Abstract
As the demand for carbon-neutral energy sources increases, so does the need to understand the impacts that these technologies have on the environment. It is well established that without appropriate planning, wind energy production can result in mortality and displacement of aerial wildlife populations. While wind farm carcass-monitoring programmes provide estimates of fatality rates, it is not well understood what these numbers represent for bird populations. We assess the potential consequences of fatalities produced by wind farms on an Endangered raptor, the Black Harrier (Circus maurus), one of the world's rarest harriers. We used information on population and ecology of this endemic species collected over 18 years, both inside and outside wind farm environments in South Africa to conduct a population viability assessment. We develop a Bayesian dynamic occupancy model that integrates life history parameters and annual reporting rates from the South African Bird Atlas Project, to explore the species population dynamics over time. This model estimates an expected population annual decline of c. 1.5 % without wind farms, but reveals that the population is sensitive to rainfall, and therefore, changes in climate could alter this trend. To study the effect of additional mortality associated with wind farms, we simulate multiple population trajectories using life history parameters estimated by our model and add different mortality rates. As expected for a long-lived species, population trajectories were highly sensitive to adult survival. Conditional on rainfall being similar to that of the study period (2008-2019), our results suggest that the Black Harrier population may collapse in under 100 years if an average of three adult birds are killed annually at South African wind farms (with probability 0.57). This declines to under 75 years if five adults are killed annually (and extinction under 100 years with probability 0.87). This level of mortality may soon exist, given that an average of three birds have been killed in two years at the best-monitored site in South Africa and many more wind farms are planned within the species' breeding distribution. Our results highlight the critical need for appropriate siting, monitoring and adaptive management of wind farms within the distribution of this (and other) endangered, range-restricted raptors. It also highlights the need for population viability assessments of the same suite of threatened birds.
Introduction
Impacts on bird populations is one of the main environmental concerns associated with the growing expansion of wind energy \citep{Schuster_2015}. While wind energy production is responsible for fewer overall fatalities than other anthropocentric activities, its impacts on long-lived species with low reproduction rates, such as raptors and other large soaring birds, has motivated scrutiny all over the world \citep{Watson_2018,Thaxter_2017}; more so when the affected species are endangered, range-restricted or endemic. Considerable efforts have been devoted to quantifying and monitoring the number of bird casualties produced by wind energy facilities, as well as to predicting collision risk (e.g. \citealt{Watson_2018,Johnson2016,Masden_2016,Loss_2013}). However, it is critical to go one step further and contextualize these numbers to investigate the repercussions of this additional mortality on bird populations, and whether these impacts are sustainable over time \cite{May_2019}.
Population level consequences of additional mortality produced by wind farms on exposed species are often complex. For example, \citet{Katzner_2016} found no population reduction in Golden Eagles Aquila chrysaetos breeding in the vicinity of the Altamont facility in California, despite migratory eagles suffering high mortality \citep{b2008}. \citet{Mart_nez_Abra_n_2011} found local effects on fecundity and survival of Griffon Vultures Gyps fulvus produced by wind farms in Spain, although they found no evidence of a direct reduction in their population numbers. A model developed by \citet{Rushworth_2014} for population trajectories of two threatened species of vulture under threat of wind farms in Lesotho (South Africa), showed that theoretically, even small increments in fatality rates due to wind farms would accelerate extinction of both species. In addition to grasp the population-level consequences of additional mortality and displacement of individuals, we need to understand how accurate our predictions are, where we can improve our understanding of ecological systems, and where we should focus our research efforts to facilitate a sustainable technological development.