Genetic Resources in Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 198

plants, animals, noncrop associates, beneficial organisms,
and so on — are adapted to local conditions and the local
variability of the environment, in addition to possessing
traits that make them specifically useful to humans.
Traditional, indigenous, and local agroecosystems con-
tain many of the genetic elements of sustainability, and we
can learn from their example. In particular, they have higher
genetic diversity within populations as well as in the crop-
ping community as a whole. Intercropping is much more
common, noncrop species and wild relatives occur within
and around cropping fields, and opportunities for genetic
diversification are abundant at the field level. In such sys-
tems, resistance to environmental stress and biotic pressures
has a much broader genetic base, genetic vulnerability is
lower, and while pests and diseases occur, catastrophic out-
break is rare. In essence, genetic change in such systems
takes place much like it does in natural ecosystems.

environment, and the human managers. Selection takes
place at all levels at the same time, rather than for single
specific characters. The more durable type of resistance
that results is termed horizontal resistance (Robinson,
1996). An example of this type of breeding program is
described in the case study Breeding for Horizontal
Resistance in Beans .
Breeding methods that provide the most durable resis-
tance rely on the use of open-pollinated, locally adapted
landraces. Open-pollinated crops are generally lower
yielding when compared to hybrid varieties, but they are
very responsive to local selection pressures because of their
genetic diversity. They also have the best average perfor-
mance in the face of the combination of all the local envi-
ronmental factors, including pests, diseases, and weeds.
The importance of system-level resistance is accepted
more easily by ecologists than by agricultural scientists.
The study of selection in natural ecosystems has repeat-
edly demonstrated the ways a wild ecotype responds to
either positive or negative selection pressures when it is
introduced into an ecosystem different from the one in
which it evolved. Selection operates simultaneously at the
level of all of the factors, biotic and abiotic, that the
organism encounters. Seen in this light, the problems asso-
ciated with genetic uniformity in crops become more
apparent. resistant v

B REEDING FOR D URABLE R ESISTANCE IN C ROP P LANTS

Agricultural plant breeding has focused mainly on creat-
ing resistance to limiting factors of the environment, be
they physical factors such as drought, poor soils, and
temperature extremes, or biological factors such as her-
bivory, disease, and competition from weeds. Remarkable
gains in yield have been achieved as a result of these
breeding programs, but as we have already noted, another
result is increased vulnerability to crop failure and increas-
ing reliance on nonrenewable inputs.
As each problem presents itself, crop breeders screen
the genetic variability of a crop until they find a resistant
genotype. This resistance is often provided by a single
gene. The gene-transfer and backcrossing techniques
described above are employed to incorporate the gene into
a specific crop pedigree. The result is sometimes called
vertical resistance. It has two weaknesses. First, the resis-
tance will continue to function only as long as the limiting
factor does not change. Unfortunately, in the case of pests,
diseases, and weeds, the limiting factor is never static for
very long because of continual natural selection. So, the
problem organism eventually develops “resistance to the
resistance,” and an outbreak or epidemic occurs. This
dynamic is the basis of the well-known crop breeders'
treadmill. Second, in the process of breeding for vertical
resistance, genes providing partial resistance to the wider
spectrum of pathogens are lost.
A more durable type of resistance is needed that does
not break down easily in the face of new strains of pests,
diseases, or weeds. Rather than directing breeding pro-
grams towards the development of specific resistances, the
idea is to manage the whole crop system. Selection for
durable resistance requires the accumulation of many
resistance characters using population-level breeding
methods, and relies on an understanding of the simulta-
neous nature of the interaction between a crop, pests, the

O N -S ITE S ELECTION AND C ONSERVATION OF
P LANT G ENETIC R ESOURCES

The concern for the erosion and loss of genetic resources led
to the establishment, in 1974, of the International Board of
Plant Genetic Resources (IBPGR). An international network
of ex situ (off-site) crop germplasm repositories was estab-
lished and genetic material from the major crop gene centers
was collected in order to establish the IBPGR system of
genebanks. Plant breeders have since relied heavily on these
genetic resources for the conventional development of
higher-yielding and resistant varieties, and the number of
genebanks of all types has increased to an FAO-estimated
1460 worldwide, which together hold more than 5.4 million
samples. In 2004, the FAO and the 15 Future Harvest Centers
of the Consultative Group on International Agricultural
Research partnered to create the Crop Diversity Trust, an
independent international organization charged with assuring
the long-term security of our most important collections of
crop diversity. In particular, the Trust seeks to salvage
collections that are at risk and to assist developing countries
with managing their collections.
Although ex situ conservation is important, it cannot
by itself stem the erosion of agricultural genetic diversity.
Limited funding for genebanks has restricted the range of
crops and regions from which material is collected, leav-
ing much of the world's crop genetic diversity out of these
reservoirs. Corn, wheat, beans, rice, and potatoes have