Agroecology:
principles and strategies for designing
sustainable farming systems.
Miguel
A. Altieri
University
of California, Berkeley
The concept of
sustainable agriculture is a relatively
recent response to the decline in the quality
of the natural resource base associated with
modern agriculture (McIsaac and Edwards
1994). Today, the question of agricultural
production has evolved from a purely
technical one to a more complex one
characterized by social, cultural, political
and economic dimensions. The concept of
sustainability although controversial and
diffuse due to existing conflicting
definitions and interpretations of its
meaning, is useful because it captures a set
of concerns about agriculture which is
conceived as the result of the co-evolution
of socioeconomic and natural systems
(Reijntjes et al. 1992). A wider
understanding of the agricultural context
requires the study between agriculture, the
global environment and social systems given
that agricultural development results from
the complex interaction of a multitude of
factors. It is through this deeper
understanding of the ecology of agricultural
systems that doors will open to new
management options more in tune with the
objectives of a truly sustainable
agriculture.
The
sustainability concept has prompted much
discussion and has promoted the need to
propose major adjustments in conventional
agriculture to make it more environmentally,
socially and economically viable and
compatible. Several possible solutions to the
environmental problems created by capital and
technology intensive farming systems have
been proposed and research is currently in
progress to evaluate alternative systems
(Gliessman 1998). the main focus lies on the
reduction or elimination of agrochemical
inputs through changes in management to
assure adequate plant nutrition and plant
protection through organic nutrient sources
and integrated pest management, respectively.
Although
hundreds of more environmentally prone
research projects and technological
development attempts have taken place, and
many lessons have been learned, the thrust is
still highly technological, emphasizing the
suppression of limiting factors or the
symptoms that mask an ill producing
agroecosystem. The prevalent philosophy is
that pests, nutrient deficiencies or other
factors are the cause of low productivity, as
opposed to the view that pests or nutrients
only become limiting if conditions in the
agroecosystem are not in equilibrium (Carrol
et al. 1990). For this reason, there still
prevails a narrow view that specific causes
affect productivity, and overcoming the
limiting factor via new technologies,
continues to be the main goal. This view has
diverted agriculturists from realizing that
limiting factors only represent symptoms of a
more systemic disease inherent to unbalances
within the agroecosystem and from an
appreciation of the context and complexity of
agroecological processes thus underestimating
the root causes of agricultural limitations
(Altieri et al. 1993).
On the other
hand, the science of agroecology, which is
defined as the application of ecological
concepts and principles to the design and
management of sustainable agroecosystems,
provides a framework to assess the complexity
of agroecosystems (Altieri 1995). The idea of
agroecology is to go beyond the use of
alternative practices and to develop
agroecosystems with the minimal dependence on
high agrochemical and energy inputs,
emphasizing complex agricultural systems in
which ecological interactions and synergisms
between biological components provide the
mechanisms for the systems to sponsor their
own soil fertility, productivity and crop
protection (Altieri and Rosset 1995).
Principles
of Agroecology
In the search
to reinstate more ecological rationale into
agricultural production, scientists and
developers have disregarded a key point in
the development of a more self-sufficient and
sustaining agriculture: a deep understanding
of the nature of agroecosystems and the
principles by which they function. Given this
limitation, agroecology has emerged as the
discipline that provides the basic ecological
principles for how to study, design and
manage agroecosystems that are both
productive and natural resource conserving,
and that are also culturally sensitive,
socially just and economically viable
(Altieri 1995).
Agroecology
goes beyond a one-dimensional view of
agroecosystems - their genetics, agronomy,
edaphology, and so on,- to embrace an
understanding of ecological and social levels
of co-evolution, structure and function.
Instead of focusing on one particular
component of the agroecosystem, agroecology
emphasizes the interrelatedness of all
agroecosystem components and the complex
dynamics of ecological processes (Vandermeer
1995).
Agroecosystems
are communities of plants and animals
interacting with their physical and chemical
environments that have been modified by
people to produce food, fibre, fuel and other
products for human consumption and
processing. Agroecology is the holitstic
study of agroecosystems, including all
environmental and human elements. It focuses
on the form, dynamics and functions of their
interrelationships and the processes in which
they are involved. An area used for
agricultural production, e.g. a field, is
seen as a complex system in which ecological
processes found under natural conditions also
occur, e.g. nutrient cycling, predator/prey
interactions, competition, symbiosis and
successional changes. Implicit in
agroecological research is the idea that, by
understanding these ecological relationships
and processes, agroecosystems can be
manipulated to improve production and to
produce more sustainably, with fewer negative
environmental or social impacts and fewer
external inputs (Altieri 1995).
The design of
such systems is based on the application of
the following ecological principles
(Reinjntjes et al. 1992) (see also Table 1):
1. Enhance
recycling of biomass and optimizing
nutrient availability and balancing
nutrient flow.
2.
Securing favorable soil conditions for
plant growth, particularly by managing
organic matter and enhancing soil biotic
activity.
3.
Minimizing losses due to flows of solar
radiation, air and water by way of
microclimate management, water harvesting
and soil management through increased
soil cover.
4. Species
and genetic diversification of the
agroecosystem in time and space.
5. Enhance
beneficial biological interactions and
synergisms among agrobiodiversity
components thus resulting in the
promotion of key ecological processes and
services.
These
principles can be applied by way of various
techniques and strategies. Each of these will
have different effects on productivity,
stability and resiliency within the farm
system, depending on the local opportunities,
resource constraints and, in most cases, on
the market. The ultimate goal of
agroecological design is to integrate
components so that overall biological
efficiency is improved, biodiversity is
preserved, and the agroecosystem productivity
and its self-sustaining capacity is
maintained. The goal is to design a quilt of
agroecosystems within a landscape unit, each
mimicking the structure and function of
natural ecosystems.
Biodiversification
of Agroecosystems
From a
management perspective, the agroecological
objective is to provide a balanced
environments, sustained yields, biologically
mediated soil fertility and natural pest
regulation through the design of diversified
agroecosystems and the use of low-input
technologies (Gleissman 1998). Agroecologists
are now recognizing that intercropping,
agroforestry and other diversification
methods mimic natural ecological processes,
and that the sustainability of complex
agroecosystems lies in the ecological models
they follow. By designing farming systems
that mimic nature, optimal use can be made of
sunlight, soil nutrients and rainfall (Pretty
1994).
Agroecological
management must lead management to optimal
recycling of nutrients and organic matter
turnover, closed energy flows, water and soil
conservation and balance pest-natural enemy
populations. The strategy exploits the
complementarities and synergisms that result
from the various combinations of crops, tree
and animals in spatial and temporal
arrangements (Altieri 1994).
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
(Altieri and Nicholls 1999). Today there is a
diverse selection of practices and
technologies available, and which vary in
effectiveness as well as in strategic value.
Key practices are those of a preventative
nature and which act by reinforcing the
"immunity" of the agroecosystem
through a series of mechanisms (Table 2).
Various
strategies to restore agricultural diversity
in time and space include crop rotations,
cover crops, intercropping, crop/livestock
mixtures, and so on, which exhibit the
following ecological features:
1. Crop
Rotations. Temporal diversity
incorporated into cropping systems,
providing crop nutrients and breaking the
life cycles of several insect pests,
diseases, and weed life cycles (Sumner
1982).
2. Polycultures.
Complex cropping systems in which tow or
more crop species are planted within
sufficient spatial proximity to result in
competition or complementation, thus
enhancing yields (Francis 1986,
Vandermeer 1989).
3. Agroforestry
Systems. An agricultural system where
trees are grown together with annual
crops and/or animals, resulting in
enhanced complementary relations between
components increasing multiple use of the
agroecosystem (Nair 1982).
4. Cover
Crops. The use of pure or mixed
stands of legumes or other annual plant
species under fruit trees for the purpose
of improving soil fertility, enhancing
biological control of pests, and
modifying the orchard microclimate (Finch
and Sharp 1976).
5. Animal
integration in agroecosystems aids in
achieving high biomass output and optimal
recycling (Pearson and Ison 1987).
All of the
above diversified forms of agroecosystems
share in common the following features
(Altieri and Rosset 1995):
a.
Maintain vegetative cover as an effective
soil and water conserving measure, met
through the use of no-till practices,
mulch farming, and use of cover crops and
other appropriate methods.
b. Provide
a regular supply of organic matter
through the addition of organic matter
(manure, compost, and promotion of soil
biotic activity).
c. Enhance
nutrient recycling mechanisms through the
use of livestock systems based on
legumes, etc.
d. Promote
pest regulation through enhanced activity
of biological control agents achieved by
introducing and/or conserving natural
enemies and antagonists.
Research on
diversified cropping systems underscores the
great importance of diversity in an
agricultural setting (Francis 1986,
Vandermeer 1989, Altieri 1995). Diversity is
of value in agroecosystems for a variety of
reasons (Altieri 1994, Gliessman 1998):
- As
diversity increases, so do
opportunities for coexistence and
beneficial interactions between
species that can enhance
agroecosystem sustainability.
- Greater
diversity often allows better
resource-use efficiency in an
agroecosystem. There is better
system-level adaptation to habitat
heterogeneity, leading to
complementarity in crop species
needs, diversification of niches,
overlap of species niches, and
partitioning of resources.
- Ecosystems
in which plant species are
intermingled possess an associated
resistance to herbivores as in
diverse systems there is a greater
abundance and diversity of natural
enemies of pest insects keeping in
check the populations of individual
herbivore species.
- A diverse
crop assemblage can create a
diversity of microclimates within the
cropping system that can be occupied
by a range of noncrop organisms -
including beneficial predators,
parasites, pollinators, soil fauna
and antagonists - that are of
importance for the entire system.
- Diversity
in the agricultural landscape can
contribute to the conservation of
biodiversity in surrounding natural
ecosystems.
- Diversity
in the soil performs a variety of
ecological services such as nutrient
recycling and detoxification of
noxious chemicals and regulation of
plant growth.
- Diversity
reduces risk for farmers, especially
in marginal areas with more
unpredictable environmental
conditions. If one crop does not do
well, income from others can
compensate.
Agroecology
and the Design of Sustainable Agroecosystems
Most people
involved in the promotion of sustainable
agriculture aim at creating a form of
agriculture that maintains productivity in
the long term by (Pretty 1994, Vandermeer
1995):
- optimizing
the use of locally available
resources by combining the different
components of the farm system, i.e.
plants, animals, soil, water, climate
and people, so that they complement
each other and have the greatest
possible synergetic effects;
- reducing
the use of off-farm, external and
non-renewable inputs with the
greatest potential to damage the
environment or harm the health of
farmers and consumers, and a more
targeted use of the remaining inputs
used with a view to minimizing
variable costs;
- relying
mainly on resources within the
agroecosystem by replacing external
inputs with nutrient cycling, better
conservation, and an expanded use of
local resources;
- improving
the match between cropping patterns
and the productive potential and
environmental constraints of climate
and landscape to ensure long-term
sustainability of current production
levels;
- working
to value and conserve biological
diversity, both in the wild and in
domesticated landscapes, and making
optimal use of the biological and
genetic potential of plant and animal
species; and
- taking
full advantage of local knowledge and
practices, including innovative
approaches not yet fully understood
by scientists although widely adopted
by farmers.
Agroecology
provides the knowledge and methodology
necessary for developing an agriculture that
is on the on e hand environmentally sound and
on the other hand highly productive, socially
equitable and economically viable. Through
the application of agroecological principles,
the basic challenge for sustainable
agriculture to make better use of internal
resources can be easily achieved by
minimizing the external inputs used, and
preferably by regenerating internal resources
more effectively through diversification
strategies that enhance synergisms among key
components of the agroecosystem.
The ultimate
goal of agroecological design is to integrate
components so that overall biological
efficiency is improved, biodiversity is
preserved, and the agroecosystem productivity
and its self-regulating capacity is
maintained. The goal is to design an
agroecosystem that mimics the structure and
function of local natural ecosystems; that
is, a system with high species diversity and
a biologically active soil, one that promotes
natural pest control, nutrient recycling and
high soil cover to prevent resource losses.
Conclusion
Agroecology
provides guidelines to develop diversified
agroecosystems that take advantage of the
effects of the integration of plant and
animal biodiversity such integration enhances
complex interactions and synergisms and
optimizes ecosystem functions and processes,
such as biotic regulation of harmful
organisms, nutrient recycling, and biomass
production and accumulation, thus allowing
agroecosystems to sponsor their own
functioning. The end result of agroecological
design is improved economic and ecological
sustainability of the agroecosystem, with the
proposed management systems specifically in
tune with the local resource base and
operational framework of existing
environmental and socioeconomic conditions.
In an agroecological strategy, management
components are directed to highlight the
conservation and enhancement of local
agricultural resources (germplasm, soil,
beneficial fauna, plant biodiversity, etc.)
by emphasizing a development methodology that
encourages farmer participation, use of
traditional knowledge, and adaptation of farm
enterprises that fit local needs and
socioeconomic and biophysical conditions.
Table 1.
Ecological processes to optimize in
agroecosystems |
- Strengthen
the immune system (proper
functioning of natural pest
control)
- Decrease
toxicity through elimination
of agrochemicals
- Optimize
metabolic function (organic
matter decomposition and
nutrient cycling)
- Balance
regulatory systems (nutrient
cycles, water balance, energy
flow, population regulation,
etc.)
- Enhance
conservation and regeneration
of soil-water resources and
biodiversity
- Increase
and sustain long-term
productivity
|
Table 2.
Mechanisms to improve agroecosystem
immunity |
- Increase
of plant species and genetic
diversity in time and space.
- Enhancement
of functional biodiversity
(natural enemies, antagonists
etc.)
- Enhancement
of soil organic matter and
biological activity
- Increase
of soil cover and crop
competitive ability
- Elimination
of toxic inputs and residues
|
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