Biodiversity under Threatby R E Hester, Geoffrey Allen, Anthony K Barbour, Don MacKay
There is much public concern about threats to global biodiversity. Industrial pollution, changes in agricultural practices and climate change, are all having a direct impact on biodiversity. In this book the Editors provide a broad view of the many pressures imposed by human-induced changes and the many threats to global biodiversity and of the policy responses
There is much public concern about threats to global biodiversity. Industrial pollution, changes in agricultural practices and climate change, are all having a direct impact on biodiversity. In this book the Editors provide a broad view of the many pressures imposed by human-induced changes and the many threats to global biodiversity and of the policy responses required to combat them. This excellent text includes the work of some 44 authors and offers a solid description of the current understanding of threats to biodiversity with a range of illustrative examples - a valuable point of reference for ecologists, environmental scientists, and students as well as, policymakers and all other environmental professionals.
- Royal Society of Chemistry, The
- Publication date:
- Issues in Environmental Science and Technology Series, #25
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Biodiversity Under Threat
By R. E. Hester, R. M. Harrison
The Royal Society of ChemistryCopyright © 2007 The Royal Society of Chemistry
All rights reserved.
Impacts of Agricultural Change on Farmland Biodiversity in the UK
NIGEL D. BOATMAN, HAZEL R. PARRY, JULIE D. BISHOP AND ANDREW G.S. CUTHBERTSON
Over the past 50 years, there has been a marked decline shown by many species closely associated with lowland farmland in the UK, which is widely considered to be a key issue in British nature conservation. Increased availability of survey data has meant it is now possible to quantify changes in biodiversity for some groups. Changes have occurred in many farming practices and these have affected biodiversity in a variety of ways. Mixed agriculture in Britain has been lost and farms have become specialised; traditional crop rotations have declined and pastoral and arable farming have become polarised. Farming has intensified; for example, wheat yields in Scotland increased by 201% during the period 1967–1999 due to more effective tillage, application of fertilisers and pesticides and plant breeding. Field sizes have increased, reducing non-crop habitat at field margins. Other changes include more autumn sowing of crops and more efficient harvesting, more non-inversion tillage, drainage and reseeding of grassland, a switch from hay to silage, increased stocking rates and the use of avermectin wormers. This has all impacted upon wildlife species that inhabit lowland farmland landscapes. Upland farming and livestock grazing has also intensified during this time. High grazing pressure, especially by sheep, has had a negative impact upon vegetation and wildlife in many upland regions of Britain.
Much of the focus on biodiversity conservation within agricultural landscapes has been on the conservation of rare or rapidly declining species, driven in part by the UK Biodiversity Action Plan, the Government's response to the 1992 Convention on Biological Diversity, and also by the adoption of a commitment as part of the Government's Public Service Agreement to "care for our natural heritage, make the countryside attractive and enjoyable for all and preserve biological diversity by reversing the long-term decline in the number of farmland birds by 2020, as measured annually against underlying trends; and bringing into favourable condition by 2010 95 per cent of all nationally important wildlife sites'. However, other issues of importance in the context of agriculture include whether or not increased biodiversity or species richness enhances ecosystem functions such as primary productivity and nutrient retention or ecosystem services such as pollination and biological control. Non- crop habitats on farmland are usually more species diverse than cropped fields or intensive grasslands, and the areas of non-crop habitat may even become islands of species richness if dispersal across suitable habitat is limited. However, over-zealous "tidying" of non-crop areas can reduce value for biodiversity and there may be a conflict here between the concept of an attractive (which for many equates with 'tidy') countryside and the wish to see it populated by diverse fauna and flora.
A review of changes in biodiversity on arable farmland concluded that around half of plant species, a third of insect species and four-fifths of bird species characteristic of farmland have declined. This chapter reviews the key changes that have occurred in agriculture during the second half of the twentieth century, set in the context of evolving policies on agricultural support and their impact on the characteristic fauna and flora of the farmed landscape. It concludes by considering the most recent reform of the EU Common Agricultural Policy and the implications of the shift away from production support and towards greater provision of incentives for environmentally sustainable management. A.D. Watt et al. present additional related and complementary material in Chapter 6 of this volume, with particular emphasis on the wider European aspects of land use change.
2 The Post-war Intensification of Agriculture
During and following the Second World War, agricultural policy focused on maximising food production. This resulted in an unprecedented level of change and intensification accompanied by subsidies ensuring prolonged price stability throughout the second half of the twentieth century. Although highly successful, this policy has been blamed for large declines in biodiversity within both the UK and Europe. Farming practices became polarised, where arable farming dominated the east of the UK and grassland/livestock farming the west, whilst mixed farming and the use of grass leys declined.
The key changes in both arable and grassland landscapes have been identified. In arable landscapes, the key changes that took place in the last forty years include simpler rotations and block cropping; a switch from spring to autumn cropping; more efficient harvesting and sealed grain storage; and recently, more non-inversion tillage. On grasslands, key changes in the last forty years include more drainage and reseeding; a switch from hay to silage; a move away from dicotyledonous fodder crops and barley grown for fodder to intensive grass and forage maize; increased stocking rates; and the introduction of avermectins. In the lowlands, there was a trend to larger, specialised farms with large fields and fewer hedgerows and increased use of pesticides and fertilisers.
2.1 Land Drainage
Land drainage grants were introduced in 1940 and abolished in 1987. The area of land drained reached a peak of 100 000 ha year in the 1970s. Most land drainage was undertaken to improve extensively managed or rough grassland but 40% was carried out for conversion to arable cropping. Remaining wet grassland habitats have also been affected by the drainage of adjacent arable land. Wet grasslands support distinctive plant communities and often contain rare species that are adapted to local conditions. Land drainage has also impacted upon wetland bird species, such as waders. Four bird species in particular have been affected: lapwing (Vanellus vanellus), redshank (Tringa tetanus), curlew (Numenius arquata) and snipe (Gallinago gallinago). Other species such as the starling (Sturnus vulgaris)have also been affected, as drier land alters the availability of soil-surface invertebrates. The decline of the water vole is probably partially due to widespread drainage and canalisation of rivers.
Drainage is often the first step in the process of improvement, followed by intensification of farming on drained land, including reseeding, ploughing and fertiliser application. This intensification has had subsequent impacts on biodiversity. More rapid and denser grass growth of competitive species as well as higher stocking densities may occur in the case of managed grassland. These changes alter the habitat properties significantly, reducing both grazing opportunities for wildfowl and the range of seed resources available for use by granivorous birds. Drainage of wetlands can also reduce soil penetrability for probing birds and access to the soil surface may be reduced by more vigorous spring plant growth, reducing access for breeding birds. Declines in bird species as a result of land drainage have occurred in both the lowlands and the uplands.
Although land drainage may have negative impacts upon biodiversity, particularly for rare wetland communities, ditches and drains themselves can provide a rich habitat within arable landscapes and can be final refuges for species that have declined due to field drainage.
2.2 Decline of Mixed Farming and Changes in Crop Rotations
The polarisation of farming, with arable farming now dominating in the east and grassland in the west, has reduced the diversity of habitats and resources associated with mixed farming. Agricultural intensification has brought about a change in the range and rotation of crops being grown. Management practices, vegetation structure and duration to harvest have altered. This has potentially had an impact upon biodiversity; however, more research is required to understand the scale at which changes are likely to have had a negative impact upon populations. A loss of crop diversity is often cited as one of the key factors in the decline of the brown hare (Lepus europeaus) although hares are still relatively common on arable farms.
The separation of pastoral and arable farming systems has led to declines in bird populations, and grassland management in arable landscapes would improve habitat diversity within farmland. Equally, it is also suggested that the presence of arable habitat within grassland landscapes can be vital to the survival of key granivorous species such as grey partridge (Perdix perdix), skylark (Alauda arvensis) and corn bunting (Miliaria calandra) in grassland regions, in order to prevent local extinctions.
The increasing availability of a large variety of pesticides and fertilisers, combined with the rising price of cereals, has led to grass leys (i.e. temporary grassland), once an important part of the arable rotation, being less popular as a means of controlling weeds and insect pests or for maintaining soil fertility. The introduction of fungicides has allowed many farmers to dispense with 'break crops' such as grass or roots, previously used to control cereal fungal diseases. As a consequence of these changes, some arable weeds have declined, whereas others, such as barren brome (Anisantha sterilis), have increased. Weed species composition and diversity within different crop types will depend upon the management, e.g. fertiliser, pesticide and harvesting/cutting requirements of that particular crop type. For example, oilseed rape often has a higher level of broadleaved weeds than cereals, because they are more expensive to control and the yield benefit does not justify the additional herbicide cost. Sugar beet is sown in late spring so is more likely to contain spring-germinating species such as fat hen (Chenopodium album), a key bird food species.
Thus, reduced habitat heterogeneity in all types of farming landscape is likely to have been an important factor in biodiversity losses over the last forty years as mixed farming has declined. However, mixed rotations, including arable crops and grassland grazed by livestock, are still key components of most organic farming systems.
Other changes in cropping practices have also had detrimental effects on biodiversity. For example, the decline in cultivation of fodder crops such as turnips in favour of field beans and, more recently, the increase in the growth of forage maize at the expense of barley in southern and western England have not been beneficial to bird populations. Changes in cropping practices may not always have a negative impact upon all species, however. For example, wood-pigeons (Columba palumbus) declined as clover was no longer sown on winter stubble during the 1960s, but increased in numbers when an alternative winter food source became available: the young leaves of autumn sown oil-seed rape.
The use of inorganic nitrogen fertiliser has doubled during the second half of the twentieth century. This has had important implications for biodiversity, particularly for plant species and associated fauna. Increased fertiliser use, often in conjunction with reseeding to competitive ryegrass (Lolium perenne), has resulted in major losses of botanical diversity in the majority of UK grasslands. One of the most important factors affecting plant diversity is nutrient availability and thus the productivity of a habitat. Since the 1940s inorganic fertilisers have increasingly been used, which allow greater concentrations of nutrients to be applied with a quicker release time into the soil, though reliance on inorganic fertilisers reduces the amount of organic matter in the soil and may affect soil chemistry. Inorganic fertilisers promote rapid growth of a few competitive species, reducing light levels within the crop and preventing the growth of other plants. Along boundaries and hedgerows, misplaced nitrogen fertiliser can alter the nutrient balance, encouraging the growth of nitrophilous annual weeds such as cleavers (Galium aparine) and common nettle (Urtica dioica).
The stimulation of grass growth by fertiliser in grasslands renders the sward unsuitable for ground-nesting birds, and eliminates many broad-leaved plant species through competition. Loss of plant species has indirect effects on birds too, as insect diversity and abundance is reduced. Rapid growth in spring stimulated by nitrogen allows early and more frequent cutting, so that birds do not have time to complete breeding before cutting destroys nests.
Other nutrients, such as phosphorus and potassium, can also lead to changes in plant species in treated fields. In trials conducted in a species-rich hay meadow on a Somerset peat moor, phosphorus was more important than nitrogen in determining both biomass production and plant species change. Grazing levels after cutting, along with fertiliser inputs, also contributed to botanical change. Liming, in order to reduce the acidity of soil before cultivation, has probably contributed to the decline of corn spurrey (Spergula arvensis L.) and corn marigold (Chrysanthemum segetum).
Highly toxic organochlorine pesticides used during the 1960s and 1970s have now been withdrawn following the well-documented decline in biodiversity that resulted from direct impacts of their use. Screening of pesticides for toxicity to non-target organisms combined with risk assessment has reduced risks to such species; for example, the number of poisoning incidents involving wild mammals and pesticides has decreased over the last 10 years, from 20 in 1996 to 12 in 2005. In 2005 only two mammal deaths were caused by the approved use of pesticides, rodenticides in both instances. This reflects the generally lower toxicity of modern products, improved education and tighter controls governing their use.
Today, indirect effects of pesticides (i.e. effects operating through the food chain) are of greatest concern for vertebrates; in particular, from broad-spectrum herbicides and insecticides. Broad-spectrum pesticides, insecticides especially, have a major long-term effect on non-target invertebrates, many of which are important as biological pest-control agents or as important links in the food chain of farmland faunal groups. In particular, the application of broad-spectrum pesticides in agricultural systems has decreased bee populations dramatically.
Herbicides impact upon bird populations by either (1) reducing the abundance of, or eliminating, non-crop plants hosting arthropod foods for birds, particularly during the breeding season, or (2) depleting or eliminating weed species, which provide food for herbivorous or granivorous species. Herbicides reduce butterfly abundance indirectly when they cause the loss of larval food-plants and nectar or pollen sources for adults, particularly in boundary vegetation.
In addition to the toxicity of the pesticide itself, the timing of pesticide application, the indirect effects of the pesticide and impacts on non-target species are all important to invertebrate survival. Pesticides may reduce invertebrate abundance through direct toxicity, but also indirectly by restricting food supply or altering habitat. Pesticides are now routinely screened for side effects against non-target invertebrates for the purpose of registration.
The direct impact of pesticides on different invertebrate groups is highly dependent upon the timing of application. For example, autumn applications tend to drift into field boundaries more because crop and marginal vegetation heights are lower and may therefore contact invertebrates over a larger area.
The importance of indirect effects of pesticides on birds was first identified for the grey partridge. Pesticides (insecticides and herbicides) were shown to be important in limiting chick survival by reducing the supply of invertebrates important in chick diet, and the implementation of conservation headlands, whereby invertebrate densities were increased by restricting use of these pesticides at the edges of cereal crops, was shown to increase chick survival. In a review of indirect effects of pesticides, grey partridge was considered to be the only species for which such effects had been conclusively demonstrated, though there was circumstantial evidence for a number of other species. Corn bunting brood condition and the probability of nest survival were both correlated with the abundance of insect food close to the nest and, furthermore, the abundance of chick-food was negatively correlated with the number of insecticide applications. More recent studies have shown similar results for yellowhammer (Emberiza citrinella.)
Excerpted from Biodiversity Under Threat by R. E. Hester, R. M. Harrison. Copyright © 2007 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Meet the Author
The series has been edited by Professors Hester and Harrison since it began in 1994.
Professor Roy Harrison OBE is listed by ISI Thomson Scientific (on ISI Web of Knowledge) as a Highly Cited Researcher in the Environmental Science/Ecology category. He has an h-index of 54 (i.e. 54 of his papers have received 54 or more citations in the literature). In 2004 he was appointed OBE for services to environmental science in the New Year Honours List. He was profiled by the Journal of Environmental Monitoring (Vol 5, pp 39N-41N, 2003). Professor Harrison’s research interests lie in the field of environment and human health. His main specialism is in air pollution, from emissions through atmospheric chemical and physical transformations to exposure and effects on human health. Much of this work is designed to inform the development of policy.
Now an emeritus professor, Professor Ron Hester's current activities in chemistry are mainly as an editor and as an external examiner and assessor. He also retains appointments as external examiner and assessor / adviser on courses, individual promotions, and departmental / subject area evaluations both in the UK and abroad.
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