Proceedings
12th EWRS Symposium, Wageningen 2002, pp. 50-51.
The
role of weeds in supporting biological diversity within crop fields
E.J.P. Marshall
Marshall Agroecology Ltd, 2 Nut Tree Cottages, Barton Winscombe,
Somerset , BS25 1DU, UK
jon.marshall@agroecol.co.uk
Introduction
Weeds are an important constraint
on crop yield in most crops. Considerable
research has been aimed at weed control, with the result that more money is spent
by growers on weed control than other crop inputs.
The global pesticide market valued at $29.2 billion is divided between
herbicides 48.1%, insecticides 27.7%, fungicides 19.6% and other products 4.6%.
Improved crop management techniques, including herbicides, have resulted in good
control of weeds and have facilitated different cropping patterns and steadily
increasing crop yields. Perhaps unsurprisingly, there have been coincident changes
in the flora of intensively managed farmland (Marshall,
2001; Marshall et al., 2001). In association
with the advances in agriculture, there is also evidence of changes in other taxa.
In the UK, previously common species associated with farmland have shown
marked reductions in range and population size over the past 30 years, most notably
amongst birds (Fuller et al., 1995). A range of
cornfield weeds, such as Ranunculus arvensis and Scandix
pecten-veneris, have declined markedly this century (Sutcliffe & Kay, 2000), to the extent that some species are now extinct in
the UK. These annual flowers are dependent on the arable
ecosystem, which is characterised by regular soil cultivation.
Most
countries are signatories to the Rio Convention on the Conservation of Biodiversity. The reasons for the conservation of biodiversity
are moral, aesthetic, social and economic. Most countries now have Action Plans for the conservation of biological
diversity. An emerging paradigm
is the conservation of species and communities within the farmed landscape as
a whole (Mineau & McLaughlin, 1996). In the UK for
example, there are almost no true wilderness areas and more than 75% of the land
surface is farmed in one way or another. With
such fragmentation of natural habitats and predominance of agriculture, there
is a need to develop practical means of maintaining diversity in the wider landscape.
Plants are key components of
terrestrial ecosystems, providing the primary production upon which food chains
are built. Different plant parts may then
provide a range of resources for associated fauna. Leaves and stems may be browsed, while pollen
and nectar provide resources for pollinating insects. Plants have other functions as well as giving
food for herbivores, by providing cover, reproduction sites and structure within
habitats. Plants may also provide environmental
heterogeneity in space and time. In agriculture and horticulture, weeds may
play all or some of these roles, providing diversity, ecosystem functions and
supporting many other species.
A
comparison of herbicide-treated and untreated plots in the headlands of winter
cereal fields in southern England (Moreby
& Southway, 1999) has demonstrated that untreated plots had greater weed
density and diversity and significantly higher numbers of many invertebrate taxa,
notably those that are important in the diet of farmland birds.
Weed management in herbicide-resistant soybean in Iowa, USA, generally
gave fewer insects than weedier fields (Buckelew
et al., 2000). The effects
were not direct impacts of herbicide, but rather indirect non-target effects,
mediated through the weed flora. Several
initiatives, notably for integrated crop management, indicate there are implications
for biological diversity within fields from different approaches to weed control
(Clements et al., 1994; Mayor &
Dessaint, 1998). The protection
of the farmers’ investment and avoidance of risk have been the driving forces
for efficient weed control in the past. However, a new paradigm is to match crop production
with conservation of biological resources (Paoletti et al., 1992) and the development of more sustainable systems. This will require the maintenance of some weeds
within fields.
Production systems are essential
for food and non-food products. Nevertheless,
threats to plants from impacts of weed management within production systems may
impact on biological diversity. Those impacts, mediated directly or indirectly
through the elimination of plants or effects for example on reproductive potential,
may affect ecosystem function by affecting soil processes, nutrient cycling and
trophic interactions via fauna, flora, microflora and fungi.
Current data indicate that some weeds need to be maintained within crops
for birds and insects. A balance is needed
between the methods of production, the demand for products and the environmental
impacts that occur.
The core concern is the balance between adequate
weed control, including the prevention of weed seed build-up, and the requirement
for some plants to support biological diversity. For some, clean crops and zero tolerance of
weeds is the approach, with non-crop areas supporting biodiversity. This may be suitable for large countries, such
as the USA. However, in western Europe,
where the landscape is almost entirely agricultural, different approaches are
required. These need to be based on integrated
weed management, though modifications to crop management in selected areas of
fields, such as conservation headlands and uncropped wildlife strips, may provide
sufficient resources for some species. Some data is available on the importance of
different weed species for beneficial and pest invertebrates, bird species and
on the relative competitive effects of the weeds (Marshall et al., 2001). On the basis
of these, it may be possible to select species and populations that may be tolerated
to achieve a sustainable ecological balance, but more comprehensive data on the
interactions between flora and fauna at appropriate spatial scales within crops
are required.
References
Buckelew, L.D. et al., 2000.J. Econ. Ent. 93, 1437-1443.
Clements, D.R. et al., 1994. Phytoprotection, 74, 1-18.
Fuller, R.J. et al., 1995. Conservation Biology, 9, 1425-1441.
Marshall, E.J.P. 2001 In 2001 Brighton Crop Protection Conference. BCPC, UK.
Marshall, J. et al., 2001. The impact of herbicides on weed abundance and biodiversity. PN0940. UK Pesticides Safety Directorate. IACR-Long Ashton Research Station, 134 pp.
Mayor, J.-P. & F. Dessaint, 1998. Weed Res. 38, 95-105.
Mineau, P. & A. McLaughlin, 1996. J. Agric. Envir. Ethics 9, 93-113.
Moreby, S.J. & S.E. Southway, 1999. Agric. Ecosys. Envir. 72, 285-297.
Paoletti, M.G. et al., 1992. Agric. Ecosys. Envir. 40, 3-23.
Sutcliffe, O.L. & Q.O.N. Kay, 2000. Biological Conservation, 93, 1-8.
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