Community Ecology Processes, Models, and Applications

We examined the biological control of the bird-
cherry oat aphidRhopalosiphum padiby the guild of
generalist predators, and could by exclusion experi-
ments roughly estimate the gain in cereal yields re-
sulting from predation on aphids and thus the
economic value of predation in one particular year
in one particular region (e.g. O ̈stmanet al. 2001, 2003).
The results are summarized in Figs 9.4 and 9.5.
First, predators had a larger impact on aphid po-
pulations in heterogeneous landscapes and on or-
ganic farms. In both cases, aphid numbers early in
the season were lower (Fig. 9.4a). This could be at-
tributed to a higher predation rate on aphids (O ̈st-
manet al. 2001), resulting in lower numbers of aphids
during the growing season. Further, exclusion of
predators resulted in a much higher abundance of
aphids (Fig. 9.4b). Because there is a direct correspon-
dence between the number of aphid days on the crop
and yield loss, predation clearly affects the yield a
farmer gets (O ̈stmanet al. 2003). However, there was
no relation between carabid diversity and predation
rates (Fig. 9.4c). This suggests that either the abun-
dance of particular species rather than predator di-
versity plays a role for predation, or predator
diversity in this area (15–35 spp.) was sufficiently
high for the relation between diversity and ecosys-
tem function to saturate towards an asymptote.
Finally, using the price of cereals we could estimate
the value of natural predators for farmers in 1999 as
approximately 40 or $37 per hectare. We do not
know whether this particular year and these land-
scapes are representative, and a generalization clearly
requires more studies. Nonetheless, because organic

farmers rely on natural predators for pest control,

they represent a substantial value for farmers, indi-

cating that landscape management is an important

tool for achieving economically sustainable farms.

However, when attempting to estimate economic

value, we encounter different perceptions of econ-

omists and ecologists. For an economist, the eco-

nomic value on the market is the marginal increase

in yield values under the present rate of service

delivery (Fig. 9.5a). This value usually levels off

when the service is available in abundance, imply-

ing that ecosystem services are worth less when

they function well. For ecologists, the value of a

service such as predation is the total effect of pre-

dators, which we measured as the effect of exclud-

ing all (or most) predators (Fig. 9.5b). This means

that economists and community ecologists may

perceive the value of an ecological service quite

differently.

9.3.4 What have we learned in the context of metacommunity ecology?

First, metacommunity theories will be important

tools in analyses of interactions between species

living in adjacent habitats, such as crop fields,

field margins and semi-natural habitats. These si-

tuations are common and crucial to understand for

applied questions such as biological control and

pollination.

Second, the ecological processes involved in main-

taining biodiversity and ecosystem services in agri-

cultural landscapes often act on larger scales than

0.3 0.4

40

Perceived ecological

value of biocontrol

(a) (b)

Marginal value =

market price

35

30

25

20

15

10

5

0

0.5

Predation rate

% yield increase

Rate of ecosystem

service

Value of product

(e.g. yield)

0.6 0.7 0.8 0.9

co

1

Figure 9.5Different perceptions of the value of ecosystem services such as biological control in ecology and economy
(a). In (b) the perceived ecological value of natural predators for biological control is indicated. Data from O ̈stmanet al.
(2001, 2003).

DIVERSITY AND ECOSYSTEM SERVICES 129