Conservation of endangered species - Ecological Applications: Toward a Sustainable World - page 132

Sugar beet has been genetically modifi ed to be resistant to the broad-spectrum
herbicide glyphosate. This allows the herbicide to be used to effectively control the
weeds that usually compete with the crop. Fat hen ( Chenopodium album ), one weed
that will be affected adversely, produces seeds that are important winter food for
farmland birds, including the skylark ( Alauda arvensis ). Watkinson and his col-
leagues take advantage of the fact that the population biology of both fat hen and
skylarks is well studied and can be readily incorporated into a model of the impacts
of GM sugar beet.
In a typical 5-year crop rotation in eastern England, sugar beet is grown every
fi fth year with cereals in intervening years. Fat hen can only establish in year 5,
when sugar beet is grown, persisting between beet crops as dormant seed banks.
Survival from germination to fl owering depends on weed control regimes (whether
traditional or involving GM sugar beet and glyphosate), whilst seed production
depends on competition for resources between weed and crop plants.
Skylarks do not feed only on fat hen, but the weed is a suitable 'model' species to
generate predictions of how GM sugar beet might affect the weed-bird interaction.
Figure 5.13 shows how skylarks aggregate in fi elds in response to seed density. Thi s

Fig. 5.13 (a) The
relationship between
density of skylarks (per
hectare) in fi elds in
Norfolk, England and
weed seed density per
square meter near the
soil surface. (From
Robinson & Sutherland
1999.) (b) Frequency
distributions of mean
seed densities across
farms before the
introduction of GM
sugar beet (solid line),
and in two situations
where the technology
has been adopted:
where the technology is
preferentially adopted
on farms where weed
density is currently
high (dotted line) and
where it is currently
low (dashed line).
(After Watkinson et al.,
2000.)

(a)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

100 1000 10,000

Weed seed density (m -2 )

(b)