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

Abstract

Summary 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. 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. 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. 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.

Mortality limits used in wind energy impact assessment underestimate impacts of wind farms on bird populations

“…The mortality thresholds commonly used to evaluate the expected effect of wind turbines on local bird populations are convenient to decision makers because they offer an apparently science-based and clear-cut method to establish whether damage to the integrity of a population will or will not occur. However, questions have recently been raised as to the validity of their use (Green, Langston, McCluskie, Sutherland, & Wilson, 2016;Horswill, O'Brien, & Robinson, 2017). Such questions refer to the general applicability of the threshold assessments, notably their uncertainty with regard to predicting impacts of additional mortality on species with different life-history strategies, as well as the cumulative impacts of wind turbine collision on population developments.…”

When is enough … enough? Effective sampling protocols for estimating the survival rates of seabirds with mark-recapture techniques

“…Thus, improved spatial representation of survival estimation has the potential to facilitate empirical demographic-based assessments in regions that are currently data limited. This is particularly pertinent for the large and expanding offshore renewables industry that is required to fully assess any negative effects to local seabird populations, but is limited to spatially restricted datasets and simplistic modelling approaches (Green et al 2016, Horswill et al 2017.…”

The sensitivity of seabird populations to density‐dependence, environmental stochasticity and anthropogenic mortality

“…Generally, seabirds are characterized as long‐lived animals that exhibit delayed reproduction, low fecundity linked to ecological constraints of resource patchiness and there is evidence of compensatory density‐dependence in several studies (Bried & Jouventin, ; Horswill et al, ; Appendix ()). There is evidence across all species that adults forego breeding when extrinsic and intrinsic conditions are sub‐optimal to maintain condition (Oro & Martinez‐Abrain, ; Weimerskirch, ).…”

“…In EIA, PVA usually contains some estimate of demographic rates, with more recent assessments attempting to include environmental stochasticity by implementing the demographic rates as stochastic processes. Although some models may include a form of density-dependence, there is ongoing debate about the practical justification for its inclusion (Green, Langston, McCluskie, Sutherland, & Wilson, 2016;Horswill, O'Brien, & Robinson, 2016). In general, PVA has been shown to be sensitive to estimates of parameters, affecting predicted outcomes (Cook & Robinson, 2016;Reed et al, 2002).…”