the breeding population. Consequently, density-independent models are often considered to provide a maximum estimate of mortality and offer a precautionary approach to impact assessment. However, this perspective overlooks other forms of density-dependence that result in contrasting population trends.

“Marine birds are one of the most threatened groups of birds in the world”. Photo credit: David Williams.

Marine birds are one of the most threatened groups of birds in the world, and in this study we collated evidence for density-dependent regulation of 31 species. We found widespread evidence for negative density-dependence. This may allow populations to offset small and infrequent losses from the breeding population associated with renewable energy developments. Here, density-independent models will offer a precautionary approach to impact assessment. However, extinctions could still occur if the number of losses is greater than the number of new recruits. Consequently, density-independent models may not be precautionary for species of higher vulnerability to renewable energy developments. Furthermore, density-dependence can change from negative to positive when numbers become depleted below a critical density. We found that positive density-dependence was prevalent in populations of marine birds, especially amongst the smaller colonial species.  Populations that are regulated by positive density-dependence will experience accelerated rates of population decline at low population densities, and the causal relationship was consistently attributed to increased predation from large gulls and corvids. For populations that are regulated by positive density-dependence, density-independent models will underestimate the projected impact and risk overlooking potential extinction events.

Scientifically robust and defensible estimates of the expected impacts of wind farms to marine bird populations are essential. The evidence for density-dependent regulation of marine bird populations indicates that density-independent models do not offer a fully precautionary approach to impact assessment. A more robust solution would be to compare the projected population size with and without the expected demographic changes associated with the proposed development, and test how this changes under a range of density-dependent structures.

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