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At the turn of the century, British farmland was mostly of a mixed character, with crop and animal husbandry coexisting on most farms. Much agricultural land fell into disuse during the economic depression of the 1930s, but with the increase in agricultural activity in the late 1940s the farming landscape became increasingly polarized, with arable predominating in the east of Britain and pasture-based farming in the west (Stoate 1995). This was largely caused by an increase in mechanization, which removed the need to grow forage crops for horses on arable farms, whilst an increase in use of agricultural chemicals removed the need for non-cereal crops and fallows, which maintained soil fertility and prevented pest build-up (Stoate 1995, 1996). These trends are continuing as the necessity for capital-intensive machinery encourages specialization in either crop or animal husbandry. Consequently, arable habitat has become increasingly scarce in pastoral landscapes, with the loss tending to be greatest in the most grassland-dominated areas (Table 1).
Table 1. Loss of arable land in grass-dominated areas (arable : grass ratio < 0·5) between 1970 and 1997 (data from MAFF statistics). The data for Scotland measure change 1978–97 and no account has been taken of slight changes in recording methods during this period
| ||% arable 1970||% arable 1997||% change|
|Upland Northern Ireland|| 6·1|| 3·1||−49·8|
|North-west England||13·8|| 7·8||−43·8|
|Wales|| 7·9|| 4·8||−40·1|
|Lowland Northern Ireland||14·6||10·7||−26·6|
|West Scotland|| 2·6|| 2·3||−11·5|
|South-west England||18·9||18·4|| −2·7|
|North-east England||29·1||32·0|| 10·0|
|East Scotland||17·6||19·5|| 11·1|
The declines in farmland bird populations, particularly granivorous passerines, are likely to have been caused by changes in arable management because arable farmland holds significant proportions of their populations (Gregory & Baillie 1998; Fuller 2000). Changes in the timing of cereal sowing, reducing the availability of seed-rich winter stubbles (Wilson, Taylor & Muirhead 1996), and reductions in food supply caused by increased herbicide use (Campbell et al. 1997), are likely to have been particularly important, although most of the changes are likely to be interrelated (Chamberlain et al. 2000). Although populations of many of these species have declined more rapidly in arable than pastoral areas, the extent of local extinctions in western Britain has been much greater than in the arable east of the country, because bird densities were initially lower in grassland areas (Chamberlain & Fuller 2000).
Vickery et al. (1999) collated data on national bird abundance to assess differences between grassland, mixed and arable habitats. They found that the majority of species occurred most abundantly in areas of mixed farming, particularly in winter. Detailed autecological studies have shown that the breeding densities of some species, notably skylark Alauda arvensis (Jenny 1990) and corn bunting (Ward & Aebischer 1994), are directly related to the diversity of crops available at a farm scale. Thus, the loss of arable habitat in grassland regions may have contributed to the declines of farmland birds in these areas and, conversely, support for arable habitat in these areas could help the conservation status of these species.
In this study we aimed to test whether increasing arable cultivation in pastoral landscapes influences the local abundance of 11 species of granivorous bird that are particularly dependent on arable habitat (skylark, chaffinch Fringilla coelebs, greenfinch Carduelis chloris, goldfinch Carduelis carduelis, linnet Carduelis cannabina, tree sparrow, house sparrow Passer domesticus, yellowhammer Emberiza citrinella, reed bunting Emberiza schoeniclus, corn bunting and grey partridge). Populations of all but chaffinch, greenfinch and goldfinch have declined severely (> 35%) since 1970 (Siriwardena et al. 1998). We also included four insectivores of similar body mass that are common on farmland: two residents, dunnock Prunella modularis and robin Erithacus rubecula, and two migrants, whitethroat Sylvia communis and blackcap Sylvia atricapilla. These were used as reference species to indicate whether arable cultivation provides additional resources specifically for the seed-eaters or some other, more general, benefits. Of these insectivores, only the dunnock has shown a slow population decline; the other three have increased in numbers since 1970. Nomenclature follows BOU (1992).
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In recent years, much has been made of the increasing intensity of management of arable systems and its impact on birds throughout Europe (Tucker 1997; Krebs et al. 1999; Aebischer et al. 2000). However, the analyses presented here show that, in grassland landscapes, increasing the amount of arable cultivation is likely to be beneficial for granivorous bird species, particularly those that have experienced severe population declines. As the availability of arable habitats in the surrounding region and in the wider landscape increases, the positive association between bird density and the local availability of arable habitat tends to weaken and in some cases is reversed. Where the latter occurs, this indicates that the presence of some grassland may be important for the persistence of some bird species in landscapes dominated by arable cultivation.
These results demonstrate the scale-dependence of bird–habitat associations, which are best shown in data sets of large geographical extent but small ‘grain’, such as the BBS. Birds are likely to select habitats hierarchically, so habitat associations will vary with scale (Kotliar & Wiens 1990). Small-scale analyses have shown that scarce features are more important in determining territory location. Thus, for example, nuthatch Sitta europaea territory occupation is correlated with the number of oak trees in coniferous woods, where they are scarce, but not in mixed deciduous woods, where they are common (Burkhardt, Schlund & Stauss 1998). This study reveals further the importance of such scale-dependence with respect to regional populations. Thus, studies of habitat associations need to consider regional context, as their strength, or even direction, may vary (Kotliar & Wiens 1990; Steele 1992).
These results support the hypothesis that range contractions (i.e. local extinctions) of some granivorous species (e.g. grey partridge, tree sparrow and corn bunting) in grass-dominated landscapes are a consequence of reduced arable cultivation. Species that have not suffered range contractions (e.g. chaffinch, greenfinch and linnet) do not seem to be strongly associated with the presence of arable habitat. This has important implications for conservation planning because it suggests that the loss of arable habitat where it is scarce may be causing declines in some areas, even though intensification of arable management is the main cause of decline elsewhere. Such distinctions present challenges to those developing agri-environmental policies adopted at the national level; very different strategies may be required in different areas.
Inspection of Fig. 2 shows that, although some species in grass-dominated landscapes tend to be found only where arable is locally common (e.g. corn bunting), others (e.g. yellowhammer, skylark and whitethroat) increase even if only small amounts of arable habitat occur within a BBS square. In western Britain, average farm size, excluding non-cropped areas such as wood and farm buildings, is around 40–50 ha (MAFF 1998). On the basis of the results presented here, if even a single field (typically 5–10 ha or c. 15% of the cropped area) on each farm was used for arable crops, farmland bird numbers could be increased.
For the granivorous species considered here, arable cultivation in grassland landscapes is likely to be beneficial through the provision of additional food sources. The major part of the winter diet of these species is the seed of arable weeds and spilt cereal grain that are available on post-harvest stubbles (Wilson et al. 1999). The annual weeds typical of arable systems are scarce in pastoral areas, particularly the highly fertilized, heavily grazed pasture common over much of lowland Britain (Wakeham-Dawson et al. 1998). A number of studies have shown that these birds prefer to forage in areas where seed density is high (Wilson, Taylor & Muirhead 1996; Robinson & Sutherland 1999). Thus, winter habitat management is likely to increase summer breeding numbers, especially for relatively sedentary species such as the yellowhammer and skylark.
None of the finches occurred in greater numbers in areas with more arable habitat. In general, finches prefer to feed on the seed of perennial weeds more associated with grassland (Wilson et al. 1999). Numbers of only one of the reference insectivores, the whitethroat, increased with the availability of arable habitat in grassland landscapes. Whitethroats use rough grassy margins and ditch banks as nesting habitat (Stoate 1999), whereas the other three insectivorous species nest in woody vegetation. Arable fields are more likely to provide this habitat because of the absence of stock grazing to the hedge base, and the difficulty of operating farm machinery in the field margins. Some of the granivorous species, notably the buntings and grey partridge, also use rough herbaceous field margins for nesting (Potts 1986; Bradbury et al. 2000; Brickle et al. 2000) and are likely to benefit in the same way.
The analyses presented here treat both arable land and agricultural grassland as though they were homogeneous habitat types. In reality, both arable and grassland vary greatly in structure, species composition and management, both within and between fields. Much recent research has shown the complexity of the mechanisms linking this variation to the species, distribution, abundance and demography of birds present (Aebischer, Green & Evans 2000). Managing arable habitat to provide over-winter stubbles, spring-sown cereals or grass margins would increase numbers of birds. Siriwardena et al. (2000) analysed the frequency of occurrence of birds breeding on British farmland and found that arable–pastoral heterogeneity per se generally did not increase bird abundance, but that particular agriculture practices were important for particular species. Further work is required to determine whether the greater importance of arable habitat in grassland habitats is because of differences in habitat management, or whether there is an interaction with the surrounding habitat. For example, pastoral farmers are unlikely to manage arable habitat as intensively as arable farmers and territories may be smaller where both grass and arable habitats are available contiguously.
The importance of over-winter stubbles has been clearly demonstrated for the cirl bunting Emberiza cirlus, whose UK population had fallen to 118 pairs, largely concentrated in south-west England. Provision of weedy stubble habitat through agri-environment schemes increased cirl bunting numbers to around 500 pairs in less than 10 years (Aebischer, Green & Evans 2000). The results presented here suggest that similar action in other pastoral landscapes could be beneficial to avian biodiversity more generally, by providing increased nesting habitat and winter food resources.
In Europe several schemes are designed to mitigate the environmental impacts of agriculture, including the Environmentally Sensitive Area (ESA) and the Less Favoured Area (LFA) schemes, which are currently being re-implemented through the Agenda 2000 reforms (Baldock et al. 2000). In addition, there are schemes specific to individual European Union (EU) member states, such as Countryside Stewardship in England. Their aim is to maintain and increase the wildlife and landscape amenity of areas under the agreements and they are largely focused on pastoral areas, where farming tends to be less profitable (Ovenden, Swash & Smallshire 1999). We suggest these schemes could be used as a basis for encouraging arable cultivation in pastoral areas; similar proposals are included in the new Welsh LFA regulations to encourage a more mixed farming landscape (Baldock et al. 2000). However, the scale of the benefits in terms of bird numbers will depend critically upon the specific management practices employed; monitoring the effect of their implementation should be a priority.
More generally, this study indicates bird species diversity and abundance are likely to fall wherever agricultural landscapes become homogeneous, whether they are dominated by intensive grass or arable management. ‘Pockets’ of arable management in grassland landscapes, and of grassland within intensively arable land, may be of great importance in allowing populations to persist at the landscape scale. The encouragement of regionally rare broad categories of farming may be of value as a general principle in agri-environmental planning.