Main patterns of habitat use
This study shows that both topographic and anthropic features determine the habitat use of the houbara bustard, while other descriptors related to vegetation structure and substrate characteristics play a minor role in its distribution over Fuerteventura. In this island, the houbara behaves as a habitat generalist according to habitat structure features, which is in agreement with the niche expansion hypothesis in island environments (Blondel, 1979; Wiens, 1989). Overall, the houbara bustard in Fuerteventura occupies similar habitats to those preferred on continental Africa: arid flat or gently undulating terrain with little shrub cover (Le Cuziat et al., 2005a,b; Hingrat et al., 2007; see also Martín & Lorenzo, 2001). This general habitat selection pattern does not change between summer and early spring. However, it should be noted that our aim was to make estimates for the whole population and thus we did not consider age or sex groups, whose habitat preferences may vary seasonally. Thus, the habitat use pattern of the species we describe in this study has to be considered as average for the whole houbara population in Fuerteventura. In the northern African population, it has been reported that habitat preferences of the females differed from those of the males and vary seasonally, because female breeding birds, during spring, select areas richer in preys (beetles) with higher vegetation (Hingrat et al., 2007). Nevertheless, Hingrat et al. (2007) did not detect seasonal variation in male habitat use. Both habitat cues and the presence of conspecifics determine the settlement patterns of bustard species (Hingrat et al., 2004; and see Lane, Alonso & Martin, 2001 for great bustard Otis tarda), so that the birds (particularly males) may show fidelity to sites somewhat regardless of the availability of suitable habitat elsewhere.
Notably, an excessive slope of the terrain and a rocky substrate were the only non-anthropic habitat features constraining the occurrence of the houbara bustard, whereas none of the four vegetation variables considered was an important predictor of the species distribution. Recently, several papers on arid environments have shown that topographic abiotic features are the primary factors determining regional bird distribution patterns, both at the species (Knight & Beale, 2005; Seoane et al., 2006; Palomino et al., 2007) and the community levels (Kaboli, Guillaumet & Prodon, 2006). In the particular case of the houbara bustard, the positive selection of the flattest plains with a compact or sandy (but not rocky) substrate could be related to the high energy demands of upward walking for such a large cursorial bird (1.2 kg mean weight) with long tarsi (Lachica & Aguilera, 2000; Daley & Biewener, 2003; Gabaldón, Nelson & Roberts, 2004). Thus, the locomotion on steep slopes, or up and down over a rocky substrata with large stones or massive rocks (e.g. ‘malpaíses’) may pose clear limits to the habitat distribution of the houbara bustard in this mountainous volcanic island.
Disturbances from human origin are currently the primary environmental effects to modify and mitigate in endangered or vulnerable houbara populations (Lavee, 1985; Osborne, Launay & Gliddon, 1997; Le Cuziat et al., 2005a,b). Fortunately, some anthropic risks have been almost eradicated for the houbara bustard in the Canary Islands (e.g. hunting, one of the most important menaces for the species in many places: Mian, 1986; Goriup, 1997; Combreau, Launay & Lawrence, 2001; Chammem et al., 2003), or are receiving particular attention (casualties on power lines: Lorenzo, 1995; and see also http://www.seo.org/lifehubara/Ingles/AccionesProyecto7.htm). This work identifies the impact of a threatening factor to be urgently corrected such as the highly uncontrolled urban sprawl and the associate road network in the flattest, steppe-like habitats of the island. The negative effect of the proximity of urban areas and the density of paved roads on the occurrence of the houbara bustard will likely increase if the prevailing models of urbanization and tourism resort penetrate the interior flat steppe-like area. Thus, there is a conflict between the development of rural areas (for urbanization and agriculture) and the houbara in Fuerteventura, both of which favour flat and vegetated plains (see also Osborne, Launay and Gliddon, 1997). Our results also show that the houbara bustard is sensitive to the extension of agricultural fields, which negatively affects its habitat use (Lavee, 1985; Goriup, 1997; Le Cuziat et al., 2005a,b; Hingrat et al., 2007; but see Medina, 1999 for partially positive influences of some agricultural practices in summer time). The density of rural tracks, regularly traversed either by tourist trekkers or sports motor-vehicles, also adversely influences the distribution pattern of the species in Fuerteventura. Uncontrolled traversing of wild and solitary areas during the breeding season (e.g. looking for mushrooms ‘criadas’ accompanied by dogs), and riding with quads and motorcycles at any time of the year, in those areas maintaining the large proportion of the Fuerteventura houbara population, should be limited and strictly regulated. This may be the reason for some of the seasonal changes observed in the geographical distribution of the species within the island, that is for example, a low relative abundance of houbaras in some places supporting large number of visitors in early spring (e.g. Jandía, Lajares-Oliva and Corralejo sandy areas; Table 3).
Population size and conservation status
Despite apparently having a large availability of potential habitat (as judged by the non-significant differences in many habitat structure variables), the mean densities and overall population of the species in Fuerteventura are low (0.11 birds per km2 and 177 birds respectively), otherwise consistent with its large body size (Carrascal & Telleria, 1991). The probable population trend of the houbara bustard in Fuerteventura can be inferred considering the previously published information. The two most exhaustive censuses of the houbara population in Fuerteventura reported 241 birds in 1994 (Martín et al., 1997) and 256 birds in 2000 (Anonymous, 2000). These estimations lay inside the 90% confidence interval, in 2005–2006 (108–258), measured in this paper. Therefore, the population size of the houbara bustard has remained stable or has probably shown a slight decrease during the last 6 years in Fuerteventura, which poses some conservation concerns according to its low population size (≤250 birds). This pattern contrasts with that observed in the nearby smaller island of Lanzarote (846 km2), separated from Fuerteventura by a narrow stretch of 11 km: mean density of 1.63 birds per km2 (the highest recorded in the whole geographic range of the species in northern Africa and central Asia), population size reaching 500 birds and average 40% increase in the last 6 years (Carrascal et al., 2006).
These contrasting differences for two subpopulations within the Canary metapopulation lead to some important ecological and conservation consequences. First, the between-island differences in population density and size can be easily explained considering the annual productivity, vegetation development and local rainfall, which are higher in Lanzarote than in Fuerteventura (Morales & Pérez-González, 2000; compare also the forb and grass covers of Fuerteventura in Table 1 in this study – 11.1% and 4.1%– with 20.6% and 6.8%, respectively, measured in Lanzarote according to Carrascal et al., 2006). Second, the contrasting population status and trends would suggest very different conservation concerns leading to a favourable protection status in Lanzarote and to a worrying conservation status in Fuerteventura and would suggest making necessary and immediate assessment of management priorities. The coexistence of disparate or antagonistic conservation status within the same Canary population weakens if the different subpopulations are viewed as several parts of the same Canary pool, and if possible inter-island movements influenced by between-years and between-areas differences in rainfall are taken into account. Indeed, it is known that the houbara bustard shows dispersive and migratory habits in other parts of its range (Cramp & Simmons, 1980; Johnsgard, 1991; Hingrat et al., 2004; Le Cuziat et al., 2005a), and that movements occur not only within but also among islands in the Eastern Canaries (Martín et al., 1997).
Four geographical strata supporting 4/5 of the whole island population (Tindaya, Triquivijate, Tefía-Ampuyenta and Fimapaire-Finimoy), are not included into the regional network of protected natural sites (http://www.gobcan.es/cmayot/espaciosnaturales/informacion/fuertev_todo.html). On the contrary, some historical areas of the species located on protected areas maintain very low densities during the breeding season (sand dunes of Corralejo and Jandía), perhaps because of being increasingly visited by tourists during the breeding season of the houbara. Therefore, it is necessary to reconsider the current design of the natural reserves in the practical conservation of the houbara bustard in Fuerteventura. For future conservation planning, we recommend that representative subset of areas in Fuerteventura be censused repeatedly to detect population trends. This census program should be carried out by means of extensive line transects using detectability estimates to correct observed population densities. Censuses should be concentrated in the most important areas for the species that include 80% of the total population in this island and only cover 250 km2 (Tindaya, Triquivijate, Tefía-Ampuyenta and Fimapaire-Finimoy; Fig. 1), thus minimizing the required number of qualified observers or economic resources available for censusing.