Annex 2
REVIEW OF THE PUBLISHED EVIDENCE OF A BIODIVERSITY
EFFECT OF ORGANIC FARMING
1. INTRODUCTION
1.1 This review looks at the published evidence,
mostly from the last decade, from studies which have sought to
test the hypothesis that organic farming benefits wildlife. None
of the studies has looked at the wildlife on a conventionally
managed farm and then followed the same farm through conversion
(or vice versa). Most of the methodology has relied instead on
the researchers pairing existing organic farms with nearby conventional
farms which are as similar as possible to the organic farm, except
for the organic management system. There are potential problems
with such a methodology. The organic farms were not selected at
random for conversion as this decision depends on the inclination
of individual farm managers. Such farms may have been managed
prior to conversion in a way which maintained healthier wildlife
populations. In this event larger or more diverse wildlife populations
on organic farms will simply have been maintained, rather than
recreated, by the organic system. There is also a paucity of research
around organic farming in the uplands. This gap needs to be addressed
by future research. Annex 1 sets out some of the theoretical reasons
for hypothesising that organic farming should be better for wildlife
than conventional farming.
1.2 There are at least three reasons, however,
for suspecting that the report results underestimate the scale
of the benefit from the organic farming system:
1.2.1 at least in the UK until very recently
organic farms have tended to be smaller family run units. In trying
to match these with similar conventional farms there is likely
to have been a tendency to avoid the bigger, most intensively
managed, conventional units which are usually assumed to support
the lowest populations of wildlife;
1.2.2 it is now accepted that the yields
on organic farms dip on conversion from conventional practice
and then gradually built over a period of about a decade as the
organic system of fertility building gradually recovers from the
withdrawal of agro-chemicals . It is likely that there is also
a time lag in the response of wildlife populations to the change.
In at least some of these studies a general shortage of organic
farms led researchers to use recently converted units which may
not yet have acquired their full "complement" of organic
wildlife;
1.2.3 most organic units exist in a "sea"
of conventionally farmed land. Island biogeography would suggest
that if there is a wildlife benefit from organic farming it will
be amplified when organic units coalesce and organic farming starts
to be practised at a landscape scale.
2. THE REVIEW
General Reviews
2.1 MAFF commissioned two studies (Unwin et al
1995 and Gardner and Brown 1998) which reviewed previous work
and examined both the direct and indirect evidence for a biodiversity
benefit from organic farming systems. Both studies concluded that
organic systems, on balance, provide more wildlife benefits than
any other available system of farming. Gardner and Brown concluded
that:
2.1.1 organic regimes were shown to have
an overall benefit for biodiversity at the farm level, both in
terms of the agricultural practices adopted and in the occurrence
and management of uncropped areas.
Plants
2.2 Recent studies (Kay and Gregory 1998
and 1999) commissioned by English Nature looked at the arable
weed flora of organic and conventionally farmed fields in the
Oxfordshire Chilterns. Although some individual species were more
common under conventional management regimes there was greater
species diversity, greater frequency of scarce and declining species
and greater overall abundance of non crop plant species in fields
managed organically.
2.2.1 Out of 23 of the rare or declining
"target" species, 18 were more common on the organic
farms with 13 of these found only on the organic farms; three
species were found only on conventional farms.
2.2.2 The organic field flora included four
species on the UK Biodiversity Action Plan list while the conventional
field flora contained only one BAP species.
2.2.3 One of the BAP species, corn buttercup,
an arable plant identified as having experienced the most rapid
decline of any British plant, was found on over a quarter of the
organic fields in the first year of the study and not found at
all on the conventional fields.
2.2.4 Importantly, many of the plants restricted
to the organic farms were known to be sensitive to herbicides.
2.3 These results are borne out by several
other studies. Hald (1999) looked at a total of 38 matched pairs
of organically and conventionally managed cereal fields over two
years and found that:
2.3.1 richness of wild plant species was
highest in organic fields;
2.3.2 all the variables tested had highest
values—and often several times higher—in organically
farmed cereals with four exceptions: total vegetation biomass,
crop biomass, proportion of a single broad leaved taxon (dead
nettles) and proportion of grasses;
2.3.3 even in spring, ie before herbicide
spraying, the densities of plants and species of the wild flora
in conventionally farmed cereals were about half the densities
in organically farmed cereals;
2.3.4 a feature of the conventional system
was the marked decrease in pre-herbicide spraying plant and species
density from the crop margin (distances <6m) to mid-field (distances
>6m). Spraying with herbicide in conventional fields reduced
the density of both plants and species even further, resulting
in a summer vegetation in conventional mid-fields being very much
poorer and with much lower density compared with organic mid-fields;
2.3.5 the two aspects used to assess the
environmental value of conventional and organic cereal fields
in relation to the herbivorous arthropod fauna (availability of
green parts of preferred taxa and accessibility of food from flowers)
showed the highest relative values in organically farmed fields.
This higher proportion of suitable arthropod food resources is
additional to the fact that the biomass of wild plants in organic
fields was five times higher than in conventional fields.
2.4 Brooks et al (1995) compared the weed
flora and stubble seed abundance in 23 pairs of cereal fields
over two years and found that:
2.4.1 the number of weed species present
on the organic fields was significantly greater than on the conventionally
managed fields;
2.4.2 overall weed cover was greater on the
organic fields;
2.4.3 there was no overall difference in
weed seed abundance between organic and conventional fields on
the post-harvest stubble, but grass seeds were more abundant on
the conventional fields and the seeds of dicotyledonous species
more abundant on organic fields.
2.5 Moreby et al (1994) looked at 59 pairs
of organic and conventional cereal fields over two years in the
UK. Sampling was restricted to points 3m in from the field margin
which, considering the findings of Hald (1999) and Redderson (1997),
was likely to minimise the differences between the two regimes.
Despite this the study found that:
2.5.1 there was a significantly higher total
weed cover (including grasses) in the organic fields in one of
the two years;
2.5.2 organic farms had significantly more
species per 0.25m2 than conventional ones in both years of the
study;
2.5.3 organic farms had significantly more
broad-leaved weed species and a significantly greater percentage
cover of broad-leaved weeds than conventional farms;
2.5.4 of the eight most commonly occurring
broadleaved species, all but one, (cleavers) had greater percentage
cover scores on the organic farms in both years. These differences
were significant in one or both years for the remaining seven
species and the category "other broadleaved species".
INVERTEBRATES
2.6 In a study which ran in parallel to
the one reported by Hald (1999) looking at flora Redderson (1997)
looked at the arthropod fauna in 38 matched pairs of fields in
two consecutive years. Key findings include:
2.6.1 in one year the conventional cereal
fields had higher total arthropod abundance than the matching
organic ones. However, this was based entirely on aphid abundance.
Excluding aphids and collembola, this study revealed a uniform
pattern of higher total arthropod abundance and biomass and of
species diversity in organic cereal fields;
2.6.2 a decline in abundance and species
diversity from field margin towards midfield was demonstrated
and tended to be steeper in conventional fields;
2.6.3 apart from the unpredictable aphid
numbers, arthropods, and particularly the species preferred as
bird food items, are a richer resource in organic cereal fields
than in conventional ones and this difference is even greater
when midfields are compared.
2.7 Moreby et al (1994) also looked at the
arthropod fauna of their fields. Again sampling was restricted
to points 3m from the field margin in late June and early July,
when migration into the crop from the field margin is likely to
have minimised any difference between the two systems. Additionally
none of the conventional fields were sprayed with insecticide
during the study. Although when all taxa were added no significant
differences between farming regimes were observed for the total
numbers of invertebrates sampled there were some qualitative differences:
2.7.1 cereal aphid densities were higher
in the conventional crops in both years and increased numbers
of parasitised aphids on conventional fields may have reflected
the differences in aphid numbers;
2.7.2 the guild of primarily weed-feeding
chick-food insects were most usually found at higher densities
in organic fields, although many of the statistical comparisons
did not achieve signficance.
2.8 Dritschilo and Wanner (1980) looked
at carabid communities in four pairs of corn fields in Illinois
and Iowa. They found that:
2.8.1 organic fields had both a greater abundance
of ground beetles and a larger variety of species in all four
field pairs during the June sampling period (when 90 per cent
of the specimens were collected). In three of the four pairs of
fields the differences in numbers collected per trap per day were
statistically significant.
2.9 Kromp (1989) looked at the carabid beetle
communities across adjacent fields of organically and conventionally
farmed winter wheat in two consecutive years in Austria. Key findings
include:
2.9.1 in the organically farmed wheat fields
higher numbers of carabid species and considerably higher numbers
of individuals were collected than in the conventionally farmed
fields;
2.9.2 two species were collected exclusively
from the organic fields. Both species are considered rare in Austria
and in this location were found in neither the conventional fields
nor in adjacent habitats; organically farmed fields possibly providing
the only habitat for these species in the region;
Kromp concluded that the proximate reasons for
these results were:
2.9.3 mainly due to the higher weediness,
greater heterogeneity of crop density and absence of pesticides
in organic wheat fields.
2.10 In a further study Kromp (1990) looked
at carabid populations in adjacent organic and conventional potato
fields over two years. Key findings include:
2.10.1 in both years the total numbers of
carabids and number of carabid species were higher in the organically
managed fields;
2.10.2 of the sixteen dominant species (representing
96 per cent of the catch):
— five species showed similar activity
densities under both types of management;
— two species showed higher activity
densities in one or both years under conventional management;
— the other nine species occurred exclusively
or at higher densities in the organically farmed fields.
2.11 Pfifner (1990) looked at the epigeal
arthropods on 12 plots comparing organic and conventional farming
in a long term study which started in 1977. Only the summary of
this paper is in English and reviewed here but key findings were:
2.11.1 the abundance of carabids, staphylinids
and spiders was greater in organic than conventional systems;
2.11.2 there was both greater species richness
and more homogeneous distribution of carabids in the organically
farmed plots.
2.12 Hokkanen and Holopainen used pitfall
trapping to look at the activity density of carabids in three
organic and four conventional cabbage fields over two years. They
found that:
2.12.1 different species of beetle tenderd
to be favoured by the different farming systems;
2.12.2 the biomass of captured carabids was
significantly (between two and 20 times) higher on the organic
fields;
2.13 Brooks et al (1995) used a combination
of soil cores and vacuum sampling (11 pairs of fields) and pitfall
trapping (13 pairs of fields) to look at invertebrate populations
on winter cereal fields and found that:
2.13.1 Earthworms were significantly more
abundant on organic than conventional fields;
2.13.2 Staphylinid beetles occurred at significantly
greater densities on the conventional fields (vacuum sampling);
2.13.3 the weevil Sitona lineatus and
the carabid Demetrias atricapillus occurred at significantly
greater densities on the organic fields (vacuum sampling);
2.13.4 of 12 carabid species analysed from
the pitfall trap samples five were significantly more abundant
on the organic fields while none were significantly more abundant
on the conventional fields.
2.14 Feber et al (1997) surveyed the butterfly
populations of 18 pairs of organic and conventional farms over
two years in England. They found that:
2.14.1 there was no significant difference
in the abundance of large white and small white (the two species
of British butterfly considered to be agricultural pests) between
the two farming systems in either year;
2.14.2 the abundance of non-pest butterflies
was significantly higher in organic than in conventional systems
in both years;
2.14.3 the management of the uncropped boundary
had a significant effect on non-pest abundance, with organic boundaries
attracting higher numbers of butterflies than conventional boundaries;
2.14.4 the abundance of non-pest butterflies
within the surveyed cropped habitats was significantly greater
on the organic farms.
2.15 Feber et al (1998) sampled the spider
fauna in nine pairs of conventional and organic winter wheat fields
in southern England. They found that:
2.15.1 both the number of spiders captured
and the species richness of spider samples were higher in organic
than conventional winter wheat fields;
They attributed this to the generally greater
"weediness" of organic crops concluding:
2.15.2 our most consistent result was the
increased abundance and species richness of spiders in our samples
with increasing abundance of understory vegetation within each
crop, both overall and within each system, within each sampling
session.
BIRDS
2.16 Farmland birds too have been shown
to benefit from organic management. Christensen et al (1996) looked
at 124 pairs of organic and conventional farms over four years
in Denmark. Of the 145 bird species recorded 92 occurred in sufficient
numbers to allow statistical comparison between the two farming
systems. Statistically significant differences were found for
34 species with three species (oystercatcher, thrush nightingale
and reed warbler) being commoner on conventional farms. The other
30 species, which included kestrel, skylark, swallow, blackbird,
song thrush, starling, linnet and corn bunting, were all significantly
more abundant on organic farms. Overall the study showed higher
bird species diversity and much higher bird densities on organic
farms. The authors concluded:
2.16.1 the results from the point censuses
were very clear: all common, characteristic farmland species occurred
with higher frequencies in organically farmed areas, the differences
between the two types of farmland often being statistically highly
significant.
2.17 Wilson et al (1997) looked specifically
at the skylark, a species which nests and feeds within the crop
and is sometimes assumed to be disadvantaged by the mechanical
weeding practices associated with organic farming and found:
2.17.1 that skylark pairs must make two to
three nesting attempts per season in order for populations to
be self-sustaining;
2.17.2 Skylarks require structurally diverse
crop mosaics in order to make multiple nesting attempts without
territory enlargement or abandonment;
2.17.3 organically-cropped fields supported
significantly higher skylark densities throughout the breeding
season than intensively cropped fields or grazed pasture.
Using the data collected from this and other
studies to construct a simple population model they further say:
2.17.4 our predictions about the demographic
status of skylark populations breeding on lowland farmland suggest
that, without structurally diverse crop mosaics which allow breeding
pairs to make multiple nesting attempts without recourse to territory
shifts or abandonments, skylark populations in arable landscapes
in lowland England are likely to be demographic "sinks",
unable to sustain their numbers in the absence of immigration.
Mixed farms, with mosaics of both winter and spring-sown cereals,
and extensively managed pastures and meadows are more likely to
support self-sustaining skylark populations, especially if farmed
organically, than farms dominated by winter cereals, broad-leaved
crops, or intensively grazed or cut grassland.
2.18 Chamberlain et al (1999) compared bird
populations from 22 pairs of organic and conventional farms over
three breeding seasons (66 comparisons) and over two autumn/winter
seasons (44 comparisons) across England and Wales. Most of the
comparisons which they made did not achieve statistical significance
however they did find that:
2.18.1 generally, organic farms held higher
densities of breeding birds in the field boundaries than conventional
farms, the density on organic farms exceeding that on conventional
farms in 43 out of 51 comparisons for individual species:
2.18.2 in field boundaries outside the breeding
season nine comparisons showed a significant result, all showing
a higher density on organic farms. Overall the density on organic
farms exceeded that on conventional farms in 56 out of 64 individual
comparisons;
2.18.3 in no species was significantly higher
bird density detected on conventional farms.
They concluded that:
2.18.4 this study has indicated that at the
whole farm level, farming practices which are characteristic of
organic agriculture would greatly benefit several species of farmland
bird.
2.19 Petersen (1994) reviewed the evidence
from a series of Danish studies comparing the abundance and ecology
of birds on organically and conventionally farmed areas. The key
results were:
2.19.1 mean breeding season densities of
birds calculated from point count data were higher on organically
farmed land than on conventionally farmed land for Lapwing, Skylark,
Swallow, Whitethroat, Starling, Linnet, Corn Bunting and Yellowhammer.
No significant difference was found for White Wagtail and Magpie.
2.19.2 in a specific study of yellowhammers,
densities were higher on transects through organically managed
farmland than through conventionally managed farmland during the
winder period (January-March).
2.20 Bradbury et al (in press) in
an intensive study of yellowhammers on four organic and five conventional
farms found:
2.20.1 that although yellow hammers nested
earlier on organic farms (and so might be able to make more nesting
attempts per season) there was no significant difference between
the two farming regimes in clutch size, hatching success or fledging
success;
As yet unpublished data from related work has
shown that:
2.20.2 when feeding nestlings, yellowhammers
forage most frequently in uncultivated field margins, but also
visit cereal crops to collect invertebrate prey. When visiting
cereal crops they are significantly more likely to use organic
rather than conventional crops.
2.21 One clear exception to trend is the
linnet. Unusually linnets feed their young entirely on seeds and
so are not reliant on summer invertebrate food supplies. Detailed
autecological study in the 1960s showed linnets feeding their
young on a range of broadleaved weed seeds deriving from plants
which have now much declined on farms. More recently Moorcroft
and Wilson (in press) have shown that linnets have learnt to compensate
for this by feeding on the unripe seed of oilseed rape, a crop
seldom included in organic rotations. The recent stabilisation
of linnet populations is almost certainly due to this adaptation
to the conventionally farmed environment. Further unpublished
work has shown much lower nesting densities and high rates of
chick starvation on conventional farmland in the absence of oil
seed rape. This does not mean that linnets will not benefit from
an increase in organic farming (Christensen et al 1996),
just that they have evolved a strategy which can allow them to
survive in some conventionally farmed environments.
3. SUMMARY
3.1 In making a change in ecosystem management
as fundamental as moving from conventional to organic farming
there are bound to be species which both benefit and lose out.
Additionally the types of study reported here are likely to give
rise to some statistical noise giving seemingly anomalous results
in both directions. Even allowing for this there are some clear
conclusions which can be drawn from the studies reviewed. When
you compare the two farming systems then on balance:
3.1.1 there are higher numbers and greater
densities of non crop species (wildlife) on organic farms;
3.2.2 there is a greater diversity of wildlife
on organic farms;
3.2.3 overwhelmingly, whether it is Birds
of Conservation Concern (BOCC), Biodiversity Action Plan arable
weeds or non pest butterflies, the wildlife species which have
suffered the greatest declines on farmland in the last fifty years
do better under an organic farming system;
3.2.4 species which appear to be unaffected
by the farming system or are favoured by conventional management
are usually still either common and widespread or, like cereal
aphids, cleavers, large white and small white butterflies, regarded
as agricultural pests;
3.2.5 where we have gained an insight into
the mechanisms by which organic farming favours wildlife the findings
are consistent with the theoretical benefits of organic farming
outlined in Annex 1.
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