326 Ecological Restoration and Design
increased in an attempt to benefit pest management, there may also be a cascade
of outcomes at these other hierarchical levels. The following examples develop this
concept of multiple-function agricultural biodiversity.
Crop scale benefits
In instances where agricultural biodiversity has reduced pest densities, the need for
pesticide inputs may be lessened although linking increased biodiversity to higher
yields through pest suppression is very difficult to demonstrate (Gurr et al, 2000).
This can have a spectrum of benefits, of which the most immediate is financial.
Probably the most detailed financial analysis of the benefits and costs associated
with introducing diversity for pest suppression is that by Thomas et al (1991). The
cost of establishing a ‘beetle bank’ in a 20ha wheat field, combined with the value
of yield lost through land taken out of production, was calculated to be US$130.
However, the value of keeping aphid pest densities below the action threshold was
estimated to be US$450, and the value of avoiding a 5 per cent aphid-induced
yield loss was US$1000. Such figures illustrate the potential for significant eco-
nomic benefits from increasing biodiversity. Potential additional advantages apply
when pesticide inputs are reduced (in an Integrated Pest Management (IPM)
scheme, for example) or almost completely removed (as in organic agriculture).
Premium prices may be attracted for produce grown under such systems (Govin-
dasamy and Italia, 1997) or market share can be increased because of the benefits
perceived by the consumer.
Even in cases where there is no change in pesticide use, there is scope for eco-
nomic benefits to the grower if crop quality and/or yield is improved. Theunissen
et al (1995), for example, investigated the effects of intercropping cabbage (Brassica
campestris L.) with white or subterranean clover (Trifolium repens L. and T. subter-
raneum L., respectively). In this work, the densities of various natural enemies were
increased in interplanted treatments compared with the monoculture control.
Densities of pests such as the cabbage moth (Mamestra brassicae L.) were lower in
intercrop treatments and, although the total yields of the cabbage were lower in
these crops as a result of plant competition, the quality of cabbages was greater as
a result of lower pest damage. The effects of the diversification were, therefore,
calculated to be economically favourable.
Aside from the direct effects on arthropods described above, diversification
may benefit crop production in a variety of ways. The impact of bird pests and
plant diseases can be reduced by aspects of vegetational diversity (Jones (1974) and
Bridge (1996) respectively). Weed densities also may be reduced, as in a recent
study of the effects of increased biodiversity in rice. When weeds were retained on
the bunds surrounding paddies, this vegetation supported not only natural ene-
mies of pests but also seed-collecting ants that had the effect of reducing numbers
of weed seeds within paddies (Upawansa, 1999). More broadly, the vegetated
bunds were more resistant to damage from floods and wild animals and were
claimed to save labour and money.