Multi-Function Agricultural Biodiversity 319(Root, 1973) and other mechanisms are responsible for this. Finch and Collier
(2000) provide a concise review of six ‘bottom-up’ hypotheses, and propose a new
one, the ‘appropriate/inappropriate landings’ hypothesis. Trap cropping (Smith and
McSorley, 2000) is another potential mechanism. Pest suppression may result also
from ‘top-down’ effects via enhancement of natural enemies (the third tropic level:
Eber, 2001); conservation biological control; or Root’s (1973) ‘enemies hypothesis’.
In a detailed, quantitative review, Andow (1991) found that although natural
enemy densities tended to be greater in polycultures than in monocultures, only
slightly more than half of the 287 herbivore species were consistently less abundant
in polycultures. One reason for this inconsistent effect of enhanced vegetational
biodiversity is that the effects of different types of plants on natural enemies can
vary markedly (Nentwig, 1992; Colley and Luna, 2000). There may also be effects
on other trophic levels (Figure 17.1). For example, plants can provide resources that
favour the pest (e.g. Baggen and Gurr, 1998) or members of the fourth trophic level
Figure 17.1 The potential effects of vegetation on higher trophic levels and
consequences for pest management