A good news story: Land use change is not necessarily detrimental to Verreaux’s Eagles
The influence of agricultural transformation on the breeding performance of a top predator: Verreaux’s Eagles in contrasting land use areas. Murgatroyd M., Underhill, L.G., Rodrigues, L. & Amar, A. 2016 The Condor 118: 238-252. View
The Verreaux’s Eagle is an iconic raptor in mountainous regions of sub-Saharan Africa. In South Africa, recent declines in the reporting rate have led to the uplisting of the regional conservation status from Least Concern to Vulnerable in the newest edition of the Red Data book. We sought to find out if agricultural development could be impacting this species.
From the widespread decline of farmland birds in the last quarter of the 20th Century (Donald et al. 2001, 2006), to the increasing worry over the conservation status of bumble bees, the negative impacts that agricultural intensification can have on biodiversity are well known in Europe. In Africa, much is still left to be found out. As the human population continues to grow, food production in Africa is expected to increase (Pretty 1999) and we can expect continued loss of natural habitat for agricultural purposes (Tilman et al. 2001).
The Verreaux’s Eagle is no stranger to loss of natural habitat due to agricultural transformation. One population in the Sandveld, near the West coast of South Africa, has experienced the loss of natural habitat for crop cultivation. The installation of electricity distribution infrastructure in the 1980s paved the way for centre-pivot irrigation supplied by underground aquifers in an otherwise dry region. Since then, the production of potatoes and other irrigated crops has increased dramatically and the Sandveld is now a nationally important agricultural area (Franke et al. 2011). Nevertheless, rocky outcrops continue to provide nesting habitat for Verreaux’s Eagles and other smaller raptors.
In contrast, the nearby Cederberg mountains are largely under the protection of CapeNature, the regional conservation body. These mountains provide an abundance of natural habitat and cliffs for breeding, and consequently support a high density of Verreaux’s Eagles. My PhD was devised to compare and contrast the ecology of Verreaux’s eagles in these two areas to find out if land use change is having any impact.
Much to our surprise, eagles seem to be fairing better in the agricultural Sandveld region than the natural Cederberg. The outcomes of 112 occupied breeding years were monitored during four breeding seasons from 2011−2014. In contrast to our expectations, Verreaux’s Eagles breeding in the Sandveld were 2.7 times more productive than those breeding in the Cederberg.
In particular, the proportion of pairs that attempted to breed was higher in the Sandveld (0.94 ±0.07 attempts/pair/year) than the Cederberg (0.48 ±0.14 attempts/pair/year). Nesting success was also higher in the Sandveld (0.80 ±0.05 fledged young/attempt/year) than the Cederberg (0.57 ±0.13 fledged young/attempt/year). One reason for this was that rainfall during the first 28 days of chick rearing negatively impacted nesting success, and there was generally more rain in the Cederberg than the Sandveld. However, this did not explain all of the differences.
We modelled the potential long-term population trends based on these productivity rates and found that the Cederberg population is unlikely to be able to sustain itself without an influx of individuals from other populations. In contrast the breeding performance of eagles in the Sandveld was high enough to potentially act as a source population. When we applied a scenario-based approach to the population viability analysis (PVA), we saw that any small increase in mortality in either area was predicted to cause a meta-population crash within 50 years. Our survival estimates were based on predictions from a stable population of Verreaux’s Eagles rather than long term monitoring observations. Therefore, although we should treat the reliability of the PVAs with caution, they demonstrate the importance of the Sandveld population and the consequences that increased mortality could have. Mitigation of threats, including power distribution infrastructure and wind farms, is vital in this population.
It is impossible to know if the breeding performance of eagles in the Sandveld has changed following agricultural development or if this area was equally productive prior to land use change. However, our findings support the notion that this top predator can maintain good breeding productivity in some agriculturally transformed areas.
Species-specific responses to land use change are variable. Agriculture can be beneficial to raptors by providing an increase in prey (Ogada 2009, Kross et al. 2013) or habitat resources (Balbontín et al. 2008, Cardador et al. 2011). The availability of these resources is usually related to the intensity of transformation and the maintenance of features such as hedgerows, field margins and areas of natural vegetation. Furthermore, the ability of individuals within the species to adapt to changes in resourses will determine their persistence in a changing environment. Within a threshold of transformation, agriculture seems to be able to play a positive role in resource provisioning.
In further research we have explored the diet and movement ecology of these eagles to uncover causative reasons for the contrasting breeding performance. Amongst these, a more varied diet and a lower energy consuming life style, due to contrasting topography and uplift availability, might benefit eagles in the Sandveld. We hope to share these results soon. The reason for national population declines of the Verreaux’s Eagle remains to be identified.
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Cardador, L., Carrete, M., and Mañosa, S. (2011). Can intensive agricultural landscapes favour some raptor species? The marsh harrier in north-eastern Spain. Animal Conservation 14: 382–390. View
Donald, P. F., Green, R. E., and Heath, M. F. (2001). Agricultural intensification and the collapse of Europe’s farmland bird populations. Proceedings of the Royal Society B: Biological Sciences 268: 25–29. View
Donald, P. F., Sanderson, F. J., Burfield, I. J. and van Bommel, F. P. J. (2006). Further evidence of continent-wide impacts of agricultural intensification on European farmland birds, 1990–2000. Agriculture, Ecosystems & Environment 116:189–196. View
Franke, A. C., Steyn, J. M., Ranger, K. S. and Haverkort, A. J. (2011). Developing environmental principles, criteria, indicators and norms for potato production in South Africa through field surveys and modelling. Agricultural Systems 104: 297–306. View
Kross, S. M., J. M. Tylianakis, and X. J. Nelson (2013). Diet composition and prey choice of New Zealand falcons nesting in anthropogenic and natural habitats. New Zealand Journal of Ecology 37: 51–59. View
Ogada, D. L. (2009). Impacts of Agriculture on the Diet and Productivity of Mackinder’s eagle owls (Bubo capensis mackinderi) in Kenya. Biotropica 41:485–492. View
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Tilman, D., Fargione, J., Wolff, B., Antonio, C. D., Dobson, A., Howarth, R., Schindler, D., Schlesinger, W. H., Simberloff, D. and Swackhamer, D. (2001). Forecasting agriculturally driven environmental change. Science 292:281–284. View
Featured image: Verreaux’s Eagle nest in the Sandveld © Megan Murgatroyd
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