Modern
Agriculture: Ecological impacts and the
possibilities for truly sustainable farming
Miguel
A. Altieri
Division of Insect Biology
University of California, Berkeley
Until about
four decades ago, crop yields in agricultural
systems depended on internal resources,
recycling of organic matter, built-in
biological control mechanisms and rainfall
patterns. Agricultural yields were modest,
but stable. Production was safeguarded by
growing more than one crop or variety in
space and time in a field as insurance
against pest outbreaks or severe weather.
Inputs of nitrogen were gained by rotating
major field crops with legumes. In turn
rotations suppressed insects, weeds and
diseases by effectively breaking the life
cycles of these pests. A typical corn belt
farmer grew corn rotated with several crops
including soybeans, and small grain
production was intrinsic to maintain
livestock. Most of the labor was done by the
family with occasional hired help and no
specialized equipment or services were
purchased from off-farm sources. In these
type of farming systems the link between
agriculture and ecology was quite strong and
signs of environmental degradation were
seldom evident (1) .
But as
agricultural modernization progressed, the
ecology-farming linkage was often broken as
ecological principles were ignored and/or
overridden. In fact, several agricultural
scientists have arrived at a general
consensus that modern agriculture confronts
an environmental crisis. A growing number of
people have become concerned about the
long-term sustainability of existing food
production systems. Evidence has accumulated
showing that whereas the present capital- and
technology-intensive farming systems have
been extremely productive and competitive,
they also bring a variety of economic,
environmental and social problems (2) .
Evidence also
shows that the very nature of the
agricultural structure and prevailing
policies have led to this environmental
crisis by favoring large farm size,
specialized production, crop monocultures and
mechanization. Today as more and more farmers
are integrated into international economies,
imperatives to diversity disappear and
monocultures are rewarded by economies of
scale. In turn, lack of rotations and
diversification take away key self-regulating
mechanisms, turning monocultures into highly
vulnerable agroecosystems dependent on high
chemical inputs.
The
expansion of monocultures
Today
monocultures have increased dramatically
worldwide, mainly through the geographical
expansion of land devoted to single crops and
year-to-year production of the same crop
species on the same land. Available data
indicate that the amount of crop diversity
per unit of arable land has decreased and
that croplands have shown a tendency toward
concentration. There are political and
economic forces influencing the trend to
devote large areas to monoculture, and in
fact such systems are rewarded by economies
of scale and contribute significantly to the
ability of national agricultures to serve
international markets.
The
technologies allowing the shift toward
monoculture were mechanization, the
improvement of crop varieties, and the
development of agrochemicals to fertilize
crops and control weeds and pests. Government
commodity policies these past several decades
encouraged the acceptance and utilization of
these technologies. As a result, farms today
are fewer, larger, more specialized and more
capital intensive. At the regional level,
increases in monoculture farming meant that
the whole agricultural support infrastructure
(i.e. research, extension, suppliers,
storage, transport, markets, etc.) has become
more specialized.
From an
ecological perspective, the regional
consequences of monoculture specialization
are many-fold:
- Most
large-scale agricultural systems
exhibit a poorly structured
assemblage of farm components, with
almost no linkages or complementary
relationships between crop
enterprises and among soils, crops
and animals.
- Cycles of
nutrients, energy, water and wastes
have become more open, rather than
closed as in a natural ecosystem.
Despite the substantial amount of
crop residues and manure produced in
farms, it is becoming increasingly
difficult to recycle nutrients, even
within agricultural systems. Animal
wastes cannot economically be
returned to the land in a
nutrient-recycling process because
production systems are geographically
remote from other systems which would
complete the cycle. In many areas,
agricultural waste has become a
liability rather than a resource.
Recycling of nutrients from urban
centers back to the fields is
similarly difficult.
- Part of
the instability and susceptibility to
pests of agroecosystems can be linked
to the adoption of vast crop
monocultures, which have concentrated
resources for specialist crop
herbivores and have increased the
areas available for immigration of
pests. This simplification has also
reduced environmental opportunities
for natural enemies. Consequently,
pest outbreaks often occur when large
numbers of immigrant pests, inhibited
populations of beneficial insects,
favorable weather and vulnerable crop
stages happen simultaneously.
- As
specific crops are expanded beyond
their "natural" ranges or
favorable regions to areas of high
pest potential, or with limited
water, or low-fertility soils,
intensified chemical controls are
required to overcome such limiting
factors. The assumption is that the
human intervention and level of
energy inputs that allow these
expansions can be sustained
indefinitely.
- Commercial
farmers witness a constant parade of
new crop varieties as varietal
replacement due to biotic stresses
and market changes has accelerated to
unprecedented levels. A cultivar with
improved disease or insect resistance
makes a debut, performs well for a
few years (typically 5-9 years) and
is then succeeded by another variety
when yields begin to slip,
productivity is threatened, or a more
promising cultivar becomes available.
A variety’s trajectory is
characterized by a take-off phase
when it is adopted by farmers, a
middle stage when the planted area
stabilizes and finally a retraction
of its acreage. Thus, stability in
modern agriculture hinges on a
continuous supply of new cultivars
rather than a patchwork quilt of many
different varieties planted on the
same farm.
- The need
to subsidize monocultures requires
increases in the use of pesticides
and fertilizers, but the efficiency
of use of applied inputs is
decreasing and crop yields in most
key crops are leveling off. In some
places, yields are actually in
decline. There are different opinions
as to the underlying causes of this
phenomenon. Some believe that yields
are leveling off because the maximum
yield potential of current varieties
is being approached, and therefore
genetic engineering must be applied
to the task of redesigning crop.
Agroecologists, on the other hand,
believe that the leveling off is
because of the steady erosion of the
productive base of agriculture
through unsustainable practices (3).
The first
wave of environmental problems
The
specialization of production units has led to
the image that agriculture is a modern
miracle of food production. Evidence
indicates, however, that excessive reliance
on monoculture farming and agroindustrial
inputs, such as capital-intensive technology,
pesticides, and chemical fertilizers, has
negatively impacted the environment and rural
society. Most agriculturalists had assumed
that the agroecosystem/natural ecosystem
dichotomy need not lead to undesirable
consequences, yet, unfortunately, a number of
"ecological diseases" have been
associated with the intensification of food
production. They may be grouped into two
categories: diseases of the ecotope, which
include erosion, loss of soil fertility,
depletion of nutrient reserves, salinization
and alkalinization, pollution of water
systems, loss of fertile croplands to urban
development, and diseases of the biocoenosis,
which include loss of crop, wild plant, and
animal genetic resources, elimination of
natural enemies, pest resurgence and genetic
resistance to pesticides, chemical
contamination, and destruction of natural
control mechanisms. Under conditions of
intensive management, treatment of such
"diseases" requires an increase in
the external costs to the extent that, in
some agricultural systems, the amount of
energy invested to produce a desired yield
surpasses the energy harvested (4).
The loss of
yields due to pests in many crops (reaching
about 20-30% in most crops), despite the
substantial increase in the use of pesticides
(about 500 million kg of active ingredient
worldwide) is a symptom of the environmental
crisis affecting agriculture. It is well
known that cultivated plants grown in
genetically homogenous monocultures do not
possess the necessary ecological defense
mechanisms to tolerate the impact of
outbreaking pest populations. Modern
agriculturists have selected crops for high
yields and high palatability, making them
more susceptible to pests by sacrificing
natural resistance for productivity. On the
other hand, modern agricultural practices
negatively affect pest natural enemies, which
in turn do not find the necessary
environmental resources and opportunities in
monocultures to effectively and biologically
suppress pests. Due to this lack of natural
controls, an investment of about 40 billion
dollars in pesticide control is incurred
yearly by US farmers, which is estimated to
save approximately $16 billion in US crops.
However, the indirect costs of pesticide use
to the environment and public health have to
be balanced against these benefits. Based on
the available data, the environmental
(impacts on wildlife, pollinators, natural
enemies, fisheries, water and development of
resistance) and social costs (human
poisonings and illnesses) of pesticide use
reach about $8 billion each year (5). What is
worrisome is that pesticide use is on the
rise. Data from California shows that from
1941 to 1995 pesticide use increased from 161
to 212 million pounds of active ingredient.
These increases were not due to increases in
planted acreage, as statewide crop acreage
remained constant during this period. Crops
such as strawberries and grapes account for
much of this increased use, which includes
toxic pesticides, many of which are linked to
cancers (6) .
Fertilizers,
on the other hand, have been praised as being
highly associated with the temporary increase
in food production observed in many
countries. National average rates of nitrate
applied to most arable lands fluctuate
between 120-550 kg N/ha. But the bountiful
harvests created at least in part through the
use of chemical fertilizers, have associated,
and often hidden, costs. A primary reason why
chemical fertilizers pollute the environment
is due to wasteful application and the fact
that crops use them inefficiently. The
fertilizer that is not recovered by the crop
ends up in the environment, mostly in surface
water or in ground water. Nitrate
contamination of aquifers is widespread and
in dangerously high levels in many rural
regions of the world. In the US, it is
estimated that more than 25% of the drinking
water wells contain nitrate levels above the
45 parts per million safety standard. Such
nitrate levels are hazardous to human health
and studies have linked nitrate uptake to
methaemoglobinemia in children and to
gastric, bladder and oesophageal cancers in
adults (7) .
Fertilizer
nutrients that enter surface waters (rivers,
lakes, bays, etc.) can promote
eutrophication, characterized initially by a
population explosion of photosynthetic algae.
Algal blooms turn the water bright green,
prevent light from penetrating beneath
surface layers, and therefore killing plants
living on the bottom. Such dead vegetation
serve as food for other aquatic
microorganisms which soon deplete water of
its oxygen, inhibiting the decomposition of
organic residues, which accumulate on the
bottom. Eventually, such nutrient enrichment
of freshwater ecosystems leads to the
destruction of all animal life in the water
systems. In the US it is estimated that about
50-70% of all nutrients that reach surface
waters is derived from fertilizers.
Chemical
fertilizers can also become air pollutants,
and have recently been implicated in the
destruction of the ozone layer and in global
warming. Their excessive use has also been
linked to the acidification/salinization of
soils and to a higher incidence of insect
pests and diseases through mediation of
negative nutritional changes in crop plants
(8).
It is clear
then that the first wave of environmental
problems is deeply rooted in the prevalent
socioeconomic system which promotes
monocultures and the use of high input
technologies and agricultural practices that
lead to natural resource degradation. Such
degradation is not only an ecological
process, but also a social and
political-economic process (9) . This is why
the problem of agricultural production cannot
be regarded only as a technological one, but
while agreeing that productivity issues
represent part of the problem, attention to
social, cultural and economic issues that
account for the crisis is crucial. This is
particularly true today where the economic
and political domination of the rural
development agenda by agribusiness has
thrived at the expense of the interests of
consumers, farmworkers, small family farms,
wildlife, the environment, and rural
communities (10).
The second
wave of environmental problems.
Despite that
awareness of the impacts of modern
technologies on the environment increased, as
we traced pesticides in food chains and crop
nutrients in streams and aquifiers, there are
those that confronted to the challenges of
the XXI century still argue for further
intensification to meet the requirements of
agricultural production. It is in this
context that supporters of "status-quo
agriculture" celebrate the emergence of
biotechnology as the latest magic bullet that
will revolutionize agriculture with products
based on natures’ own methods, making
farming more environmentally friendly and
more profitable for the farmer. Although
clearly certain forms of non-transformational
biotechnology hold promise for an improved
agriculture, given its present orientation
and control by multinational corporations, it
holds more promise for environmental harm,
for the further industrialization of
agriculture and for the intrusion of private
interests too far into public interest sector
research (11).
What is ironic
is the fact that the biorevolution is being
brought forward by the same interests
(Monsanto, Novartis, DuPont, etc.) that
promoted the first wave of
agrochemically-based agriculture, but this
time, by equipping each crop with new
"insecticidal genes", they are
promising the world safer pesticides,
reduction on chemically intensive farming and
a more sustainable agriculture.
However, as
long as transgenic crops follow closely the
pesticide paradigm, such biotechnological
products will do nothing but reinforce the
pesticide treadmill in agroecosystems, thus
legitimizing the concerns that many
scientists have expressed regarding the
possible environmental risks of genetically
engineered organisms.
So far, field
research as well as predictions based on
ecological theory, indicate that among the
major environmental risks associated with the
release of genetically engineered crops can
be summarized as follows (12):
- The
trends set forth by corporations is
to create broad international markets
for a single product, thus creating
the conditions for genetic uniformity
in rural landscapes. History has
repeatedly shown that a huge area
planted to a single cultivar is very
vulnerable to a new matching strain
of a pathogen or pest;
- The
spread of transgenic crops threatens
crop genetic diversity by simplifying
cropping systems and promoting
genetic erosion;
- There is
potential for the unintended transfer
to plant relatives of the
"transgenes" and the
unpredictable ecological effects. The
transfer of genes from herbicide
resistant crops (HRCs) to wild or
semidomesticated relatives can lead
to the creation of super weeds;
- Most
probably insect pests will quickly
develop resistance to crops with Bt
toxin. Several Lepidoptera species
have been reported to develop
resistance to Bt toxin in both field
and laboratory tests, suggesting that
major resistance problems are likely
to develop in Bt crops which through
the continuous expression of the
toxin create a strong selection
pressure;
- Massive
use of Bt toxin in crops can unleash
potential negative interactions
affecting ecological processes and
non-target organisms. Evidence from
studies conducted in Scotland suggest
that aphids were capable of
sequestering the toxin from Bt crops
and transferring it to its
coccinellid predators, in turn
affecting reproduction and longevity
of the beneficial beetles;
- Bt toxins
can also be incorporated into the
soil through leaf materials and
litter, where they may persist for
2-3 months, resisting degradation by
binding to soil clay particles while
maintaining toxic activity, in turn
negatively affecting invertebrates
and nutrient cycling;
- A
potential risk of transgenic plants
expressing viral sequences derives
from the possibility of new viral
genotypes being generated by
recombination between the genomic RNA
of infecting viruses and RNA
transcribed from the transgene;
- Another
important environmental concern
associated with the large scale
cultivation of virus-resistant
transgenic crops relates to the
possible transfer of virus-derived
transgenes into wild relatives
through pollen flow.
Although there
are many unanswered questions regarding the
impact of the release of transgenic plants
and micro-organisms into the environment, it
is expected that biotechnology will
exacerbate the problems of conventional
agriculture and by promoting monocultures
will also undermine ecological methods of
farming such as rotations and polycultures.
Because transgenic crops developed for pest
control emphasize the use of a single control
mechanism, which has proven to fail over and
over again with insects, pathogens and weeds,
transgenic crops are likely to increase the
use of pesticides and to accelerate the
evolution of "super weeds" and
resistant insect pest strains. These
possibilities are worrisome, especially when
considering that during the period 1986-1997,
approximately 25,000 transgenic crop field
trials were conducted worldwide on more than
60 crops with 10 traits in 45 countries. By
1997 the global area devoted to transgenic
crops reached 12.8 million hectares.
Seventy-two percent of all transgenic crop
field trials were conducted in the USA and
Canada, although some were also conducted in
descending order in Europe, Latin America and
Asia (13). In most countries biosafety
standards to monitor such releases are absent
or are inadequate to predict ecological
risks. In the industrialized countries from
1986-1992, 57% of all field trials to test
transgenic crops involved herbicide tolerance
pioneered by 27 corporations including the
world’s eight largest pesticide
companies. As Roundup and other broad
spectrum herbicides are increasingly deployed
into croplands, the options for farmers for a
diversified agriculture will be even more
limited.
The array
of alternatives to conventional agriculture.
Reduction and,
especially, elimination of agrochemical
require major changes in management to assure
adequate plant nutrients and to control crop
pests. As it was done a few decades ago,
alternative sources of nutrients to maintain
soil fertility include manures, sewage sludge
and other organic wastes, and legumes in
cropping sequences. Rotation benefits are due
to biologically fixed nitrogen and from the
interruption of weed, disease and insect
cycles. A livestock enterprise may be
integrated with grain cropping to provide
animal manures and to utilize better the
forages produced. Maximum benefits of pasture
integration can be realized when livestock,
crops, animals and other farm resources are
assembled in mixed and rotational designs to
optimize production efficiency, nutrient
cycling and crop protection.
In orchards
and vineyards, the use of cover crops improve
soil fertility, soil structure and water
penetration, prevent soil erosion, modify the
microclimate and reduce weed competition.
Entomological studies conducted in orchards
with ground cover vegetation indicate that
these systems exhibit lower incidence of
insect pests than clean cultivated orchards.
This is due to a higher abundance and
efficiency of predators and parasitoids
enhanced by the rich floral undergrowth (14).
Increasingly,
researchers are showing that it is possible
to provide a balanced environment, sustained
yields, biologically mediated soil fertility
and natural pest regulation through the
design of diversified agroecosystems and the
use of low-input technologies. Many
alternative cropping systems have been
tested, such as double cropping, strip
cropping, cover cropping and intercropping,
and more importantly concrete examples from
real farmers show that such systems lead to
optimal recycling of nutrients and organic
matter turnover, closed energy flows, water
and soil conservation and balanced
pest-natural enemy populations. Such
diversified farming exploit the
complementarities that result from the
various combinations of crops, trees and
animals in spatial and temporal arrangements
(15).
In essence,
the optimal behavior of agroecosystems
depends on the level of interactions between
the various biotic and abiotic components. By
assembling a functional biodiversity it is
possible to initiate synergisms which
subsidize agroecosystem processes by
providing ecological services such as the
activation of soil biology, the recycling of
nutrients, the enhancement of beneficial
arthropods and antagonists, and so on. Today
there is a diverse selection of practices and
technologies available, and which vary in
effectiveness as well as in strategic value.
The
barriers for the implementation of
alternatives
The
agroecological approach seeks the
diversification and revitalization of medium
size and small farms and the reshaping of the
entire agricultural policy and food system in
ways that are economically viable to farmers
and consumers. In fact, throughout the world
there are hundreds of movements that are
pursuing a change toward ecologically
sensitive farming systems from a variety of
perspectives. Some emphasize the production
of organic products for lucrative markets,
others land stewardship, while others the
empowerment of peasant communities. In
general, however, the goals are usually the
same: to secure food self-sufficiency, to
preserve the natural resource base, and to
ensure social equity and economic viability.
What happens
is that some well-intentioned groups suffer
from "technological determinism",
and emphasize as a key strategy only the
development and dissemination of low-input or
appropriate technologies as if these
technologies in themselves have the
capability of initiating beneficial social
changes. The organic farming school that
emphasizes input substitution (i.e. a toxic
chemical substituted by a biological
insecticide) but leaving the monoculture
structure untouched, epitomizes those groups
that have a relatively benign view of
capitalist agriculture. Such perspective has
unfortunately prevented many groups from
understanding the structural roots of
environmental degradation linked to
monoculture farming (16).
This narrow
acceptance of the present structure of
agriculture as a given condition restricts
the real possibility of implementing
alternatives that challenge such a structure.
Thus, options for a diversified agriculture
are inhibited among other factors by the
present trends in farm size and
mechanization. Implementation of such mixed
agriculture would only be possible as part of
a broader program that includes, among other
strategies, land reform and redesign of farm
machinery adapted to polycultures. Merely
introducing alternative agricultural designs
will do little to change the underlying
forces that led to monoculture production,
farm size expansion, and mechanization in the
first place.
Similarly,
obstacles to changing cropping systems has
been created by the government commodity
programs in place these last several decades.
In essence, these programs have rewarded
those who maintained monocultures on their
base feed grain acres by assuring these
producers a particular price for their
product. Those who failed to plant the
allotted acreage of corn and other
price-supported crops lost one deficit
hectrage from their base. Consequently this
created a competitive disadvantage for those
who used a crop rotation. Such a
disadvantage, of course, exacerbated economic
hardship for many producers (17). Obviously
many policy changes are necessary in order to
create an economic scenario favorable to
alternative cropping practices.
On the other
hand, the large influence of multinational
companies in promoting sales of agrochemicals
cannot be ignored as a barrier to sustainable
farming. Most MNCs have taken advantage of
existing policies that promote the enhanced
participation of the private sector in
technology development and delivery,
positioning themselves in a powerful position
to scale up promotion and marketing of
pesticides. Realistically then the future of
agriculture will be determined by power
relations, and there is no reason why farmers
and the public in general, if sufficiently
empowered, could not influence the direction
of agriculture along sustainability goals.
Conclusions
Clearly the
nature of modern agricultural structure and
contemporary policies have decidedly
influenced the context of agricultural
technology and production, which in turn has
led to environmental problems of a first and
second order. In fact, given the realities of
the dominant economic milieu, policies
discourage resource-conserving practices and
in many cases such practices are not
privately profitable for farmers. So the
expectation that a set of policy changes
could be implemented for a renaissance of
diversified or small scale farms may be
unrealistic, because it negates the existence
of scale in agriculture and ignores the
political power of agribusiness corporations
and current trends set forth by
globalization. A more radical transformation
of agriculture is needed, one guided by the
notion that ecological change in agriculture
cannot be promoted without comparable changes
in the social, political, cultural and
economic arenas that also conform
agriculture. In other words, change toward a
more socially just, economically viable, and
environmentally sound agriculture should be
the result of social movements in the rural
sector in alliance with urban organizations.
This is especially relevant in the case of
the new biorevolution, where concerted action
is needed so that biotechnology companies
feel the impact of environmental, farm labor,
animal rights and consumers lobbies,
pressuring them to re-orienting their work
for the overall benefit of society and
nature.
References
Altieri, M.A.
1992. Agroecological foundations of
alternative agriculture in California.
Agriculture, Ecosystems and Environment 39:
23-53.
Altieri, M.A.
1995. Agroecology: the science of sustainable
agriculture. Westview Press, Boulder
Altieri, M.A.
and P.M. Rosset 1995. Agroecology and the
conversion of large-scale conventional
systems to sustainable management.
International Journal of Environmental
Studies 50: 165-185.
Audirac, Y.
1997. Rural sustainable development in
America. John Wiley and Sons, N.Y.
Buttel, F.H.
and M.E. Gertler 1982. Agricultural
structure, agricultural policy and
environmental quality. Agriculture and
Environment 7: 101-119.
Conway, G.R.
and Pretty, J.N. 1991. Unwelcome harvest:
agriculture and pollution. Earthscan
Publisher, London.
Gliessman,
S.R. 1997. Agroecology: ecological processes
in agriculture. Ann Arbor Press, Michigan.
James, C.
1997. Global status of transgenic crops in
1997. ISAA Briefs, Ithaca, N.Y.
Krimsky, S.
and R.P. Wrubel 1996. Agricultural
biotechnology and the environment: science,
policy and social issues. University of
Illinois Press, Urbana.
Liebman, J.
1997. Rising toxic tide: pesticide use in
California, 1991-1995. Report of Californians
for Pesticide Reform and Pesticide Action
Network. San Francisco.
Mc Guinnes, H.
1993. Living soils: sustainable alternatives
to chemical fertilizers for developing
countries. Unpublished manuscript, Consumers
Policy Institute, New York.
Mc Isaac, G.
and W.R. Edwards 1994. Sustainable
agriculture in the American midwest.
University of Illinois Press, Urbana.
Pimentel, D.
and H. Lehman 1993. The pesticide question.
Chapman and Hall, N.Y.
Rissler, J.
and M. Mellon 1996. The ecological risks of
engineered crops. MIT Press, Cambridge.
Rosset, P.M.
and M.A. Altieri 1997. Agroecology versus
input substitution: a fundamental
contradiction in sustainable agriculture.
Society and Natural Resources 10: 283-295.
Endnotes
(1) Altieri,
M.A. 1995. Agroecology: the science of
sustainable agriculture. Westview Press,
Boulder
(2) Conway,
G.R. and Pretty, J.N. 1991. Unwelcome
harvest: agriculture and pollution. Earthscan
Publisher, London.
(3) Altieri,
M.A. and P.M. Rosset 1995. Agroecology and
the conversion of large-scale conventional
systems to sustainable management.
International Journal of Environmental
Studies 50: 165-185.
(4) Gliessman,
S.R. 1997. Agroecology: ecological processes
in agriculture. Ann Arbor Press, Michigan.
(5) Pimentel,
D. and H. Lehman 1993. The pesticide
question. Chapman and Hall, N.Y.
(6) Liebman,
J. 1997. Rising toxic tide: pesticide use in
California, 1991-1995. Report of Californians
for Pesticide Reform and Pesticide Action
Network. San Francisco.
(7) Conway,
G.R. and Pretty, J.N. 1991. Unwelcome
harvest: agriculture and pollution. Earthscan
Publisher, London.
(8) Mc
Guinnes, H. 1993. Living soils: sustainable
alternatives to chemical fertilizers for
developing countries. Unpublished manuscript,
Consumers Policy Institute, New York.
(9) Buttel,
F.H. and M.E. Gertler 1982. Agricultural
structure, agricultural policy and
environmental quality. Agriculture and
Environment 7: 101-119.
(10) Audirac,
Y. 1997. Rural sustainable development in
America. John Wiley and Sons, N.Y.
(11) Krimsky,
S. and R.P. Wrubel 1996. Agricultural
biotechnology and the environment: science,
policy and social issues. University of
Illinois Press, Urbana.
(12) Rissler,
J. and M. Mellon 1996. The ecological risks
of engineered crops. MIT Press, Cambridge.
(13) James, C.
1997. Global status of transgenic crops in
1997. ISAA Briefs, Ithaca, N.Y.
(14) Altieri,
M.A. 1992. Agroecological foundations of
alternative agriculture in California.
Agriculture, Ecosystems and Environment 39:
23-53.
(15) Altieri,
M.A. 1995. Agroecology: the science of
sustainable agriculture. Westview Press,
Boulder
(16) Rosset,
P.M. and M.A. Altieri 1997. Agroecology
versus input substitution: a fundamental
contradiction in sustainable agriculture.
Society and Natural Resources 10: 283-295.
(17) Mc Isaac,
G. and W.R. Edwards 1994. Sustainable
agriculture in the American midwest.
University of Illinois Press, Urbana.
