Richness of the raptor community in the Sahel
Overall richness (15) and abundance (47 individuals 100 km−1) of raptors in the Sahel was similar to that found in other arid and semiarid ecosystems such as the Patagonian Monte, the central Asia steppes, or Baja California (Ellis, Glinshi & Smith, 1990; Donázar et al., 1993; Rodríguez-Estrella, Donázar & Hiraldo, 1998; Sánchez-Zapata et al., 2003; Carrete et al., 2009). However, this result can be misleading because of the presence of a very abundant migrant species, the black kite. Indeed, when excluding this species from analysis, total abundance drops to 7 individuals 100 km−1. Furthermore, even not taking into account black kites, most of the raptors censused were trans-Saharan wintering species (5 individuals 100 km−1; six species), in contrast with the lower abundance of resident raptors (2 individuals 100 km−1; eight species). The richness of residents raptors was thus much lower than expected, considering the large-bodied resident birds of prey that, although potentially present in the Sahel (hooded Necrosyrtes monachus, African white-backed Gyps africanus, Rüppell's Gyps rueppellii, lappet-faced Torgos tracheliotus, white-headed vultures Trigonoceps occipitales and martial eagles Polemaetus bellicosus, Thiolay, 2006a,b, 2007), were totally absent from our roadside counts despite the significant sampling effort and the high visibility of these species. These absences were unlikely due to sampling bias, because road transects have been successful in detecting large-bodied raptors in many other studies carried out in open systems such as our one (Donázar et al., 1993; Sánchez-Zapata et al., 2003; Thiolay, 2006a,b, 2007; Carrete et al., 2009). The increasing rarity of large vertebrates was also reported in other parts of the western Sahel (Mali, Burkina Faso and Niger), where the persistence of large raptors, ungulates and carnivores is restricted to a few protected areas (Thiollay, 2006a,b, 2007) and reflects the deep impoverishment suffered by biological communities in this area. As discussed below, this decline could be linked to the increasing density of human population. This effect may be more serious in the case of sedentary species that complete their whole life cycle in the Sahel and thus suffer more constant human pressure than the wintering species that breed in the Western Palaearctic.
Factors driving raptor richness in arid landscapes
Environmental available energy and environmental heterogeneity are major drivers of species richness at macroecological scales (Davies et al., 2007). In our study, patterns at a large scale (25 km resolution) between people, productivity and species richness only partially agree with expected patterns. The raw explicative power of productivity over raptor richness (16–21%) was larger than values reported by other works for taxa with these characteristics (i.e. high mass, top predator; Evans, Greenwood & Gaston, 2005). However, as revealed by deviance-partitioning analysis, this effect was mainly due to a combined effect of productivity and land-uses. Actually, the positive effects of crops and the negative effect of deserts (independent of habitat productivity) were the main factors shaping raptor richness at macroecological scales in the Sahel. This result is remarkable taking into account our current knowledge about species–energy relationships. Many studies dealing with this subject implicitly assume that, because descriptors of environmental available energy (e.g. temperature, NDVI) likely affects basic resources or conditions, the detected species–energy relationships reflect ecological processes (e.g. Evans et al., 2005; but see Chase & Leibold, 2002 and Evans, James & Gaston, 2006). Our results thus support studies suggesting that part of the influence of productivity on species richness arises from its correlation habitat type (Evans et al., 2005, 2006).
At a smaller scale (5 km resolution), deviance-partitioning analysis showed that population, land-uses and productivity show a higher explanatory power for sedentary compared with wintering species, the latter being rather low (17%). Different macroecological responses depending on species attributes and scale have been described previously (e.g. Evans et al., 2005, 2008). In our case, it is likely that the distribution of wintering species may be more loosely related to environmental factors and resources, largely responding to unpredictable abundant feeding sources (Sánchez-Zapata et al., 2007). On the other hand, the lower percentage of deviance explained by population variables highlights the lower vulnerability of wintering species to continuous human pressure compared with sedentary species (Thiollay, 2007), that undergo their entire life cycle in the Sahel. In sedentary raptors, we detected a larger effect of people compared with land-uses and productivity. The increase in human effects on species richness as we scale down agrees with described general patterns (Pautasso, 2007).
Habitat, measured here as land-uses, played a central role in raptor richness similar to that described for other arid ecosystems (Rodríguez-Estrella, 2007; Carrete et al., 2009). Crops are positively related to wintering raptor richness at both macroecological and more local scales, whereas the response of resident raptors to crops was less consistent, shifting between scales. These patterns generally agree with those found by Thiollay (2006b) at a 5 km scale in Burkina Faso and are also similar to that described for raptors in European landscapes, where trans-Saharan migrant raptors are more abundant in extensive agroforestal mosaics and resident raptors more common in more natural areas (Sánchez-Zapata & Calvo, 1999). The presence of wintering raptors was also linked to deserts, probably because black kites, the most abundant species, were common in arid landscapes. Overall, the response of richness to deserts was negative.
At the smaller scale (5 km), where the effects of human population are relevant, resident raptor richness showed a negative response to towns. This result indicates a negative effect of human population, particularly at higher densities. The effect of human population relying on subsistence economies and the extensive use of the territory, independent of land-uses, is likely to be a major force shaping the distribution of sedentary raptors in the Sahel. The main processes behind this negative effect on biodiversity could be direct persecution of raptors as well as harvesting of their main basic resources such as prey or nesting material (Thiollay, 2006b). Indeed, bushmeat for human consumption is a key contributor to local economies throughout the developing world (Milner-Gulland, Bennett & the SCB 2002 Annual Meeting Wild Meat Group, 2003; Brashares et al., 2004). Other activities such as overgrazing or firewood collection have also been pointed out as leading causes of degradation of dry-lands, and thus, as having a negative effect on biodiversity (Wezel & Rath, 2002; Darkoh, 2003). This scenario resembles the situation in rural Europe before industrialization, when high densities of dispersed human populations and, more importantly, their negative effects through persecution and poisoning of wild animals, increased extinction rates at both local and global scales (Thompson & Jones, 1999; Brashares et al., 2001; Carrete et al., 2007). This situation might be exacerbated in the Sahel by the lack of natural refuges for wildlife, others than a few protected areas.
Once the most sensitive species to human perturbations have become virtually extinct (i.e. vultures and eagles), the resulting community of resident raptors mainly includes medium and small body-size species. This impoverished community may find adequate habitats in landscapes with low to intermediate human disturbance, as shown by the positive response of resident raptors to villages (<100 houses). Finally, opposite to resident species, wintering raptors were positively related to towns, likely indicating the lower impact of harvesting on wintering species, because they do not complete their annual cycle in the Sahel, and thus are not dependent on territories and nests, which are extremely vulnerable.
Furthermore, most birds related to towns are to some extent scavengers, and thus are not intensively harvested because they are considered impure as a food source by the Muslim culture (e.g. Levitic 11, 13–19). Additionally, this result may be magnified by black kites, by far the most common species in our transects (Table 1), which is a scavenger partially relying on human waste and refuse and thus linked to human centres and their surroundings (Sánchez-Zapata et al., 2007).
General conservation insights
Much attention has been focused on habitat loss and fragmentation and on urbanization as major drivers of biodiversity loss due to human population growth (Millennium Ecosystem Assessment, 2005). Our work indicates that subsistence economies in systems with low natural environmental heterogeneity and with human populations over carrying capacity, such as the Sahel, may alone lead to exhausted biological systems even in the absence of significant agricultural or urban land-use changes, as shown by the striking impoverishment of our sedentary raptor community. This situation is likely to be occurring in many systems of developing countries with high population growth and low carrying capacity, a situation particularly common in the arid and semiarid portions of Africa (Le Blanc & Perez, 2008).
Reductions in species richness, particularly of large body-size vertebrates and top predators, are also likely to have far-reaching consequences on the ecosystem (Rooney et al., 2006) and on the services provided to humans (Sekercioglu, Daily & Erhlich, 2004). Non-scavenger raptors may play an important role in controlling prey populations and thus in the control of pest outbreaks (Crooks & Soulé, 1999; Sergio et al., 2008). Particularly in the Sahel, raptors seem to act as a biological control of locust outbreaks (Sánchez-Zapata et al., 2007). Scavenger raptors are mainly responsible for the disposal of carcasses and their functional extinction may yield increases in small scavenger-opportunist species and disease outbreaks (Pain et al., 2003). The current extreme rarity of large vultures in the Sahel detected in this work supports the severe decline of this group in many areas worldwide (Koenig, 2006).
In light of the devastating droughts and famines that occurred between the late 1960s and early 1990s, there has been sustained interest in recent decades in the study of environmental change in the Sahel (e.g. Zeng, 2003; Olsson, Eklundh & Ardö, 2005). Some authors highlight that climate change and plant cover decrease may yield an scenario of expansion of the Sahara desert (Zeng, 2003; Zeng & Yoon, 2009). Because habitat and productivity seem to play a relevant role in driving species richness, our results suggest that changes in climate, and thus in the distribution of habitats and productivity, may have a major effect on this raptor community.