The impact of power line-related mortality on the Cape Vulture Gyps coprotheres in a part of its range, with an emphasis on electrocution

Abstract

SummaryThe global population of the Cape Vulture Gyps coprotheres, a threatened southern African endemic, is known to be impacted by electrocutions and collisions on power line infrastructure, but to date this impact has not been estimated or quantified. Using data in a national database from the period prior to our study, conducted in the Eastern Cape Province of South Africa, we estimated a mean annual mortality rate from power line-related mortality of around 14 vultures per year. After applying an adjusted rate based on the results of a landowner survey, this estimate increased to around 80 vultures per year (i.e. a 5.7 fold increase). For a number of reasons, the estimated mean annual mortality rate is considered to under-represent the true situation, and must therefore be considered a minimum value. A simple model was constructed and run to investigate the potential impact of the mortality rate from electrocution on the study population. It distinguishes between vulture subpopulations in areas of high and low electrocution threat, and a migratory subpopulation that moves between these two areas. The model, simulated over 50 years and applying a constant theoretical maximum annual growth rate of 2%, indicates positive growth of the population in those areas where the electrocution threat from power lines is low, whereas the population in those areas where this threat is high is predicted to crash to extinction, from electrocution mortality alone, within a 20–35 year period. The regional population is predicted to show positive growth over the 50 year period. However, for a number of reasons that relate to the nature of certain parameters used in the model, the simulations must be considered to be conservative, at best. In addition, other unnatural mortality factors (notably inadvertent poisoning, drowning in high-walled farm reservoirs, harvesting for the traditional medicine trade, local food shortage), which are additive to power line-related mortality have not been taken into account. Management recommendations aimed at obtaining an improved estimate of the mean annual mortality rate from power lines, and at ameliorating the impact of electrocutions on the regional Cape Vulture population, are briefly mentioned. These address the former by documenting ways to improve the quantity and quality of the field data, and the latter by identifying areas where urgnt action needs to be taken to reduce or avoid the electrocution of vultures, by mitigating extant ‘unsafe’ power line infrastructure, and by ensuring that that only ‘safe’ infrastructure is used for new power lines.