Multi-Function Agricultural Biodiversity 327In cases where biodiversity has been increased by the introduction of a legumi-
nous plant (e.g. Theunissen et al, 1995) atmospheric nitrogen will be fixed. This
important plant nutrient will be available to the diversified crop or the next crop
in the rotation (Altieri, 1994). As well as directly benefiting plant growth, this
effect may also be involved in the effects on host plant quality that were concluded
to be important in accounting for reduced densities of Thrips tabaci Lindeman on
leeks interplanted with clover (Belder et al, 2000). The structure and management
of agricultural vegetation may also have effects on soil flora and benefit soil pro-
ductivity (Hawksworth, 1991). Agroforestry, for example, supports a greater abun-
dance of mycorrhizal fungi than does monoculture (Boddington and Dodd, 2000),
and biological control of plant pathogens and nematodes can be enhanced by ‘sup-
pressive soils’ – those with high microbiological activity (Whipps and Davies,
2000).
Another benefit of vegetational biodiversity is the conservation of pollinating
insects that are important in many dicotyledonous crop species (Mineau and
McLachlin, 1996). The occupancy rate of bumblebee domiciles was greatest in
more diverse habitats (Barron et al, 2000). Overall, biodiversity can have a variety
of benefits at this level of a specific diversified crop.
Farm scale benefits
The next level at which biodiversity may be beneficial is beyond the specific crop
being manipulated. It considers other components of the farming system such as
the intrinsic value of the introduced vegetation, impacts on adjacent but un-ma-
nipulated crops and effects on livestock within the farming system.
In instances where biodiversity is increased by adoption of some form of inter-
cropping, the additional crops will have an added value, through their harvest
being economically worthwhile. Examples include hay made from lucerne strips
grown within cotton (Mensah and Kahn, 1997) or timber from agroforestry (Chi-
tra and Solanki, 2000). There may also be complementary production benefits,
such as supporting apiculture, sericulture or lac culture (Chitra and Solanki, 2000).
At least some of these may apply even when the non-crop vegetation is not for
harvest, for example in the case of hedges or woodland. It is also recognized that
integrating trees into pastoral systems may have benefits for welfare of livestock
animals via shelter from wind and provision of shade (Sinclair, 1999), as well as
providing aesthetic (e.g. landscape), recreational, conservation and other advan-
tages.
Landscape scale benefits
At this level, biodiversity may have a variety of benefits. Beetle banks and conserva-
tion headlands, both examples of vegetational diversity where increases in natural
enemy density have been recorded (Thomas et al, 1991; Hassall et al, 1992;
Cowgill et al, 1993) provide considerable conservation benefits. A variety of birds