Miguel A. Altieri
Department of Environmental Science, Policy, and Management
University of California, Berkeley

and

Clara Ines Nicholls
University of California Cooperative Extension
Alameda County, California

Agroecology is the scientific discipline that provides the basic ecological principles for how to study, design and manage agroecosystems that are both productive and natural resource conserving, and are also culturally sensitive, socially just and economically viable. Agroecology provides the guidelines to restore and enhance the resiliency, sustainability and health of agroecosystems. Biotic constraints stressing agroecosystems are understood as imbalances; therefore the goal of the agroecological treatments is to recover balance and enhance the "immunity"of the agricultural system. Agroecologists contend that the links between healthy soils and healthy plants is fundamental to ecollogically based pest management. Also agroecologists promote biodiversification as the primary technique to evoke self-regulation and sustainability. However, ecological health and sustainability is not possible without preserving the cultural diversity that nurtures local agricultures. In addition to a proper balance of crops, soils, nutrients, arthropods, etc, stable production must take place in the context of social organization that protects the integrity of natural resources and encourages the harmonious interaction of humans, the agroecosystem and the overall environment.

The ultimate goal of all farmers should be to grow healthy and productive plants and/or animals, while maintaining the ecological integrity of the resource base of their farms. For agroecologists, such goals would translate into healthy agroecosystems exhibiting a high degree of integrity, a strong capacity to respond/adapt and high levels of efficiency, productivity, stability and self-dependence (Altieri and Nicholls, 1999).

Sustainable yield in the agroecosystem derives from the proper balance of crops, soils, nutrients, sunlight, moisture, and other coexisting organisms. The agroecosystem is productive and healthy when this balance and rich growing conditions prevail, and when crop plants remain resilient to tolerate stress and adversity. Occasional disturbances can be overcome by vigorous agroecosystems, which are adaptable, and diverse enough to recover once the stress has passed (Altieri and Rosset, 1995). If the cause of disease, pests, soil degradation, etc. is understood as imbalance (Table 1), the goal of agroecological treatment is to recover balance: the agroecosystems natural tendency toward repairing itself. This tendency is known in ecology as homeostasis, the maintenance of the system's internal functions and defense to compensate for external stress factors. But achieving and maintaining such state of homeostasis requires a deep understanding of the nature of agroecosystems and the principles by which they function. Fortunately there are new integrative scientific approaches that allow for such understanding. Among them is agroecology which has emerged as a discipline that provides basic ecological principles on how to study, design, and manage agroecosystems that are both productive and natural resource conserving (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 on focusing on one particular component of agroecosystem, agroecology emphasizes the interrelatedness of all agroecosystem components and the complex dynamics of ecological processes such as nutrient cycling and pest regulation (Gliessman, 1998).

From a management perspective, the agroecological objective is 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 (Altieri, 1994). The strategy is based on ecological principles that lead management to optimal recycling of nutrients and organic matter turnover, closed energy flows, water and soil conservation and balanced pest-natural enemy populations. The strategy exploits the complementarities that result from the various combinations of crops, trees and animals in spatial and temporal arrangements (Altieri and Nicholls, 1999). These combinations determine the establishment of a planned and associated functional biodiversity which when correctly assembled, plays key ecological services which subsidize agroecosystem processes that underlie agroecosystem health (Table 2).

In other, words, ecological concepts are utilized to favor natural processes and biological interactions that optimize synergies so that diversified farms are able to sponsor their own soil fertility, crop protection and productivity through the activation of soil biology, the recycling of nutrients, the enhancement of beneficial arthropods and antagonists, and so on.

In this article, we provide an agroecological framework to achieve crop health through agroecosystem diversification and soil quality enhancement, key pillars of agroecosystem health. The main goal is to set in motion all mechanisms that enhance the "immunity" of the agroecosystem (Table 3)

Many scientists today agree that conventional modern agriculture faces an environmental crisis. Land degradation, salinization, pesticide pollution of soil, water and food chains, depletion of ground water, genetic homogeneity and associated vulnerability, all raise serious questions regarding the sustainability and health of modern agroecosystems. The loss of yields due to pests in many crops, despite the substantial increase in the use of pesticides is a symptom of the environmental crisis affecting agriculture (Altieri and Rosset, 1995). It is well known that cultivated plants grown in genetically homogeneous monocultures do not possess the necessary ecological defense mechanisms to tolerate out breaking 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 (Robinson, 1996). On the other hand, modern agricultural practices negatively affect natural enemies (predators and parasites), which do not find the necessary environmental resources and opportunities in monocultures to effectively suppress pests (Altieri, 1994). While the structure of monocultures is maintained as the structural base of modern agricultural systems, pest problems will continue to be the result of a negative treadmill that reinforces itself (Figure 1). Thus the major challenge for those advocating ecologically based pest management (EBPM) is to find strategies to overcome the ecological limits imposed by monocultures.

Integrated Pest Management (IPM) approaches have not addressed the ecological causes of the environmental problems in modern agriculture. There still prevails a narrow view that specific causes affect productivity, and overcoming the limiting factor (i.e. insect pest) via new technologies, continues to be the main goal. In many IPM projects the main focus has been to substitute less noxious inputs for the agrochemicals that are blamed for so many of the problems associated with conventional agriculture. Emphasis is now placed on purchased biological inputs such as Bacillus thuringiensis, a microbial pesticide that is now widely applied in place of chemical insecticides. This type of technology pertains to a dominant technical approach called "input substitution". The thrust is highly technological, characterized by a limiting factor mentality that has driven conventional agricultural research in the past. Agronomists and other agricultural scientists have for generations been taught the "law of minimum" as a central dogma. According to this dogma, at any given moment there is a single factor limiting yield increases, and that factor can be overcome with an appropriate external input. Once the hurdle of the first limiting factor has been surpassed - nitrogen deficiency, for example, with urea as the correct input - then yields may rise until another factor - pests, say - becomes limiting in turn due to increased levels of free nitrogen in foliage. The factor then requires another input - pesticide in this case - and so on, perpetuating a process of treating symptoms rather than dealing with the real causes that evoked ecological unbalance (Altieri and Rosset, 1995).

Emerging biotechnological approaches do not differ as they are being pursued to patch up problems (e.g. pesticide resistance, pollution, soil degradation, etc.) caused by previous agrochemical technologies promoted by the same companies now leading the bio-revolution. Transgenic crops developed for pest control closely follow the paradigm of using a single control mechanism (a pesticide) that has been 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 pests (Rissler and Mellon, 1996).

The 'one gene-one-pest' approach has been proven to be easily overcome by pests that are continuously adapting to new situations and evolving detoxification mechanisms. There are many unanswered ecological questions regarding the impact of the release of transgenic plants and microorganisms into the environment (Snow and Moran, 1997). Among the major environmental risks associated with genetically engineered plants are the unintended transfer to plant relatives of the 'transgenes' and the unpredictable ecological effects on non-target organisms. Given the above considerations, agro-ecological theory predicts that biotechnology will exacerbate the problems of conventional agriculture (Altieri, 1998). By promoting monocultures, it will also undermine ecological methods of farming, such as rotations and polycultures, key strategies to break the homogeneous nature of monocultures. As presently conceived, biotechnology does not fit into the broad ideals of sustainable agriculture.

Nowhere are the consequences of biodiversity reduction more evident than in the realm of agricultural pest management. The instability of agroecosystems, which is manifested as the worsening of most insect pest problems, is increasingly linked to the expansion of crop monocultures at the expense of the natural vegetation, thereby decreasing local habitat diversity (Altieri and Letourneau, 1982). Plant communities that are modified to meet the special needs of humans become subject to heavy pest damage and generally the more intensely such communities are modified, the more abundant and serious the pests (Andow, 1991). The inherent self-regulation characteristics of natural communities are lost when humans modify such communities through the shattering of the fragile thread of community interactions. Agroecologists maintain that this breakdown can be repaired by restoring the shattered elements of community homeostasis through the addition or enhancement of biodiversity (Altieri and Nicholls, 1999).

The key is to identify the type of biodiversity that is desirable to maintain and or enhance in order to carry out ecological services, and then to determine the best practices that will encourage the desired biodiversity components. Figure 2 shows that there are many agricultural practices and designs that have the potential to enhance functional biodiversity, and others that negatively affect it. The idea is to apply the best management practices in order to enhance or regenerate the kind of biodiversity that subsidizes the health and sustainability of agroecosystems by providing ecological services such as biological pest control, nutrient cycling, water and soil conservation, etc. (Gliessman,1998). As depicted in Figure 3 crop health can be achieved by regulating insect pests through two routes: a healthy soil and a rich natural enemy biodiversity harbored by a diversified agroecosystem.

A key feature of modern cropping systems is the frequency of soil disturbance regimes, including periodic tillage and pesticide applications, which reduce soil biotic activity and species diversity in agroecosystems. Such soil biodiversity reductions are negative because the recycling of nutrients and proper balance between organic matter, soil organisms and plant diversity are necessary components of a productive and ecologically balanced soil environment (Hendrix et al., 1990). The ability of a crop plant to resist or tolerate pests is tied to optimal physical, chemical and biological properties of soils. Adequate moisture, good soil tilth, moderate pH, right amounts of organic matter and nutrients and a diverse and active community of soil organisms all contribute to plant health. Organic rich soils generally exhibit good soil fertility as well as complex good webs and beneficial organisms that prevent infection by disease causing organisms as Pythium and Rhizoctonia. On the other hand, farming practices that causes nutrition imbalances can lower crop resistance. High nitrogen fertilizer levels can enhance the incidence of diseases such as Phytophtora and Fusarium and stimulate outbreaks of Homopteran insects such as aphids and leafhoppers (Campbell, 1989). In fact there is increasing evidence that crops grown in organic rich and biologically active soils are less susceptible to pest attacks. Many studies suggest that the physiological susceptibility of crops and pathogens may be affected by the form of fertilizer used (organic vs. chemical fertilizer). Studies documenting lower diversity of several insect herbivores in low-input systems, have partly attributed such reduction to a low nitrogen content in organically farmed crops (Magdoff, 1992). In California, a series of comparative experiments conducted on various growing seasons between 1989-1996 where broccoli was subjected to varying fertilization regimes (conventional vs. organic) showed that agroecological techniques can reduce the abundance of key insect pests, cabbage aphid (Brevicoryne brassicae) and flea beetle (Phyllotreta cruciferae), while sustaining yields. Lower herbivore numbers in organically managed plots were attributed to low foliage nitrogen content in compost fed broccoli plants (Altieri, 1994).

In Japan, density of immigrants of the planthopper Sogatella furcifera was significantly lower while the settling rate of female adults and survival rate of immature stages of ensuing generations were lower in organic rice fields. The number of eggs laid by a female of the invading and following generations was smaller, and the percentage of brachypterous females in the next generation was also lower. Consequently, the density of nymphs and adults in the ensuing generations decreased in organically farmed fields (Kajimura, 1995).

In England, conventional winter wheat fields developed a larger infestation of the aphid Metopolophiunt dirhodum than its organic counterpart. This crop also had higher levels of free protein amino acids in its leaves during June, which were believed to have resulted from a nitrogen top dressing of the crop in early April. However, the difference in the aphid infestations between crops was attributed to the aphid's response to relative proportions of certain non-protein to protein amino acids in the leaves at the time of aphid settling in the crops (Kowalski and Visser,1979). In greenhouse experiments when given a choice of maize grown on organic versus chemically fertilized soils, European corn borer females preferred to significantly lay more eggs in chemically fertilized plants (Phelan et al., 1995).

Nutrition is also important in determining susceptibility or resistance of plants to pathogens. Mineral nutrients are essential metabolic regulators of plant growth, and most studies indicate that nutrition affects pathogens and diseases indirectly. Biological activity in the soil becomes very intense in response to organic amendments and increase fungistasis as well as populations of existing microbial antagonists. Composts of diverse organic materials have proven effective in controlling diseases. Many types of compost host beneficial organisms that feed directly on pathogens, compete with them for nutrients or produce antibiotics (Tjamos et al., 1992)

Balanced soil nutrition helps plants stay more vigorous, increase the growth rate, make better use of soil water, and improve anatomical or histological characteristics. These factors enable plants to produce greater numbers of roots, allowing more surface area for root absorption and nutrient uptake, and possibly, shortening susceptible stages of plant growth. This allows plants to function more efficiently even when some roots are infected. Histological changes strengthen plants and possibly create barriers more difficult for pathogens to breach. A healthier plant may also increase quality of exudates, which can stimulate increases in populations of antagonistic microorganisms. These, in turn, may compete with pathogens for nutrients or possibly produce toxins that directly affect pathogen development and survival (Palti, 1981).

Application of nutrients (especially N) may increase competitive suppression of crops by weeds, as most weeds, (especially C₄ species) are often more responsive to application of nitrogen than other crops. A study reported that application of chemical N fertilizer to wild oat-spring wheat mixtures increased wild oat growth and decreased wheat yields. On the other hand other studies have shown that N fertilizer can improve the competitive status of crops. What seems critical in determining the outcome of competitive interactions, is the timing of nutrient availability relative to crop and weed demands upon nutrient supplies (Liebman and Gallandt,1997). Emerging research shows that the species composition and general ecology of weeds is radically different in organic versus conventionally fertilized systems. Yield reductions of wheat due to interference from Italian ryegrass (Lolium multiflorum Lam.) were greater under conventionally fertilized conditions than under organic fertilized conditions. In many cases weed suppression is related to delayed N release from the organic N source compared to nitrate fertilizer. In other cases soil incorporated organic residues increase phytotoxicity or pathogen activity, which suppress weed seed and/or seedlings (Liebman and Ohno, 1998).

Diversified cropping systems, such as those based on intercropping and agroforestry or cover cropping of orchards, have been the target of much research recently. This interest is largely based on the new emerging evidence that these systems are more stable and more resource conserving (Vandermeer, 1995). Much of these attributes are connected to the higher levels of functional biodiversity associated with complex farming systems. In fact, an increasing amount of data reported in the literature documents the effects that plant diversity have on the regulation of insect herbivore populations by favoring the abundance and efficacy of associated natural enemies (Altieri and Letourneau, 1984). One of the key elements in diversified agroecosystems is the presence of flowering plants that provide pollen and nectar that serve as alternative food for natural enemies. A number of studies have shown that many parasitoids require nectar for normal fecundity and longevity, and thus spectacular parasitism increases have been observed in annual crops and orchards with rich floral undergrowth (Leius, 1967).

Flowering plants can also increase populations of non-pestiferous herbivores (neutral insects) in crop fields. Such insects serve as alternative hosts or prey to entomophagous insects thus improving the survival and reproduction of these beneficial insects in the agroecosystem. Many researchers have reported that the presence of neutral insects on flowering plants within or near crop fields increase predation and parasitism of specific crop pests (Altieri and Whitcomb, 1979).

Our research in northern California vineyards confirms the importance of maintaining full season floral diversity to enhance natural enemy abundance and diversity in the grape agroecosystem. Growing summer cover crops of buckwheat and sunflower had a substantial impact on the abundance of western grape leafhoppers and associated natural enemies. During two consecutive years (1996-1997) vineyard systems with flowering cover crops, were characterized by lower densities of leafhoppers (Figure 4). Such reduced pest numbers in diversified vineyards were due to the impacts of generalist predators. With increased plant diversity, insect pests remained at lower levels than in clean cultivated vineyards, partly because the summer cover crop vegetation harbored pollen, nectar and neutral insects that served as alternate food and hosts for important predators along with the parasitic wasp Anagrus.

Plant diversity around crop fields is also important in determining the diversity and abundance of natural enemies within agroecosystems. Emerging data demonstrate that there is enhancement of natural enemies and more effective biological control where wild vegetation remains at field edges in close association with crops (Altieri, 1994). These habitats are important as overwintering sites for predators, or they may provide increased resources, such as pollen and nectar for parasitoids from flowering plants (Thies and Tscharntke, 1999). The presence of plant rich habitats enhances predator colonization and abundance of adjacent crop fields but this influence is limited to the distance to which natural enemies disperse into the vineyard (Corbett and Plant, 1993). A corridor however could amplify this influence by allowing enhanced and timely circulation and dispersal movement of predators from edges into the center of crop fields.

In northern California, taking advantage of an existing 600 m corridor connected to a riparian forest and that cut across a monoculture vineyard, we tested the idea whether such a corridor served as a biological highway for the movement and dispersal of natural enemies from the forest into the center of the vineyard. The goal was to evaluate if the corridor acted as a consistent, abundant and well dispersed source of alternative food (pollen, nectar, and neutral insects) for a diverse community of generalist predators and parasitoids.

During 1996 and 1997, a 2.5 hectares organic vineyard dissected by a corridor composed of 65 flowering plants species which was connected to the surrounding riparian habitat, was monitored to assess the distributional and abundance patterns of the Western grape leafhopper and its parasitoid Anargus spp., and generalist predators. In both years, leafhopper adults and nymphs tended to be more numerous in middle rows of the vineyard and less abundant in border rows close to the forest and corridor where predators were more abundant (Figures 5 and 6). The complex of predators supported by the corridor moved to the adjacent vine rows and exerted a regulatory impact on herbivores present in such rows. Although it is suspected that the leafhopper parasitic wasp Anagrus depended on food resources of the corridor, it did not display a gradient from this rich flowering area into the middle of the field. Likewise no differences in rates of egg parasitism of leafhoppers could be detected in vines near the corridor or in the vineyard center. The presence of riparian habitats enhanced predator colonization and abundance on adjacent vineyards, although this influence was limited by the distance to which natural enemies disperse into the vineyard. However, the corridor, amplified this influence by enhancing timely circulation and dispersal movement of predators into the center of the field (Figure 7).

Achieving health in agroecosystems requires that management be directed at improving soil and plant quality, as the link between healthy soils and healthy plants is fundamental to EBPM. Of key importance is also the realization that the level of internal regulation of function in agroecosystems is largely dependent on the level of plant and animal biodiversity present. In agroecosystems, biodiversity performs a variety of ecological services beyond the production of food, including recycling of nutrients and regulation of pest populations. For this reason agroecologists promote multifunctional technologies that enhance biodiversity, as their adoption usually means favorable changes in various components of the farming systems at the same time. For example, legume based crop rotations; one of the simplest forms of biodiversification can simultaneously optimize soil fertility and pest regulation. It is well known that rotations improve yields by the known action of interrupting weed, disease and insect lifecycles. However, they can also have subtle effects such as enhancing the growth and activity of soil biology, including vesicular arbuscular mycorrhizae (VAM), which allow crops to more efficiently use soil nutrients and water, and thus better resist pest attack.

Many scientists are concerned that, with accelerating rates of habitat simplification through monoculture expansion, the contribution to pest suppression by biocontrol agents using these habitats will decline (Fry, 1995). For this reason, many researchers have proposed options to rectify this decline by increasing vegetational diversity within agroecosystems and throughout agricultural landscapes. Options include diversifying cropping systems with polycultural designs and/or maintaining wild vegetation adjacent to crop fields (Thomas et al., 1991).

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, that is, systems that include:

1. 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.
2. A regular supply of organic matter through the regular addition of organic matter (manure, compost, and promotion of soil biotic activity).
3. Nutrient recycling mechanisms through the use of crop rotations, crop livestock systems based on legumes, etc.
4. Pest regulation assured through enhanced activity of biological control agents achieved by introducing and/or conserving natural enemies and antagonists.

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Table 1. Causes of agroecosystem immune dysfunction

Table 2. Routes to agroecosystem health

Table 3. Mechanisms to improve agroecosystem immunity

Figure 4. Densities of adult leafhoppers E. elegantula in cover cropped and monoculture vineyards in Hopland, California, in two growing seasons. Mean densities (number ofadults per yellow sticky trap) and standard errors of two replicate means are indicated. In some cases error bars were too small to appear in the figure.

Figure 5.Seasonal patterns of leafhopper nymphs in both vineyard blocks, as influenced by the presence or absence of forest edges and the corridor (Hopland, California. 1996)

Figure 6. Seasonal patterns of predator catches (numbers per yellow sticky trap) in both vineyard blocks, as influenced by the presence or absence of forest edge and the corridor (Hopland, California. 1997)

Figure 7. Spatial extent of forest's influence on natural enemy colonization and abundance (A) and spatial extent of corridor's influence on natural enemy colonization and abundance (B)