Genetic Resources in Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 195

the genetic and environmental mechanisms of resistance
by breeding crops of specific traits rather than general
fitness, and by planting crops in large single-species popu-
lations at the same time in the same place. This creates
an environment that is more uniform and predictable than
it might otherwise be, setting the stage for outbreaks to
occur.
Moreover, changes independent of agriculture are
increasing the threat of serious outbreaks of disease and
pests. The interconnections of global commerce give
pathogens even more vectors for expansion into new areas,
and climate change threatens to both allow pathogens to
move into areas where the climate formerly excluded
them, and to put stresses on crop plants that make them
more vulnerable to pest and pathogen attack.
The increasing uniformity of the genetic base in live-
stock production also makes poultry, cattle, swine, goats,
and sheep more vulnerable to the spread of disease. In
more traditional pastoral systems, livestock breeds vary
regionally and are well adapted to local conditions,
providing good resistance to disease. In contrast, modern
confined animal feeding operations (CAFOs) pack large
numbers of genetically similar animals together in an arti-
ficial context that is the perfect setting for the rapid spread
of a pathogen.
One of the most well known examples of the dangers
of genetic uniformity is the Irish Potato Blight. In 1846
the late-blight fungus ( Phytophthora infestans ) destroyed
half of Ireland's potato crop, causing widespread famine
and forcing a quarter of the population to emigrate. The
blight occurred because Irish potato farmers had devel-
oped a dependence on only two potato genotypes that had
been brought to the country over 300 years before and
then vegetatively propagated; the blight had such a pro-
found impact because the country had become overly
dependent on the carbohydrate-rich potato as a food
source. The fungus was well adapted to the cool, moist
conditions of the region, and once the disease arrived and
got established, there was no stopping it. Interestingly, the
same fungus is also found in the place of origin of the
potato, the Andes of South America, but the great genetic
diversity of potatoes there, combined with ongoing natural
selection, ensures that a large proportion of the crop will
be resistant.
Another well-known example is the 1970 to 1971
outbreak of southern corn leaf blight ( Helminthosporium
maydis ), which destroyed almost the entire corn crop in
areas of Illinois and Indiana and resulted in the loss of
more than 15% of the corn crop in the U.S. as a whole
(Ullstrup, 1972). This outbreak was linked to the genetic
factors for cytosterility bred into the lines of corn used to
produce hybrid seed. These factors produced male sterility
and eliminated the need for expensive hand detasseling,
but they also increased the hybrid's susceptibility to southern
corn leaf blight. When a new strain of the blight appeared,

Mass-selected, open-pollinated variety

Genetic diversity of the variety

ABCEF LOQRTX

ACF

BEF FLO RAQ BXT

Genes and gene combinations in individuals' genotypes

Hybrid variety

Genetic diversity of the variety

ACF Q

ACFQ ACFQ ACFQ ACFQ

ACFQ

Genes and gene combinations in individuals' genotypes

FIGURE 14.8 Genetic diversity in a mass-selected crop variety
and a hybrid crop variety. In a mass-selected variety, overall genetic
diversity is much greater than that of any individual; in a hybrid
variety, any individual contains all the genetic diversity of the variety.

GE crops. This situation, along with the other types of
loss of diversity, makes our crops increasingly vulnerable
to the age-old enemies of agriculture — pests, diseases,
and unusual weather.

G ENETIC V ULNERABILITY

This consequence of the loss of genetic diversity in crop
plants and livestock deserves further discussion. Genetic
vulnerability is the susceptibility of the narrowed genetic
stock of plants and animals to attack by pests and diseases,
or to losses caused by extremes in the weather. The basic
problem is that when a crop variety or livestock breed is
genetically uniform over a large area, the ideal conditions for
the rapid outbreak of a pest or disease population are in place.
Pest and disease populations evolve at a relatively
rapid rate, in part, because of their short generation time.
With this capacity for rapid genetic change, they can adapt
quickly to changes in their hosts' defenses — or to factors
(such as pesticides) introduced into the environment by
humans. For this reason, pests and diseases in agriculture
have been able to (and might always be able to) overcome
just about everything agricultural science has thrown at
them, from pesticides and antibiotics to resistant varieties
to new practices.
In traditional agroecosystems, where crop plants are
subjected to both natural and human-imposed selection
pressures and the system retains many of the characteris-
tics of a natural ecosystem, crop plants have a fighting
chance to stay one step ahead of pathogens and herbivores.
But with modern plant breeding, large-scale monocul-
tures, and uniformity of farming practices, we have given
pests and diseases the advantage. We strive to change both