Community Ecology Processes, Models, and Applications

spiders have also been studied in this context
(Schmidt and Tscharntke 2005; O ̈berget al. 2007).
The basic system is shown in Fig. 9.3. In crop
fields, aphids on cereals can be serious pests, but
they are preyed upon by predators which require
more permanent habitats like field margins for sur-
vival and overwintering. The predators also feed on
other prey, both in the field and in the margins.
Presented in this way, this clearly is a system in
which the basic theories for trophic interactions in
spatially structured habitats developed by Holt
(2002), Oksanen (1990) and Poliset al.(1997)are
applicable. It is a metacommunity problem where
the strength of trophic interactions depends on local
and regional (landscape) factors. The strength of

biological control will be affected by the produc-

tivity of predators in the second habitat (field mar-

gins) and their movements, which can depend on

distance but also on the abundance of other prey in

the crop.

For control of pests such as aphids with multiple

generations during a season, it is crucial that the

predators are present in the fields when aphids

colonize (Ekbomet al. 1992). Aphid abundance in

one year does not depend on particular fields and

their margins; their dynamics are on a much larger

scale. Hence aphids can be considered as ‘donor

controlled’ and we cannot expect the predators in

a field to regulate aphids, only keep their numbers

below damage thresholds.

100

0.08

0.07

0.06

0.05

0.04

0.03

200 300 400 500

4

4

3

3

2

1

1

2

5

5

600

(a)

Organic

Conventional

Perimeter/area ratio

Aphid establishment

(winged aphids/tiller)

–4 –2 0

1

0.8

0.6

0.4

0.2

246

r = –0.47, n=8, ns

(c)

Diversity (residuals from

landscape–carabid diversity regression)

Predation rate on aphids

100

0

1 Conv1 Org2 Conv2 Org3 Conv3 Org4 Conv4 Org5 Conv5 Org

200

300

400

500

(^600) Without predation
(b)
Heterogeneous
landscapes
Number of aphid days
With predation
Figure 9.4Effects of organic farming and landscape heterogeneity on aphid establishment in spring cereal fields, and
the total number of aphid days (an indicator of yield loss) on 10 farms in central Sweden in 1999. Landscape
heterogeneity was measured as the perimeter-to-area ratio, high values indicating mosaic landscapes. Organic farms are
indicated with closed symbols, conventional farms with open symbols. (a) Aphid establishment during the early
colonization phase. (b) The number of aphid days in areas within (hatched+black) and outside (black) predator
exclosures. Effects of organic farming and landscape were both significant (O ̈stmanet al. 2001). (c) Predation rates on
aphids glued on paper and placed in the field in relation to diversity. Diversity was measured as the residuals from the
regression of carabid diversity (y-axis) versus landscape heterogeneity (x-axis). Extracted from data in O ̈stmanet al.
(2001) and Weibullet al.(2003).
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