Conservation and Sustainable Use of the Biosphere - WBGU

Agrobiodiversity: functions and threats under global change D 3.4
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little information about the genetics of farmers’ varieties
(Zeven, 1998). Using barley as an example, it
can be demonstrated that the results are dependent
on the selection of material and method. Petersen et
al (1994) found molecular markers in wild barley
(ssp. spontaneum) for a greater genetic diversity than
in the investigated range of modern varieties (ssp.
vulgare), but the diversity in the farmers’ variety collection
was lower than in the current varieties. Nevo
et al (1986), in contrast, found that the cultivated barley
varieties of the Middle East possessed a higher
degree of diversity of morphological features than
wild barley. In actual fact, the genetic diversity within
a single variety or between varieties in a region does
not have an influence per se on the genetic vulnerability.
What are decisive are the characteristics in
each case, not the ‘degree of relationship’: not morphological
or molecular markers, degrees of relationship
or data on heterogeneity; rather the actual features,
eg resistances. Genetic diversity as is present in
and between historic farmers’ varieties, does not by
itself mean improved resistance or tolerance. However,
since farmers’ varieties were adapted to the specific
conditions at a given site by continuous selection
over long periods of time, they often have the respective
resistances and tolerances.
In countries with highly developed seed systems,
genetic diversity in the field replaced ‘temporal’
diversity (Duvick, 1984). One variety is replaced
after a number of years, for example, when its resistances
are no longer effective. Geographic biodiversity
is thus replaced by temporal diversity.
The value of genetic diversity in the agro-ecosystem
has changed over the course of the process. Systematic
plant breeding combines the most valuable
characteristics from genetically diverse material and
thus generally achieves higher levels of resistance (eg
via genetic pyramiding). Genetic diversity then fulfils
its function within the agrarian system, that is to
reduce disease and vulnerability to stress, but no
longer in simultaneous, geographically distributed
cultivation. Instead, it becomes a reservoir of individual
valuable characteristics that can be combined and
recombined. It becomes, in short, a genetic resource.
As the example of the Southern Corn Leaf Blight
(Box D 3.4-1) demonstrates, low genetic diversity can
lead to increased incidence of disease and massive
spread of damaging agents. Thus, to ensure yields
today the spatial arrangement of several varieties is
necessary.
Consequences
There are three different categories of functions that
the individual components of agrobiodiversity can
fulfil, namely
1. ecological functions on site (current biodiversity),
2. functions as suppliers of various products and services
(current biodiversity),
3. functions as genetic resources, ie as repositories of
information or ‘raw materials’ eg for the breeding
process (latent biodiversity).
Whereas current agrobiodiversity provides positive
contributions directly to the productivity of the system
and, therefore, is cultivated by the farmer, this is
generally not the case with latent agrobiodiversity
(unless the farmer is also a breeder). The more agrobiodiversity
is separate from active use, the greater
the effort that has to be invested in maintaining it as
the genetic resource that it has become. At the same
time, the ecosystem services of biodiversity that are
now missing must be replaced. At the level of agricultural
production this happens via ‘external inputs’,
such as pesticides that ultimately only become necessary
when large areas are covered by a few varieties
of just one crop species. At the level of manufacturing
industry it is similar: the greater the amounts of
food that are not consumed fresh, but rather in industrially
processed form, the greater is the demand for
large uniform amounts of ‘raw material’. Loss of taste
intensity, for example, is often a consequence of
breeding for maximum yield and can be replaced in
industrial processes with additives.
The less diversity there is in the industrial structure
of the purchaser, the lower the diversity in procedures
and production methods, which also in turn
reduces the demand for diversity in quality.
Measures to reduce the negative external effects
of agriculture therefore often have a direct impact on
the use of agrobiodiversity, as do trade initiatives
with the aim of supplying high-quality fresh produce.
Box D 3.4-1
Genetic vulnerability
In the fifties and sixties in the USA the large-scale cultivation
of varieties of hybrid maize began, with 80 per
cent of the hybrids containing what was known as the
Texas cytoplasm. It gives the inert plant male sterility
and is required for the production of hybrids. In 1969/70
the varieties with Texas cytoplasm were stricken with an
epidemic-like spread of Southern Corn Leaf Blight. The
maize varieties with normal cytoplasm were not
affected. It was found that male sterility and vulnerability
to the agent Bipolaris maydis, race T, could both be
traced back to the product of a mitochondria gene, Turf
13.
In this example the problem was neither the hybrid
technology per se nor uniformity or close genetic proximity
of the hybrids, rather it was the broad spread of
Turf 13. The genetic disposition of the maize hybrids lay
here in the existence of a ‘genetic monoculture’ in relation
to their vulnerability to Southern Corn Leaf Blight.