The construction of offshore wind farms is rapidly increasing as governments aim to reduce carbon emissions. However, since there is a growing body of evidence linking offshore wind farms to increased rates of mortality and displacement in seabirds, many countries require a full assessment of any potential impacts to seabird populations before giving consent for wind farm development. These impact assessments follow the precautionary principle, so that consent decisions are based on estimates of the maximum harm that could be caused to seabird populations. Such assessments typically exclude any consideration of density-dependent processes on the population concerned. Density-dependence can be both positive and negative. Negative density-dependence occurs when, for example, the death of breeding adults frees up space in the population for young birds to join it and become breeders themselves. This process can therefore offset the loss of individuals from a breeding population. Positive density dependence can take place once a population falls below a critical threshold. Small populations might not have the ‘safety in numbers’ benefits of seeing off predators, or have a sufficient numbers of potential mates to make the population sustainable. Populations can change from negative to positive density-dependence should they become too depleted.

Seabirds are one of the most threatened bird groups in the world, so understanding the implications of wind farm impact assessments for their breeding populations is crucial. This research by BTO and the JNCC collated evidence for density-dependent population processes in 31 species. The results found widespread evidence for negative density-dependence, especially amongst the large gulls and auks, meaning that populations may be able to withstand small and infrequent losses associated with renewable energy developments. However, extinctions will still occur if the number of losses is greater than the number of new breeding recruits, and species that are highly vulnerable to wind farm developments include large gulls, small gulls, Gannets, seaducks and divers.

The study also showed that positive density-dependence was prevalent in the smaller species that breeding in colonies, such as terns, small gulls and auks. These species are likely to experience accelerated rates of population decline at low densities due to increased predation from large gulls and corvids. For such populations impact assessments that ignore density-dependent processes will underestimate the projected impact of a wind farm and could therefore overlook potential extinction events.

Scientifically robust estimates of the expected impacts of wind farms on seabird populations are critical. The evidence for density-dependence in seabird populations indicates that the current impact assessment process does not offer a fully precautionary approach. A more robust approach would be to compare the projected population size with and without the expected population changes associated with the proposed development, and test how this changes under a range of density-dependent scenarios.

For more information about this research, please see this blog for the Journal of Applied Ecology.

1. Although density-dependent regulation of population growth is thought to be relatively widespread in nature, density-independent models are often used to project the population response to drivers of change. Such models are often considered to provide a maximum estimate of mortality and therefore offer a precautionary approach to impact assessment. However, this perception assumes that density dependence operates as compensatory (negative density dependence), and overlooks that other forms of density dependence, such as depensation (positive density dependence), would generate a contrasting population response. 

2. Currently, there is debate about including density-dependent mechanisms in models that assess the impact of offshore wind farms on marine bird populations. Density dependence is considered poorly understood for this group of species. Consequently, it is either excluded from assessments, or incorporated in a compensatory form that has little empirical validation. 

3. We reviewed the evidence for compensatory and depensatory regulation of 31 marine bird species, and conducted a meta-analysis to examine the functional shape of density-dependent population growth. The evidence was also evaluated in relation to established species-specific indices of wind farm vulnerability in order to assess whether compensatory mechanisms are likely to offset losses associated with collision or displacement. 

4. Compensatory regulation was reported across all of the demographic processes and focal groups considered, and was attributed to a variety of causal mechanisms. The strength of compensatory population growth appeared consistent between colonies; however, the regulation of productivity was highly context-dependent with a similar number of studies reporting compensatory, depensatory and insignificant effects. Depensation was consistently attributed to increased rates of predation at lower population densities. 

Synthesis and applications. We conclude that among marine bird species with high vulnerability to wind farms, compensatory regulation is unlikely to offset large and sustained losses from the breeding population. In addition, depensation has the potential to accelerate population declines and generate local or regional extinctions, especially in smaller colonial species. Consequently, density-independent models will not offer a consistently precautionary approach for assessing the potential impact of wind farms on marine bird populations. Instead, assessments should examine the potential population response using a range of density-dependent structures.