APPENDIX
POSSIBLE IMPACTS OF THE WIDESPREAD ADOPTION
OF WINTER COVER CROPS ON ARABLE BIODIVERSITY
ASSUMPTIONS
Up to 20% of tillage area will be covered with
a sown cover crop as opposed to being bare land/stubble (note:
not sure if estimates include land managed as naturally-regenerating
rotational set-aside, a major source of stubbles). The cover crop
will be established by mid-September and destroyed in the following
January/February, after which a spring crop is grown. The stubbles
following late harvested arable crops (eg sugar beet/maize) will
not be planted with cover crops. The use of cover crops might
reduce the use of organic manure on arable fields.
IMPACTS ON
FARMLAND BIRDS
Crop stubbles that are retained over-winter
(especially those that have a rich weed cover) are the key foraging
habitat for granivorous (seed-eating) farmland birds. For example,
Evans and Smith (1994) and Donald and Evans (1994) found that
Cirl Buntings and Corn Buntings selected winter stubbles, and
Wilson et al (1996) found strong selection of stubbles
by Greenfinches, Reed Buntings, Linnets, Chaffinches, Yellowhammers,
Goldfinches, Pied Wagtails, Skylarks, Grey Partridges and Meadow
Pipits on mixed farmland in Oxfordshire. The latter study recorded
88% of all finches, sparrows and buntings on stubbles. Even more
extreme selection was noted by Mason and Macdonald (1999) who
recorded 96% of these birds on stubbles in Essex farmland. A larger
scale study by BTO examined the breeding population changes of
a range of common farmland birds between 1994 and 2003 across
over 600 1-km squares in lowland Britain and related these to
the area of over-winter stubble: for both Skylark and Yellowhammer,
the population trends were significantly more positive in squares
that held a large area of stubble (20 ha) than those that held
little or no stubble (Gillings et al 2005).
In view of the above, it is proposed that the
widespread replacement of over-wintered stubbles fields with planted
cover crops could significantly reduce the availability of foraging
habitats for granivorous farmland birds with likely detrimental
impacts on the population trends of what are already declining
species in many cases. Any population impacts are likely to be
worsened given the recent decision to set a 0% set-aside rate,
which is likely to dramatically reduce the area of set-aside stubbles,
known to be a vital foraging habitat for these birds (eg Buckingham
et al 1999).
The impacts on seed-eating birds could be reduced
if the planted cover crops could provide a suitable seed source
that could be accessed by the target bird species. This might
be provided by stubble turnips or mustard but the R&D evidence
is not available to confirm this. Undersowing the previous cereal
crop with a grass/legume mix can have some benefits for invertebrates
that over-inter in the soil (eg sawflies) which area a key food
source for some farmland birds in the following breeding season
(Grey Partridge), although undersown stubbles do not provide the
same quality of winter foraging habitat for birds.
A further issue of concern for farmland birds
relates to the potential reduction in manuring of arable fields—there
is some evidence that manured fields increase the abundance or
availability of invertebrates (eg earthworms) for insectivorous
farmland birds (eg thrushes and plovers).
IMPACT ON
OTHER ARABLE
BIODIVERSITY
The impact on other biodiversity associated
with arable fields is less clear. Most scarce arable plants are
spring germinating and so are unlikely to be affected. However,
there are some priority species that are autumn germinating and
so are likely to suffer from the application of cover crops. In
addition, some scarce lower plants (eg bryophytes) inhabit over-winter
stubbles and so could lose a significant part of their habitat.
Brown hares are most abundant in diverse farmland
landscapes (Vaughan et al 2003), and stubbles fields form
an important component of this habitat heterogeneity. The impact
on brown hares will therefore depend on the extent to which the
cover crops can be used for foraging/cover.
CONCLUSION
The widespread replacement of over-winter stubbles
fields by planted cover crops could drastically reduce the availability
of winter foraging habitats for declining seed-eating bird species
and, hence, greatly inhibit our efforts to meet the PSA target
which seeks to reverse the decline in farmland birds by 2020.
There are also concerns over other taxa associated with arable
fields, though we are less certain of the impacts on these.
REFERENCES
Buckingham, DL, Evans, AD, Morris, AJ, Orsman, CJ
and Yaxley, R 1999. Use of set-aside in winter by declining
farmland bird species in the UK. Bird Study 46: 157-169.
Donald, PF and Evans, AD 1994. Habitat selection
by Corn Buntings Miliaria calandra in winter. Bird Study 41:
199-210.
Evans, AD and Smith, KW 1994. Habitat selection
of Cirl Buntings wintering in Britain. Bird Study 41: 81-87.
Gillings, S, Newson, SE, Noble, DG and Vickery, JA
2005. Winter availability of cereal stubbles attracts declining
farmland birds and positively influences breeding population trends.
Proceedings of the Royal Society B272: 733-739.
Mason, CF and Macdonald, SM 1999. Winter bird
numbers and land-use preferences in an arable landscape in eastern
England. Bird Conservation International 9: 119-127.
Vaughan N, Lucas E-A, Harris S, White P C L 2003.
Habitat associations of European hares Lepus europaeus in England
and Wales: implications for farmland management. Journal of
Applied Ecology 40: 163-175.
Wilson, JD, Taylor, R and Muirhead, LB 1996b. Field
use by farmland birds in winter: an analysis of field type preferences
using resampling methods. Bird Study 43: 320-332.
| 
