- Top of page
The Little Bustard has undergone a steep reduction of its Western Palaearctic range over the last century. In the west of France, breeding populations declined by 96% from 1978 to 2008 in cultivated areas where grasslands have been converted into intensively managed annual crops. Little Bustard abundance and nest productivity have been monitored since 1995 in a 450-km2 site in western France. We assessed the proximate causes of the decline of Little Bustards in French farming landscapes and quantified the effectiveness of conservation measures that aimed to reverse the decline. The decline of Little Bustard, from about 65 males in 1995 to just six males in 2003, could be related to a near absence of recruitment over this period. Since 2004, the establishment of more than 1300 ha of specifically targeted agri-environment schemes (AES) in the study site has led to a sharp increase in female productivity, mainly associated with nesting in AES fields. By imposing constraints on mowing dates, AES have prevented nest destruction and female mortality during mowing and, by increasing plant species diversity, provided chicks with a higher abundance of grasshoppers. This has contributed to reversing the trend, and increasing the population to around 30 males in 2009. Conservation strategies involving specifically targeted AES based on the identification of limiting factors can help to reverse the decline of threatened species.
Since 1994, agri-environment schemes (AES) have been widely used in Europe to counteract the general decline of farmland bird populations (Kleijn & Sutherland 2003). While some authors have shown positive effects (e.g. Berendse et al. 2004), evidence of the effectiveness of AES remains controversial due to the lack of quantitative assessment, inappropriate statistical tests or experimental designs (Kleijn & Sutherland 2003, Kleijn et al. 2004, 2006). AES specifically designed for threatened birds have yielded mixed and sometimes even negative results (e.g. Kleijn et al. 2001) that may pertain to a lack of precise understanding of the mechanisms of the decline prior to implementation.
The Little Bustard Tetrax tetrax is a medium-sized Palaearctic steppe bird in the Otididae (Del Hoyo et al. 1996) that is considered near-threatened worldwide (IUCN 2008). In Europe, Little Bustards occur in natural steppes as well as in agricultural landscapes, though the species’ range has greatly reduced over the 20th century (Goriup 1994), becoming extinct in at least 10 European countries (Cramp & Simmons 1980, Tucker & Heath 1994). Formerly continuously distributed from Iberia to Russia, western European populations (of the nominate subspecies T. t. tetrax) are now restricted to Portugal, Spain, France and Italy. Although these countries still harbour an estimated 110 000–280 000 individuals (Birdlife International 2004), populations have declined throughout the current range (Jolivet & Bretagnolle 2002, Cabral et al. 2005, Petretti 2006, García de la Morena et al. 2006). Western France now holds the last migratory population of the nominate subspecies (Villers et al. 2010), although some populations breeding in northern Spain show partial migration towards southern and central Spain (García de la Morena et al. 2004, 2006). The estimated French population size was 8500 displaying males in 1978–79, falling to 1400 males in 1995 (Jolivet 1996) and to 1300 males in 2000 (Jolivet & Bretagnolle 2002). The decline has only affected the migratory population breeding in cultivated areas, whereas the non-migratory population breeding in the natural steppe area of La Crau, southern France, has recently increased (Wolff et al. 2002, Jolivet et al. 2007). The core population inhabiting western farmlands has undergone one of the steepest declines recently documented for a European bird: from 7800 males in 1978 to 390 in 1996 (95% decline over 18 years; Bretagnolle & Inchausti 2005), and to 300 in 2008, with an estimated extinction risk of 45% over the next 30 years (Inchausti & Bretagnolle 2005).
In 1996, a LIFE project started in France that aimed to collect data on Little Bustard breeding biology, which was largely unknown at that time (Cramp & Simmons 1980), to identify the causes of this decline and to propose conservation measures to reverse the trend. Here, we analyse the trends of a Little Bustard population from a large (450 km2), intensively managed cereal agro-ecosystem of western France that has shown an 80% decline in just 8 years. We assess the causes of the decline, especially in relation to the loss of suitable breeding habitat and the decline in food availability (grasshoppers, Acrididae). We also show how the implementation of targeted AES aimed at mitigating the underlying causes of the Little Bustard decline has reversed the population trends in this population.
- Top of page
European farmed landscapes have traditionally consisted of complex mosaics of extensive crops that sustained high levels of biodiversity (Potter 1997, Walk & Warner 2000). Over the last 50 years, however, farmlands of western European countries have experienced dramatic changes, mainly through the intensification of farming techniques (Fuller et al. 1995, Siriwardena et al. 2000, Robinson & Sutherland 2002). Plants, insects and birds have declined at the community level (Tucker & Heath 1994, Pain & Dixon 1997, Bouma et al. 1998, Söderström & Part 2000, Chamberlain et al. 2000).
In the case of the Little Bustard, habitat loss and degradation, a result of agricultural intensification and increasing application of agro-chemicals that reduce food availability, were suspected to be responsible for the species decline (Goriup 1994). Our 15-year study has largely confirmed those threats and quantified their effects on Little Bustard productivity. Food reduction, mostly in arthropods, which are an essential resource for fledging success (Jiguet 2002), may be a limiting factor for Little Bustard productivity in agricultural habitats (Traba et al. 2008). Indeed, grasshoppers constitute the bulk of Little Bustard chick diet (Jiguet 2002). Data obtained in captivity have revealed that Little Bustard chicks require around 200 grasshoppers per day (Vincent Bretagnolle, unpubl. data). With an average grasshopper density of < 1/m2 (in non-AES fields), female Little Bustards must forage either in the richest plots or over large areas. Little Bustards rarely nested in crops other than alfalfa in our study site. Over half of Little Bustard nests were destroyed during the mowing of alfalfa, at least until AES were implemented. Before AES, there was a strong implication that a significant proportion of females did not breed, and for those that laid eggs, half of the nests were destroyed by alfalfa harvesting during incubation. For the remaining clutches that hatched, brood reduction, probably due to food shortage, resulted in almost complete chick loss. Consequently, between zero and two fledglings joined the post-breeding gatherings each year, resulting in almost no recruitment into the breeding stock between 1995 and 2001.
A first phase of the conservation strategy (1995–2000) consisted of gaining an understanding of Little Bustard breeding biology in agricultural landscapes and the consequences of farming activities on productivity and survival. Only thereafter could conservation measures start to be formalized and implemented on our study site. Little Bustard conservation efforts in France have been targeted at increasing insect availability (Jolivet & Bretagnolle 2002) and decreasing nest destruction through agri-environmental measures and, to a lesser extent, preventing female mortality during the mowing of grasslands. The main result of the conservation actions undertaken on our study site has been a complete reversal of the population trend, with male population size in 2008 being similar to that in 1998. It is not known whether the population increased due to local recruitment or immigration, as the ringing of a large number of wild chicks is technically difficult and questionable given the conservation status of the species. Large-scale monitoring of the entire western French population of male Little Bustards every 4 years since 1996 (Jolivet et al. 2007) has revealed a strong decline in non-SPA zones and, at the same time, either stable or increasing Little Bustard populations in SPAs. Thus in 2009, 85% of males were in SPAs in the region Poitou Charentes (compared with only 50% in 2000). This suggests that immigration might have contributed to the observed population increase (associated with the attractiveness of the SPA relative to surrounding areas), in addition to local improvements in productivity. Unfortunately, data on male dispersal are scarce and we can only propose a scenario that would need to be verified through data analysis on factors affecting male movement at a local scale, such as habitat quality and the presence of conspecifics. On the other hand, the observed rate of population increase (20% between 2003 and 2008) is still compatible with the recruitment of fledglings produced locally, with no need for immigration. In other words, the sharp increase in local productivity could be a direct consequence of a beneficial effect of AES on Little Bustard dynamics, as nests were increasingly located in fields under AES after 2004, the latter allowing higher survival for both females and chicks compared with years before the settlement of AES. However, it was not technically possible to quantify those effects directly on female or chick survival. In summary, even if the relationship between the increase in the number of chicks fledged and the settlement pattern in AES fields is largely correlative, the improvement in local resource abundance (see below) is very likely to have contributed to the increase in local Little Bustard productivity.
AES, at least for those cropped with alfalfa, also benefited other components of biodiversity. This perennial cover allowed higher diversity of arable weeds and grasshopper biomass compared with other annual crops where grasshoppers are almost absent. In addition, AES alfalfa showed higher diversity of weeds, and high abundance of grasshoppers, compared with non-AES alfalfa, suggesting that herbicide and insecticide bans are important practical measures to restore functional biodiversity in intensively managed agro-ecosystems. Although the AES involving alfalfa and its associated management practices were specifically designed for the protection of an endangered species, alfalfa seems to play a key role in intensive agricultural landscapes, at least at the regional level. We believe that alfalfa constitutes a semi-permanent habitat with relatively few impacts from management operations, resulting in higher associated biodiversity than in surrounding annual crops and helping to maintain the functioning of trophic chains in intensive agroecosystems. The implementation of AES and their costs have recently been the focus of much criticism (Kleijn & Sutherland 2003, Kleijn et al. 2006). Overall, it has been found that the results of AES in terms of their effect on farmland biodiversity were at best equivocal, and at worse negative. Based on our data on the population dynamics of Little Bustards as well as on more general benefits of AES in our study site, we argue that AES relying on ecological studies and therefore using ecological processes as proximate mechanisms for targeting measures can be implemented at reasonable costs with major benefits for farmland biodiversity.