Robert J. Fuller

Benefits of organic farming to biodiversity vary among taxa

Habitat and biodiversity differences between matched pairs of organic and non-organic farms containing cereal crops in lowland England were assessed by a large-scale study of plants, invertebrates, birds and bats. Habitat extent, composition and management on organic farms was likely to favour higher levels of biodiversity and indeed organic farms tended to support higher numbers of species and overall abundance across most taxa. However, the magnitude of the response varied; plants showed larger and more consistent responses than other taxa. Variation in response across taxa may be partly a consequence of the small size and isolated context of many organic farms. Extension of organic farming could contribute to the restoration of biodiversity in agricultural landscapes.

Changes in sheep numbers in Britain: implications for bird populations

Abstract Large increases in sheep numbers occurred in most regions of Britain, both upland and lowland, between the mid 1970s and late 1980s, followed by a drop or levelling in numbers. In the mid 1990s numbers of sheep generally remained far higher than in the mid 1970s. Wales stands out as showing an increase throughout the period since 1950 and as a region of exceptionally high sheep density. These broad trends conceal much local variation. There is concern about the implications for bird populations, especially in the Welsh uplands, and the mechanisms are reviewed by which the increase in sheep may have affected birds, especially in semi-natural upland habitats. Grazing can potentially affect bird populations through a large number of mechanisms but principal ones are likely to be: (1) loss of preferred vegetation types, (2) alteration of food supplies and (3) alteration of predation pressure. The recent increases in sheep have almost certainly caused reduction in habitat quality for ground-nesting birds such as grouse and waders in some regions. However, firm evidence linking declines in upland birds to increases in sheep is largely unavailable because (a) there is inadequate monitoring of upland birds, (b) habitat-specific information on changes in grazing pressure is generally lacking, (c) there has been little relevant research on mechanisms, (d) effects of severe grazing cannot be readily isolated from other factors such as acidification and changes in predation pressure. It is suggested that the apparent collapse in Welsh upland bird populations may be a response to this recent increase in grazing pressure superimposed on a long-term reduction in carrying capacity.

Assessing the impacts of agricultural intensification on biodiversity: a British perspective

Agricultural intensification is best considered as the level of human appropriation of terrestrial net primary production. The global value is set to increase from 30%, increasing pressures on biodiversity. The pressures can be classified in terms of spatial scale, i.e. land cover, landscape management and crop management. Different lowland agricultural landscapes in Great Britain show differences among these pressures when habitat diversity and nutrient surplus are used as indicators. Eutrophication of plants was correlated to N surplus, and species richness of plants correlated with broad habitat diversity. Bird species diversity only correlated with habitat diversity when the diversity of different agricultural habitats was taken into account. The pressures of agricultural change may be reduced by minimizing loss of large habitats, minimizing permanent loss of agricultural land, maintaining habitat diversity in agricultural landscapes in order to provide ecosystem services, and minimizing pollution from nutrients and pesticides from the crops themselves. While these pressures could potentially be quantified using an internationally consistent set of indicators, their impacts would need to be assessed using a much larger number of locally applicable biodiversity indicators.

A comparison of bird populations on organic and conventional farm systems in southern Britain

Abstract Field boundaries and fields on 22 pairs of organic and conventional farms in England and Wales were surveyed over three breeding seasons (April–July) and two autumn (September–November) and winter (December–February) periods in order to ascertain whether organic and conventional farms differed in the size and diversity of their associated bird populations. Species diversity was significantly higher on organic farms in the 1994 breeding season, but in no other year or season. Of 18 species, eight showed a significantly higher density on organic field boundaries in at least one season/year, with a greater number of significant results being detected in the autumn. There were very few significant differences in bird density in fields outside the breeding season. The density of breeding skylarks Alauda arvensis, the principal field nesting species, was significantly greater on organic farms in one breeding season. Hedges tended to be higher and wider, field boundaries tended to have more trees and field sizes tended to be smaller on organic farms. Canonical correspondence analysis indicated that components of habitat structure were important in explaining differences in bird density between farm types for certain species.

The potential value of managed cereal field margins as foraging habitats for farmland birds in the UK

Abstract Many farmland bird populations have exhibited marked declines in the last 20–30 years and there is growing evidence to link these declines to agricultural intensification. Field margin management is frequently proposed as a way of enhancing bird populations but there have been few attempts to assess the relative value of different management options. This paper aims to provide a preliminary assessment of the relative value of six different cereal field margin management practices in providing foraging habitats for farmland birds. It also briefly compares these with the benefits afforded by field and whole farm approaches such as set-aside and organic farming, to wildlife conservation on arable land. The field margins considered are: grass margins (separated into grass-only strips and grass/wildflower strips), naturally regenerated set-aside margins, uncropped wildlife strips, game cover crops and conservation headlands. Sympathetically managed field margins can provide a range of plant and invertebrate food resources for birds both in summer and winter. In general, the best winter food supplies (mainly seeds) will be provided by game cover crops and naturally regenerated rotational set-aside strips. The most abundant summer food supplies (invertebrates and seeds) will be provided by a diverse sward; grass/wildflower strips, uncropped wildlife strips and naturally regenerated rotational set-aside strips followed by conservation headlands. Field margin swards that are less diverse in terms of species composition and structural heterogeneity and that consequently support fewer invertebrates, can, nonetheless, provide higher quality foraging habitats for birds than an intensively managed crop up to the hedge base. A number of bird species such as yellowhammer ( Emberiza citrinella ) and tree sparrow ( Passer montanus ), prefer to forage in margins in winter and summer and many of the benefits of whole field approaches such as set-aside, overwinter stubbles and undersown cereals, could be gained from margins under the same management. Whole-field approaches are, however, required for boundary-avoiding species such as skylark ( Alauda arvensis ) and lapwing ( Vanellus vanellus ). Less is known about the use other birds make of margins, relative to their use of field centres in winter. Whole farm approaches such as organic farming, whilst being highly beneficial to birds, are likely to remain rather localised in the UK, whereas field margin management can be relatively easily incorporated into the farmed landscape on an extensive scale.

Local extinctions and changes in species richness of lowland farmland birds in England and Wales in relation to recent changes in agricultural land-use

Changes in agricultural land-use may have been responsible for contractions in range that have occurred in a number of bird species over the past three decades. This was considered by examining spatial change in the ranges of 21 farmland bird species at the scale of 10 km squares in relation to spatial change in agricultural land-use variables between the late 1960s and the late 1980s in lowland England and Wales. Seven species showed range declines (local extinction) exceeding 5% over this period and analyses focused on these: Grey Partridge Perdix perdix, Lapwing Vanellus vanellus, Turtle Dove Streptopelia turtur, Yellow Wagtail Motacilla flava, Tree Sparrow Passer montanus, Corn Bunting Miliaria calandra and Reed Bunting Emberiza schoeniclus. Individual species loss and change in species richness tended to be related to variables with strong regional trends, reflecting greater losses in western 10 km squares characterized by pastoral agriculture. It is unlikely that many of the variables selected in the regression models are, in themselves, the causal factors behind changes in the ranges of species and changes in species richness. Lapwing was an exception, agricultural variables associated with grassland being consistently selected. Principal Components Analysis (PCA) was applied to the agricultural variables. This identified a major gradient of change in cropping patterns, involving large increases in areas of wheat Triticum spp. and oilseed rape Brassica napus at the expense of barley Hordeum spp., bare fallow and grass. Local extinctions and change in species richness were consistently related to the first PCA axis, showing that local extinctions have occurred most in those squares where there had been relatively little change in crop types. The greater rate of local extinctions in pastoral regions may be associated with a number of factors, including changes in the management of grass and livestock, edge of range effects (where species in less favoured habitats are more likely to become locally extinct) and source-sink effects. These results highlight the need for further research into the effects of agricultural management on birds in pastoral systems. ©2000 Elsevier Science B.V. All rights reserved.

Estimating numbers of birds by point counts: how long should counts last?

In the British habitats studied, most species and pairs were seen within 10 minutes. Hence longer point counts are a poor investment of time.

Effects of forest management and grazing on breeding bird communities in plantations of broadleaved and coniferous trees in western England

Management options in commercial forestry include choice of conifers or broadleaves, rotation length, stand size and grazing regime. Each factor potentially affects the conservation value of woodland for birds. Relationships between these factors and the structure and composition of breeding bird communities were examined in 69 stands distributed across a range of plantations composed of predominantly native broadleaved and non-native coniferous trees in the Forest of Dean, western England, in 1992 and 1993. Each stand was classified as one of three high forest types: broadleaved, coniferous or mixed broadleaves and conifers. Stand size had no effect on bird communities. Species richness and overall bird abundance increased with forest age when all forest types were combined. Within stands of similar ages, there were no consistent differences in species richness or overall bird abundance between the three forest types or between grazed and ungrazed stands. However, bird communities in mixed stands were intermediate in their overall species composition to those in broadleaved and coniferous stands. Regression and gradient analyses (CCA and PCA) revealed that major gradients in the species composition of the bird communities were associated with stand age and with tree species composition. The proportion of individuals contributed by hole-nesting species was higher in broadleaved than coniferous stands and increased with stand age. The proportion of individuals contributed by migrants was higher in especially the early years, but also in the late years of the rotation. The proportion of migrants was higher in ungrazed than in grazed stands and increased with openness of the canopy and development of low vegetation. The diversity of stands in terms of tree sizes and tree species was positively correlated with both number of bird species and overall bird abundance. The relevance of these findings is discussed in relation to the integration of bird conservation into coniferous forestry, focusing particularly on the value of broadleaved stands and the effects on bird communities of stand structure and grazing pressure.

Changes in bird populations on sample lowland English farms in relation to loss of hedgerows and other non-crop habitats

Abstract Farmland bird population trends were examined on a sample of lowland English farms to assess the relative importance of habitat loss and habitat degradation. Data were extracted from 11 farms surveyed by territory mapping between 1966 and 1986 as part of the British Trust for Ornithologys Common Birds Census. The population size of 38 bird species was quantified for each farm in each year. The extents of five non-crop habitats were measured at 4-yearly intervals on each farm. The farms were selected because some had undergone extensive removal of non-crop habitats while others had undergone little or none. Although declines were commonest on farms where the severest habitat loss had taken place, we found no evidence that habitat loss was the main factor causing population declines: all 11 farms had significant numbers of declining species, even where habitat loss was minimal. Furthermore, general linear modelling found no significant effects of habitat loss on population trends and principal-components analysis found limited effects of habitat extent on community composition. These results suggest that habitat loss is of secondary importance in causing farmland bird population declines. We suggest that other processes, such as habitat degradation, may have caused a baseline population decline in at least 10 farmland bird species and that declines may have been exacerbated by localised habitat loss.

Agronomic and ecological costs and benefits of set-aside in England

Abstract Around 11% of arable land in England was set-aside in the mid-1990s as part of the Common Agricultural Policy (CAP). The scheme allowed both annual and longer-term exclusion from cropping. The most widespread green cover was natural regeneration, followed by non-food crops and sown grass covers. Impacts of set-aside management on agronomy and ecology were investigated using a questionnaire for farmers and field studies of plants, invertebrates and breeding birds on up to 200 set-aside fields, half rotational and half non-rotational. More detailed studies were conducted on crop diseases, vegetation dynamics and breeding birds. Plant species diversity on set-aside was greater in the west of England than in the east, and, on non-rotational naturally-regenerated set-aside, plant communities became increasingly dominated by grassland species with age. Gradients of powdery mildew and septoria leaf blotch infection levels were observed within cereal fields adjacent to rotational set-aside. Invertebrate pest species were more frequent in crops than in neighbouring set-aside, regardless of type. All groups of birds studied were found least on winter cereals, and most were found preferentially on rotational set-aside. Set-aside (especially rotational) has provided suitable habitats for breeding birds over very large areas, and may have helped to compensate for other changes in the farmed landscape detrimental to birds, without causing major agronomic problems. It is important that any scheme replacing set-aside should be over large enough total areas so that the benefits to farmland birds across landscapes are not reduced.