The influence of agricultural transformation on the breeding performance of a top predator: Verreaux's Eagles in contrasting land use areas

Abstract

Breeding productivity frequently shows variation across a species' range or locally between different habitat types. Agricultural transformation generally has negative effects on biodiversity and often results in reduced prey abundance or increased foraging effort in top predators and, consequently, often reduces breeding productivity. Major factors that affect reproductive success also include climatic variables, breeding density, and timing of breeding. We explored the influence of agricultural transformation on a specialist raptor, Verreaux's Eagle (Aquila verreauxii). From 2011 to 2014, we examined productivity in 2 adjacent populations in the Western Cape Province, South Africa: an unspoiled area of Fynbos vegetation with little human development (the Cederberg Mountains) and an agriculturally transformed area (the Sandveld region). Counterintuitively, breeding productivity was higher in the agricultural than in the natural site. In particular, the proportion of pairs that attempted to breed (i.e. breeding rate) was higher in the Sandveld (0.94 ± 0.07 attempts pair-1 yr-1) than in the Cederberg (0.48 ± 0.14 attempts pair-1 yr-1). Nesting success was also higher in the Sandveld (0.80 ± 0.05 fledged young attempt-1 yr-1) than in the Cederberg (0.57 ± 0.13 fledged young attempt-1 yr-1), and the probability of nesting successfully was related to the lay date (decreased success with later laying) and to the total cumulative rainfall up to 28 days after hatching (decreased success with increasing rainfall). Using the site-specific breeding rates to produce a population model, we found that in isolation, the Cederberg population is unlikely to be self-sustaining, but Verreaux's Eagles breeding in the agriculturally developed Sandveld region are likely to be an important source population, despite occurring at a much lower density. These results, contrary to our expectations, suggest that Verreaux's Eagle may be more adaptable to agricultural transformation than previously thought, with breeding performance in the agricultural site adequate to maintain the population.