Eagles as indicators of ecosystem health: Is the distribution of Martial Eagle nests in the Karoo, South Africa, influenced by variations in land-use and rangeland quality?

We examined the relationship between the distribution of Martial Eagles Polemaetus bellicosus (and other large eagle species) breeding on electricity transmission pylons in the central and southwestern Karoo, South Africa, and the general environmental health of commercially managed farmland in that region. Eighty-three eagle territories were located along 1400 km of surveyed pylon line. The influence of habitat quality on eagle distributions was assessed in terms of (i) patterns of dispersion and spacing between territories, (ii) macro-scale differences between the habitat composition of eagle territories and an equivalent array of random plots, and (iii) micro-scale differences in habitat quality assessed at points within and outside of eagle territories. Territories were generally irregularly distributed along the pylon line (p<0.025). At the landscape scale, Martial Eagles significantly favoured habitats with lower and/or less predictable winter rainfall and lower primary productivity, while Tawny Eagles Aquila rapax preferred habitats with higher and/or more predictable summer rainfall and higher primary productivity. All eagles avoided areas of highest primary productivity and were absent from cultivated areas. On a local scale, habitat qualities within eagle territories were generally not significantly different to those recorded outside territories. However, eagle densities in areas stocked with indigenous game were significantly higher than in areas supporting only domestic stock (nearest neighbour distances, 7 km vs. 12 km, p=0.025). This study shows that (i) large eagle distributions in the Karoo are significantly influenced by both macro- and micro-scale variation in habitat quality, and by the nature of land management, and (ii) that the Martial Eagle is potentially suitable as an indicator of ecosystem quality and functionality in the Karoo.