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10 February 2016 The influence of agricultural transformation on the breeding performance of a top predator: Verreaux's Eagles in contrasting land use areas
Megan Murgatroyd, Les G. Underhill, Lucia Rodrigues, Arjun Amar
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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.

INTRODUCTION

Transformation of land for agriculture is a key driver of biodiversity loss (Haines-Young 2009). However, the effects of agriculture on biodiversity depend largely on the intensity of land use (Reidsma et al. 2006) and the ability of a species to adapt to changes in resource availability (Butet et al. 2010). A well-documented example of the negative effects of agriculture on biodiversity has been the collapse of farmland bird populations in Europe during the late 20th century, a period characterized by agricultural intensification (Fuller et al. 1995, Donald et al. 2001). However, some species apparently benefit from cultivated landscapes, often associated with the provision of food or breeding resources in areas where they were previously scarce or unavailable (Wolff et al. 2001, Moreno-Mateos et al. 2009, Cardador et al. 2011).

Raptors are considered good indicators of ecosystem health because of their position as apex predators and their sensitivity to a changing environment (Sergio et al. 2006). Agricultural transformation can degrade or destroy preferred habitats and deplete prey resources (Donázar et al. 1993, Amar and Redpath 2005, Jenkins et al. 2013), resulting in decreased breeding productivity (Arroyo et al. 2002), reduced offspring condition (Almasi et al. 2015), or delayed egg laying (Costantini et al. 2014). Breeding performance can be further influenced by climatic variables. In a study of Mauritius Kestrels (Falco punctatus), agricultural transformation exacerbated negative impacts of high rainfall on nesting success by reducing the availability of native prey species around nest sites (Cartwright et al. 2014). However, for some raptorial species, there is evidence that agricultural land use can increase breeding performance, perhaps linked to increases in prey availability or the abundance of suitable nesting areas (Coates et al. 2014). Cultivation of oilseed rape (Brassica napus) has been positively correlated with vole abundance and with breeding productivity of Common Buzzards (Buteo buteo; Panek and Hušek 2014). In Western Marsh-Harriers (Circus aeruginosus), increases in breeding productivity, egg size (Sternalski et al. 2013), and nest-site occupancy (Cardador et al. 2011) have been correlated with increased agricultural land use.

Verreaux's Eagle (Aquila verreauxii) is a long-lived raptor that reproduces slowly, producing a maximum of 1 young yr−1 (Gargett 1990; Figure 1). Verreaux's Eagle is generally considered a specialist predator, with a single prey type (hyraxes: Procavia and Heterohyrax spp.) comprising 88–98