Agriculture Reference
In-Depth Information
CHAPTER 10
DIVERSIFICATION STRATEGIES
MARIA R. FINCKH AND MARTIN S. WOLFE
10.1 INTRODUCTION
From the early 1980s, black leg on oilseed rape (caused by Leptosphaeria maculans )
became a serious disease problem in Canada. It was encouraged by a sudden export
market opportunity which triggered a huge increase in the area grown to a single
genotype of rape (Busch et al., 1994; Juska et al., 1997). This illustrates an
important side-effect of industrialised agriculture (Clunies-Ross and Hildyard, 1992)
which, over the past 50 years, has led to a sharp reduction in the number of different
crops produced and in the number of crop varieties.
In contrast, the plant species and genotypes that survive in natural ecosystems do
so because they are able to withstand, simultaneously, the depredations of many
competitors, diseases and pests in addition to all other selective pressures. The
defence mechanisms involved are still poorly understood but it seems clear that
there are many and that they operate at different levels from the individual plant to
the plant community. Nevertheless, infections or infestations may be sometimes
severe, which can lead to local extinction of the host. Importantly, pure stands of
species are rare in natural ecosystems; where they do occur, the individuals within
the stand are thought to be genetically distinct (e.g. Wills, 1996). Indeed, genetic
diversity within and among species appears to be a concomitant of survival and of
stability in communities (see review by McCann, 2000).
The origins of monoculture are buried in our agricultural past but the
concentration and isolation of crop plants for convenience of planting and harvesting
was a development that defined and separated early agriculture from food-gathering.
The emergence of species, variety and gene monocultures in agricultural ecosystems
represented a further and even more radical departure from nature. Although the
potential for yield gain is probably limited by the likelihood that inter-plant
competition for resources is maximal in uniform stands, the convenience of
monoculture outweighed the disadvantages.
Until recently, crop species were numerous and relatively undeveloped. Then, as
the crop range decreased and human population increased, the disadvantages of
monoculture in terms of greater and more consistent attacks by diseases and pests
became more apparent because of the ease of spread of the organisms involved. For
the major crops, there has also been a tendency to increase the harvest index for
large-scale production, resulting usually in dwarf forms of the plant which may be
less competitive towards weeds than their taller predecessors. Moreover, splash
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