Agriculture Reference
In-Depth Information
10.3 BENEFITS FROM SPATIAL DIVERSIFICATION: SMALL SCALE
The expected benefits from intercropping were summarised by Vandermeer (1989),
although similar summaries have been published widely, for example in agroforestry
(Nair, 1993):
•
increased productivity and yield;
•
better use of resources such as land, labour, time, water and nutrients;
•
reduction of losses due to diseases, pests and weeds;
•
socio-economic benefits such as stability of production, economic gain and
nutrition.
To achieve such benefits requires great attention to management on the one hand,
and to choice of crop partners on the other. Diversity simply as an end in itself may
lead to losses in production and productivity. Partners need to be selected that have
different useful characteristics, that occupy non-identical ecological niches and that
are complementary (Tilman
et al
., 2001).
10.3.1 Diversity and disease control
Mechanisms
Diversity can play an important role in the control of pests and pathogens (Wolfe,
1985; Wolfe and Finckh, 1997; Mundt, 2002) provided such diversity is functional.
Functional diversity
is diversity that limits pathogen and pest expansion and that is
designed to make use of knowledge about host-pest/pathogen interactions to direct
pathogen evolution (Schmidt, 1978; Mundt and Browning, 1985a).
Several mechanisms may contribute to changes in disease incidence or severity
(usually a reduction) in host populations that are diverse for resistance (e.g., Van der
Plank, 1968; Browning and Frey, 1969; Barrett, 1978; Burdon and Chilvers, 1982;
Mundt and Browning, 1985a; Wolfe, 1985; Wolfe and Finckh, 1997; Mundt, 2002).
Boudreau and Mundt (1997)
argued that the most important mechanisms in variety
mixtures are those that affect dispersal since competitive differences among the host
components are relatively small (but see Finckh and Mundt, 1992b; Finckh
et al.
,
1999). On the other hand, they found that in species mixtures, competitive
interactions among the components are relatively large and probably more important
in determining disease levels.
The first four mechanisms apply to all variety mixtures, whether or not there is
pathogen specialisation to the host in question:
1. Disease reduction due to
increased distance
between plants of the most
susceptible component in the mixture. Burdon and Chilvers (1976) confirmed,
experimentally, density dependence of the spread of powdery mildew (caused by
Blumeria graminis
f.sp
. hordei
) of barley (
Hordeum vulgare
). This space effect
depends on the proportion of spores available for allo-infection, the density of
planting and the number of components in the mixture. The first two of these factors
