Juliet A. Vickery

Farmland biodiversity: is habitat heterogeneity the key?

Agricultural intensification has led to a widespread decline in farmland biodiversity measured across many different taxa. The changes in agricultural practices affect many different aspects of the farmland habitat, but agricultural industry, policy and much previous research has tended to be concerned with specific sectors or practices (e.g. pesticide use or cereal husbandry). Here, we review the empirical literature to synthesize the research effort that has been directed to investigate specific practices or goals to make general statements regarding the causes and consequences of farmland biodiversity decline. We argue that the loss of ecological heterogeneity at multiple spatial and temporal scales is a universal consequence of multivariate agricultural intensification and, therefore, that future research should develop cross-cutting policy frameworks and management solutions that recreate that heterogeneity as the key to restoring and sustaining biodiversity in temperate agricultural systems.

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.

Winter availability of cereal stubbles attracts declining farmland birds and positively influences breeding population trends

Many studies have demonstrated the selection of stubble fields by farmland birds in winter, but none have shown whether provisioning of this key habitat positively influences national population trends for widespread farmland birds. We use two complementary extensive bird surveys undertaken at the same localities in summer and winter and show that the area of stubble in winter attracts increased numbers of several bird species of conservation concern. Moreover, for several farmland specialists, the availability of stubble fields in winter positively influenced the 10 year breeding population trend (1994–2003) whereas hedgerow bird species were less affected. For skylarks and yellowhammers, initially negative trends showed recovery with 10–20 ha of stubble per 1 km square. Thus, agri-environment schemes that promote retention of over-winter stubbles will attract birds locally and are capable of reversing current population declines if stubbles are available in sufficient quantity.

Habitat switching by dark-bellied brent geese Branta b. bernicla (L.) in relation to food depletion

Seasonal changes in the distribution and feeding behaviour of dark-bellied brent geese Branta b. bernicla (L.) and the biomass of their food plants were studied in three successive winters on the Norfolk coast. The data was used, in conjunction with published information, to show how depletion, productivity and mortality of food plants drive the pattern of habitat switching in this species. It is then possible to explain the habitat shifts observed over the last 35 years and predict future changes. On arrival, geese fed first on algal beds and then on salt marsh, grass and arable fields before returning to feed entirely on the salt marsh in spring. The biomass of green algae, and subsequently the salt marsh vegetation, declined during the autumn and this could be attributed to depletion through goose grazing and natural mortality. As depletion occurred the geese fed more intensively, for a greater percentage of time and with an increasing pace rate, the net result, however, was a declining intake rate (as measured by defaecation rate). The algal biomass at which the geese switched from the algal beds to salt marsh was consistent between years, with heavy storm-induced loss of algae in one year resulting in an earlier switch. That the timing of habitat switches may be explained by depletion of food plants was further supported by historical data: the number of brent geese wintering at the site has increased dramatically over the last 30–35 years and the time of switching from algal beds to salt marsh and from salt marsh to salt marsh and fields has become progressively earlier, as expected from the increased depletion. The expected further increase in brent goose numbers will increase the rate of depletion of intertidal vegetation so that the switches between habitats will be more rapid and the geese will move inland earlier and remain inland longer. The expected increase in the brent goose population will thus result in a disproportionate increase in the levels of conflict between brent geese and agriculture.

Conserving the Birds of Uganda’s Banana-Coffee Arc: Land Sparing and Land Sharing Compared

Reconciling the aims of feeding an ever more demanding human population and conserving biodiversity is a difficult challenge. Here, we explore potential solutions by assessing whether land sparing (farming for high yield, potentially enabling the protection of non-farmland habitat), land sharing (lower yielding farming with more biodiversity within farmland) or a mixed strategy would result in better bird conservation outcomes for a specified level of agricultural production. We surveyed forest and farmland study areas in southern Uganda, measuring the population density of 256 bird species and agricultural yield: food energy and gross income. Parametric non-linear functions relating density to yield were fitted. Species were identified as “winners” (total population size always at least as great with agriculture present as without it) or “losers” (total population sometimes or always reduced with agriculture present) for a range of targets for total agricultural production. For each target we determined whether each species would be predicted to have a higher total population with land sparing, land sharing or with any intermediate level of sparing at an intermediate yield. We found that most species were expected to have their highest total populations with land sparing, particularly loser species and species with small global range sizes. Hence, more species would benefit from high-yield farming if used as part of a strategy to reduce forest loss than from low-yield farming and land sharing, as has been found in Ghana and India in a previous study. We caution against advocacy for high-yield farming alone as a means to deliver land sparing if it is done without strong protection for natural habitats, other ecosystem services and social welfare. Instead, we suggest that conservationists explore how conservation and agricultural policies can be better integrated to deliver land sparing by, for example, combining land-use planning and agronomic support for small farmers.

Numbers and distribution of Northern Lapwings Vanellus vanellus breeding in England and Wales in 1998

This paper presents the results of the BTO/RSPB survey of breeding Northern Lapwings Vanellus vanellus in England and Wales carried out in 1998. Numbers of breeding Lapwings and land-use types were recorded by volunteers in 1312 randomly selected tetrads using identical methods to those adopted in a previous survey in 1987. The results suggest a population of 62 923 breeding pairs (95% confidence intervals 55 268 to 74 499), representing a 49% decline in 11 years. The declines were greatest in southwest England and Wales, resulting in a marked contraction in range. Both arable and pastoral farmland was found to be important for Lapwings, highlighting their vulnerability to changes in a range of agricultural systems. Recent major changes in land management practices, including the switch from spring to autumn sowing and the conversion of rough to intensively managed, improved grassland is likely to have reduced both the area and the suitability of these habitats for Lapwings. Grassland within arable areas is known to provide important chick rearing habitat, and spring tillage with adjacent grassland was significantly more likely to hold Lapwings than spring tillage that did not abut grass fields. The prevalence of grass and spring tillage fields in close proximity has declined in recent years with the loss of traditional mixed farming and the polarization of pastoral systems in the west of Britain and arable in the east. The implications of these results for the conservation of Lapwings are discussed and it is recommended that, in the light of the scale of the decline recorded, Lapwings should be afforded much higher conservation priority in Britain than they are at present. The range contraction has resulted in high numbers being concentrated on reserves, highlighting the importance of sensitive management of these areas for this species. However, wider countryside measures should also be considered, such as specific options within agri-environment schemes, designed to enhance farmland habitats for Lapwings.

Changes in the numbers of breeding waders on lowland wet grasslands in England and Wales between 1982 and 2002

Capsule Lapwing, Snipe, Curlew and Redshank decreased significantly between 1982 and 2002, while over the same period Oystercatcher increased. Aims To provide current status and population trends for waders breeding on lowland wet grassland in England and Wales. Methods Waders were counted and mapped on three visits to 1051 lowland wet grassland sites, between April and June 2002. Data were compared with those gathered using identical methods in a survey in 1982 and related to land use characteristics and conservation designation. Results Between 1982 and 2002 there were significant declines of 38% for Lapwing Vanellus vanellus, 61% for Snipe Gallinago gallinago, 40% for Curlew Numenius arquata and 29% for Redshank Tringa totanus. Oystercatcher Haematopus ostralegus increased significantly by 47% in the same period. Populations of all species were highly aggregated with most of the birds found on a small number of key sites. Site designation was found to be an important predictor of breeding wader distribution, density and density change. Conclusions The marked population declines of four grassland wader species over the last 20 years are of considerable concern. Careful management of nature reserves and other key sites is essential to halt these declines, while wider scale and more carefully targeted agri-environment schemes may hold the key to reversing declines in the longer term. The efficacy of site designation and current agri-environment schemes for conserving breeding waders on wet grasslands needs to be reviewed.

Densities and population estimates of breeding Skylarks Alauda arvensis in Britain in 1997

We present the results of a national survey of breeding Skylarks Alauda arvensis in Britain in 1997 carried out by the British Trust for Ornithology (BTO). Numbers of Skylarks and land-use types were recorded by volunteers in 608 1-km squares, selected using random stratification based on the Institute of Terrestrial Ecologys (ITE) Landscape Classification to avoid over- or under-sampling particular habitat types. The results suggest a maximum national population of around 1000 000 pairs. This agrees extremely well with the national population estimate of around 1046 000 pairs derived from the Breeding Bird Survey (BBS) of the BTO/Joint Nature Conservation Committee/Royal Society for the Protection of Birds, and provides further evidence for the robustness of the BBS as a national monitoring scheme for common and widespread species. The figure is approximately half that of the most recent published estimate of 2000 000 in 1988–91. However, the scale of this discrepancy is likely to reflect a bias in Atlas field methodology and site selection since there has been a real decline of approximately 9% since 1990. Arable squares supported the highest densities of Skylarks; 4.6–6.0 pairs per km2, and 46–49% of the British breeding population was associated with arable areas. Marginal upland and upland areas supported lower densities but still accounted for approximately 34% of the estimated national breeding population. Differences in density at broad habitat scales were reflected in geographical differences across Britain, with southern and eastern arable regions supporting much higher densities than western and northern ones. At a finer scale, Skylarks occurred at highest densities on, and showed highest habitat preferences for, set-aside and various types of ungrazed grassland. Winter cereal, improved grassland and set-aside held the highest proportion of the Skylark population on farmland in England and Wales; grazed pasture, winter cereals and spring cereals held the highest proportion in Scotland.

Determinants of bird species richness in public green spaces

Capsule Bird species richness showed consistent positive correlations with site area and rough grass. Aim To determine those features of parks and other public green spaces that are of most importance in promoting bird species richness and to use this information to make recommendations for management of these areas. Methods Birds were counted in parks and other public green spaces within Greater London. Three survey visits were made in each of the periods summer 2002 and 2003 and winter 2002/03 and 2003/04. Habitat data were also collected. Results Site area was the most consistently significant variable related (positively) to total species richness. There were also positive effects of waterbodies and rough grass (including nettles and weed-rich areas). Species richness increased with a greater proportion of adjacent gardens in sites <1 ha in size. There were negative associations with the presence of buildings. When considered by species group (woodland species, farmland species and habitat generalists), results were very similar to those for total species richness, except that there were no significant associations with presence of waterbodies. Conclusion Bird species richness is likely to be enhanced by increasing site area, and providing rough grass and waterbodies in urban green spaces. Such information could be especially useful in designing green spaces in new residential developments to maximize avian diversity. Surrounding habitats are also crucial determinants of bird species richness in parks.

A preliminary assessment of some factors influencing the density and distribution of palearctic passerine migrants wintering in the Sahel zone of West Africa

Population densities of Palearctic migrant and African birds were estimated from 444 point counts, that estimated minimum density, at 10 sites in the Sahel zone of northern Nigeria during December–January 1993/94. In total, 15 Palearctic species were recorded during point counts averaging 5 species per site, and 76 African species averaging 24 species per site. The densities of Palearctic migrants varied from 1 bird ha-1 in semi-desert and highly degraded woodland, up to 8 birds ha-1 in dense Sahelian woodland. Common Whitethroats Sylvia communis were most abundant (up to 0.7 birds ha-1) in sites rich in Piliostigma reticulata trees; Subalpine Warblers S. cantillans were most abundant (up to 5.9 birds ha-1) in sites rich in Acacia spp. and Cassia sieberiana; Lesser Whitethroats S. curruca and Redstarts Phoenicurus phoenicurus were associated with high overall tree densities (both up to 0.6 birds ha-1); Northern Wheatears Oenanthe oenanthe occurred at densities of about 1 bird ha-1 at very low tree density...