Abstract
Globally, conversion of natural habitats to farmland poses the greatest extinction risk to birds, its consequences being especially pervasive in the case of large predators and scavengers, whose declines may trigger extensive cascading effects. Human population growth in sub-Saharan Africa is expected to drive a vast expansion in agricultural land by 2050, largely at the expense of pastoral land and savanna. The greatest expanse of suitable land yet to be converted to agriculture lies in East and Central Africa, including South Sudan, DRC and Tanzania. To gauge the effects of land conversion on raptor populations in this region we used road survey data from neighbouring Uganda, from which we determined linear encounter rates (birds seen 100 km−1; n = 33 species), and species richness (from 53 species). Encounter rates were much lower in pastoral land than in protected savanna (median difference: −41%; 23 species), and lower still in agricultural land (−90%; 24 species). These disparities were influenced by diet and body mass. For large eagles and vultures, encounter rates in agricultural land were 97% lower than in protected savanna (median of 12 species), whereas for smaller raptors they were 30% lower (12 species). Large, apex consumers were thus more vulnerable to farmland expansion, and this was reflected in the mean body mass of species encountered in savanna (1740 g), pastoral (995 g) and agricultural land (856 g). Body mass differences remained significant when vultures were excluded. Since threat status is linked to body mass, encounter rates for globally threatened and near-threatened species likewise showed a more pronounced deficit in farmland than those of least concern. Accordingly, pastoral and agricultural transects were less species-rich (10.6 and 6.7 raptor species 100 km−1, respectively) than savanna transects (13.2 species). Our findings suggest that the expansion of agricultural land in sub-Saharan Africa will reduce raptor populations in pastoral land and savanna by c. 50% and 90%, respectively. We propose that conservation efforts focus on identifying the causes of raptor population deficits in farmland, and on safeguarding tracts of unprotected, intact savanna, together with existing protected areas.
Highlights
Land use conversion is considered to be the single biggest driver of biodiversity loss in the tropics (Foley et al 2005, Jung et al 2017)
A much greater disparity was evident between protected savanna and agricultural land; 19 out of 24 278 species were less abundant in the latter, and the median difference in their encounter rates was
We show that raptor encounter rates were 41% lower in pastoral land and 90% lower in agricultural land than in protected savanna
Summary
Land use conversion is considered to be the single biggest driver of biodiversity loss in the tropics (Foley et al 2005, Jung et al 2017). In Africa, these effects include the potential loss of ecosystem services provided by vultures and other avian scavengers, which are likely to 66 inhibit disease transmission, through the rapid disposal of carcasses (Ogada et al 2012). The loss of this service in India has been described in a well-documented trophic cascade, wherein the 68 collapse of vulture populations was followed by a substantial rise in the feral dog population, which in turn contributed to a $34 billion increase in healthcare costs associated with rabies 70 treatment in humans (Sudarshan et al 2007, Markandya et al 2008)
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