Designing species-rich, pest-suppressive agroecosystems through habitat management

Clara I. Nicholls and Miguel A. Altieri

Division of Insect Biology
University of California, Berkeley

Ninety-one percent of the 1.5 billion hectares of cropland worldideare under annual crops, mostly monocultures of wheat, rice, maize, cotton, and soybeans (Smil 2000). This process represents an extreme form of simplification of nature’s biodiversity. Monocultures in addition to being genetically uniform and species-poor systems, advance at the expense of natural vegetation, a key landscape component that provides important ecological services to agriculture such as natural mechanisms of crop protection (Altieri 1999). Since the onset of agricultural modernization, farmers and researchers have been faced with a major ecological dilemma arising from the homogenization of agricultural systems: an increased vulnerability of crops to insect pests and diseases, which can be devastating when infesting uniform-crop, large-scale monocultures (Adams et al l971, Altieri and Letourneau l982/l984). Monocultures may have temporary economic advantages for farmers, but in the long term they do not represent an ecological optimum. Rather, the drastic narrowing of cultivated plant diversity has put the world’s food production in greater peril (NAS l972, Robinson 1996).

In this chapter, we explore practical steps to break the non-diverse nature of monocultures and thus reduce their ecological vulnerability, by restoring agricultural biodiversity at the field and landscape level. The most obvious advantage of diversification is a reduced risk of total crop failure due to invasions by unwanted species and subsequent pest infestations (Altieri l994). The chapter focuses on ways in which biodiversity can contribute to the design of pest-stable agroecosystems by creating an appropriate ecological infrastructure within and around cropping systems. Selected studies reporting the effects of intercropping, cover cropping, weed management, agroforestry and manipulation of crop-field border vegetation are discussed, special attention to understanding the mechanisms underlying pest reduction in with diversified agroecosystems. This reflection is fundamental if habitat management through vegetation diversification is to be used effectively as the basis of Ecologically Based Pest Management (EBPM) tactics in sustainable agriculture.

 

Biodiversity in agroecosystems: types and roles

Biodiversity refers to all species of plants, animals, and microorganisms existing and interacting within an ecosystem, and which play important ecological functions such as pollination, organic matter decomposition, predation or parasitism of undesirable organisms and detoxification of noxious chemicals (Gliessman l998). These renewal processes and ecosystem services are largely biological; therefore their persistence depends upon maintenance of ecological diversity and integrity. When these natural services are lost due to biological simplification, the economic and environmental costs can be quite significant. Economically, in agriculture the burdens include the need to supply crops with costly external inputs, since agroecosystems deprived of basic regulating functional components lack the capacity to sponsor their own soil fertility and pest regulation. Often the costs also involve a reduction in the quality of life of rural communities due to decreased soil, water, and food quality when pesticide, nitrate or other type of contamination linked to industrial agriculture occurs (Conway and Pretty l991).

Biodiversity in agroecosystems can be as varied as many crops, weeds, arthropods, or microorganisms involved, according to geographical location, climatic, edaphic, human, and socioeconomic factors. In general the degree of biodiversity in agroecosystems depends on several features of the agroecosystem. Higher levels of biodiversity are expected in systems that (Altieri 1994):

The biodiversity components of agroecosystems can be classified in relation to the role they play in the functioning of cropping systems. According to this, agricultural biodiversity can be grouped as follows (Altieri l994, Gliessman l998):

The above categories of biodiversity can further be recognized as two distinct components (Vandermeer and Perfecto 1995). The first component, planned biodiversity, includes the crops and livestock purposely included in the agroecosystem by the farmer, which will vary depending on the management inputs and crop spatial/temporal arrangements. The second component, associated biodiversity, includes all soil flora and fauna, herbivores, carnivores and decomposers that colonize the agroecosystem from surrounding environments and that will thrive in the agroecosystem depending on its management and structure. The relationship of both types of biodiversity components is illustrated in Figure 1. Planned biodiversity has a direct function, as illustrated by its connection with the ecosystem function box. Associated biodiversity also has a function, but it is mediated through planned biodiversity. Thus, planned biodiversity also has an indirect function, illustrated by the dotted arrow in the figure, which is realized through its influence on the associated biodiversity. For example, the trees in an agroforestry system create shade, which makes it possible to grow only sun intolerant crops. So, the direct function of this second species (the trees) is to create shade. Yet along with the trees might come wasps that seek out the nectar in the tree’s flowers. These wasps may in turn be the natural parasitoids of pests that normally attack understory crops.The wasps are part of the associated biodiversity. The trees then create shade (direct function) and attract wasps (indirect function) (Vandermeer and Perfecto 1995).

The optimal behavior of agroecosystems depends on the level of interactions among 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, and all important components that determine the sustainability of agroecosystems (Altieri and Nicholls 2000).

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. There are many agricultural practices and designs that have the potential to enhance functional biodiversity, and others that affect it negatively (Figure 2). The idea is to apply the best management practices in order to enhance or regenerate the kind of biodiversity that can best subsidize the sustainability of agroecosystems by providing ecological services such as biological pest control, nutrient cycling, water and soil conservation. The role of agroecologists should be to encourage those agricultural practices that increase the abundance and diversity of above- and below-ground organisms, which in turn provide key ecological services to agroecosystems.

Thus, a key strategy of EBPM should be to exploit the complementarity and synergy that result from the various combinations of crops, trees, and animals in agroecosystems that feature spatial and temporal arrangements such as polycultures, agroforestry systems and crop-livestock mixtures. In real situations, the exploitation of these interactions involves agroecosystem design and requires an understanding of the numerous relationships among soils, microorganisms, plants, insect herbivores, and natural enemies to guide proper management.

 

Linking biodiversity and agroecosystem stability

In general, natural ecosystems appear to be more stable and less subject to fluctuations in populations of the organisms making up the community compared to cultivated systems. Ecosystems with higher diversity are more stable because they exhibit:

    1. Higher resistance, or the ability to avoid or withstand disturbance, and

    2. Higher resilience, or the ability to recover following disturbance.

The community of organisms becomes more complex when a larger number of different kinds of organisms are included, when there are more interactions among organisms, and when the strength of these interactions increases. As diversity increases, so do opportunities for coexistence and beneficial interference between species that can enhance agroecosystem sustainability (van Emden and Williams 1974). Diverse systems encourage complex food webs which entail more potential connections and interactions among members, and many alternative paths of energy and material flow through it. For this and other reasons a more complex community exhibits more stable production and less fluctuations in the numbers of undesirable organisms (Power 1999).

Recent studies conducted in grassland systems suggest that there are no simple links between species diversity and ecosystemic stability. What is apparent is that functional characteristics of component species are as important as the total number of species. The experiments on grassland plots suggest that functionally different roles represented by plants are at least as important as the total number of species in determining processes and services in ecosystems (Tilman et al. 1997).

This latest finding has practical implications for agroecosystem management. If it is easier to mimic specific ecosystem processes rather than duplicate all the complexity of nature, then the focus should be placed on incorporating a specific biodiversity component that plays a specific role, such as a plant that fixes nitrogen, provides cover for soil protection or harbors resources for natural enemies. In the case of farmers without major economic and resource limitations and who can afford a certain risk of crop failure, a crop rotation or a simple crop association may be all it takes to achieve a desired level of stability. But in the case of resource-poor farmers, where crop failure is intolerable, highly diverse polyculture systems would probably be the best choice. The obvious reason is that the benefit of complex agroecosystems is low risk; if a species falls to disease, pest attack or weather, another species is available to fill the void and maintain full use of resources. Thus there are potential ecological benefits to having several species in an agroecosystem: compensatory growth, full use of resources and nutrients, and pest protection (Ewel l999).

 

Plant diversity and insect pest regulation

Throughout the years, many ecologists have conducted experiments testing the hypothesis that decreased plant diversity in agroecosystems that allows greater chance for invasive species to colonize, subsequently leads to enhanced herbivorous insect pest abundance. Many of these experiments have shown that mixing certain species with the primary host of a specialized herbivore gives a fairly consistent result: specialized species usually exhibit higher abundance in monoculture than in diversified crop systems (Andow 1983).

Several reviews have been published documenting the effects of within-habitat diversity on insects (Altieri and Letourneau 1984; Risch et al. 1983). Two main ecological hypotheses (natural enemy hypothesis and the resource concentration hypothesis), have been offered to explain why insect communities in agroecosystems can be stabilized by constructing vegetational architectures that support natural enemies and/or directly inhibit pest attack. The literature is full of examples of experiments documenting that diversification of cropping systems often leads to reduced herbivore populations. In the review by Risch et al., (1983) 150 published studies of the effect of diversifying an agroecosystem on insect pest abundance were summarized; 198 total herbivore species were examined in these studies. Fifty-three percent of these species were found to be less abundant in the more diversified system, 18% were more abundant in the diversified system, 9% showed no difference, and 20% showed a variable response. In another analysis of 50 studies, it was concluded that monophagous (specialist) insects are more susceptible to crop diversity then polyphagous insects. The author cautions about the increased risk of pest attack if the dominant herbivore fauna in a given agroecosystem is polyphagous (Andow 1991). The reduction in pest numbers was for monophagous insects almost twice (53.5% of the case studies showed lowered numbers in polycultures) than for polyphagous insects (33.3% of the cases).

Both empirical data and theoretical arguments suggest that differences in pest abundance between diverse and simple annual cropping systems can be explained by the movement and reproductive behavioral responses of herbivores when confronted with plant diversity and in many cases by mortality imposed by natural enemies. Of thirty five insect pest species investigated in one study, the majority in the orders Lepidoptera, Coleoptera and Homoptera, natural enemy action accounted for 30% of the control mechanisms of the various pests, and the remaining species were controlled by a variety of factors including lowered resource concentration, trap-cropping, diversionary mechanisms and plant physical obstruction (Barbosa l998).

Recent reviews concerned with agroecology, habitat management and conservation biological control overwhelmingly state that higher pest losses should be expected in more vulnerable ecosystems, usually mechanized, large-scale monocultures (Altieri 1994, Barbosa 1998, Pickett and Bugg 1998). Such systems represent highly disturbed systems exhibiting ecological conditions that may be more susceptible to colonization by invasive species. Herbivores with a narrower host range are more likely to colonize crops grown in pure stands and thus attain pest states in simplified agroecosystems (Smith and McSorely 2000). Moreover, as a result of frequent and intense disturbance regimes, monocultures are difficult environments for natural enemies to colonize and survive in, thus predators and parasitoids reach low abundance levels and exhibit poor effectiveness in such systems. The ubiquity of pesticide use negatively impacts natural enemies and high rates of synthetic chemical fertilizer may render crops more susceptible to pests. Effects of transgenic crops on non-target organisms will not be as localized or as transient as initially anticipated. Rather, studies suggest that the effects of transgenic crops might spread via wind, via trophic webs and persist in the soil, in many cases compounding pest problems (Marvier 2001).

 

Designing biodiverse pest-suppressive agroecosystems

Monoculture agriculture is a futile attempt to impose agronomic simplicity on ecosystems that are inherently complex and possess high biotic intricacy. Accordingly, many agroecologists have proposed that a better land use management strategy is to imitate the structure and function of the natural communities of each region (Ewel l986). Successional communities offer several traits of potential value to agriculture ( Soule and Piper 1992, Ewel l999):

A strategy to bring such benefits to agricultural systems is to use successional ecosystems as templates for the design of agroecosystems, a strategy that has been used for centuries by traditional tropical small farmers in the design of polycultures, agroforestry and complex home gardens. In modern agricultural systems, the same strategy can be utilized following key ecological guidelines:

Recent case studies confirm that adoption of some form of diversification following the key agroecological principles outlined above can lead to enhanced pest regulation. Many of these studies have transcended the research phase and have found wide applicability to regulate specific pests:

  1. Researchers working with farmers in ten townships in Yumman, China, covering an area of 5350 hectares, encouraged farmers to switch from rice monocultures to planting variety mixtures of local rice with hybrids. Enhanced genetic diversity reduced blast incidence by 94% and increased total yields by 89%. By the end of two years, it was concluded that fungicides were no longer required (Zhu et al. 2000, Wolfe 2000).

  2. In Africa, scientists at ICIPE developed a habitat management system to control stemborers and striga, which uses two kinds of crops that are planted together with maize: a plant that repels borers (the push) and another that attracts (pulls) them acting as a trap crop (Khan et al. 1998). The push-pull system has been tested on over 450 farms in two districts of Kenya and has now been released for uptake by the national extension systems in East Africa. Participating farmers in the breadbasket of Trans Nzoia are reporting a 15-20 percent increase in maize yield. In the semi-arid Suba district – plagued by both stemborers and striga – a substantial increase in milk yield has occurred in the last four years, with farmers now able to support grade cows on the fodder produced. When farmers plant maize, napier and desmodium together, a return of US$ 2.30 for every dollar invested is made, as compared to only $1.40 obtained by planting maize as a monocrop. Two of the most useful trap crops that pull in the borers’ natural enemies are napier grass (Pennisetum purpureum) and Sudan grass (Sorghum vulgare sudanese), both important fodder plants; these are planted in a border around the maize. Two excellent borer-repelling crops which are planted between the rows of maize are molasses grass (Melinis minutifolia), which also repels ticks, and the leguminous silverleaf (Desmodium). This plant can also suppress the parasitic weed Striga by a factor of 40 compared to maize monocrops; its N-fixing ability increases soil fertility; and it is an excellent forage. As an added bonus, sale of Desmodium seed is proving to be a new income-generating opportunity for women in the project areas.

  3. Several researchers have introduced flowering plants as strips within crops as a way to enhance the availability of pollen and nectar, necessary for optimal reproduction, fecundity and longevity of many natural enemies of pests. Phacelia tanacetifolia strips have been used in wheat, sugar beets and cabbage leading to enhanced abundance of aphidophagous predators especially syrphid flies, and reduced aphid populations. In England in an attempt to provide suitable overwintering habitat within fields for predators of cereal aphids, researchers created "beetle banks" sown with perennial grasses such as Dactylis glomerata and Holcus lanatus. When these banks run parallel with the crop rows, great enhancement of predators (up to 1500 beetles per square meter) can be achieved in only two years (Landis et al 2000).

  4. In perennial cropping systems the presence of flowering undergrowth enhances the biological control of a series of insect pests. The beneficial insectary role of Phacelia in apple orchards was well demonstrated by Russian and Canadian researchers more than 30 years ago (Altieri 1994). In California organic vineyards, the incorporation of flowering summer cover crops ( buckwheat and sunflower) led to enhanced populations of natural enemies which in turn significantly reduced the numbers of leafhoppers and thrips (Nicholls et al 2000).

  5. In Washington State, researchers reported that organic apple orchards managed with lower inputs and that retained some level of plant diversity in the form of weeds mowed as needed, gave similar apple yields to conventional and integrated orchards. Their data showed that the organic system ranked first in environmental and economic sustainability as this system exhibited higher profitability, greater energy efficiency and lower negative environmental impact (Reganold et al 2001).

  6. In Central America, Staver et al (2001) designed pest-suppressive multistrata shade-grown coffee systems, selecting tree species and associations, density and spatial arrangement as well as shade management regimes with the main goal of creating optimum shade conditions for pest suppression. For example, in low-elevation coffee zones, 35-65% shade promotes leaf retention in the dry seasons and reduces the pathogen Cercospora coffeicola, weeds and the insect Planococcus citri; at the same time, it enhances the effectiveness of microbial and parasitic organisms without contributing to increased leafminer (Hemileia vastatrix) levels or reducing yields.

  7. Several entomologists have concluded that the abundance and diversity of predators and parasites within a field are closely related to the nature of the vegetation in the field margins. There is wide acceptance of the importance of field margins as reservoirs of the natural enemies of crop pests, although depending on plant composition certain hedgerows may also harbor pests. Many studies have demonstrated increased abundance of natural enemies and more effective biological control where crops are bordered by wild vegetation from which natural enemies colonize. Parasitism of the armyworm, Pseudaletia unipunctata, was significantly higher in maize fields embedded in a complex landscape than in maize fields surrounded by simpler habitats. In a two-year study researchers found higher parasitism oflarvae of the lepidopteran pest, Ostrinia nubilalis by the parasitoid Eriborus terebrans in edges of maize fields adjacent to wooded areas, than in field interiors (Landis et al 2000). Similarly in Germany, parasitism of rape pollen beetle was about 50% at the edge of the fields, while at the center of the fields parasitism dropped significantly to 20% (Thies and Tscharntke 1999).

  8. One way to introduce the beneficial biodiversity from surrounding landscapes into large-scale monocultures is by establishing vegetationally diverse corridors that allow the movement and distribution of useful arthropod biodiversity into the center of monocultures. Nicholls et al (2001) established a vegetational corridor which connected to a riparian forest and cut across a vineyard monoculture in northern California. The corridor allowed natural enemies emerging from the riparian forest to disperse over large areas of otherwise monoculture vineyard systems. The corridor provided a constant supply of alternative food for predators effectively decoupling predators from a strict dependence on grape herbivores and avoiding a delayed colonization of the vineyard. This complex of predators continuously circulated into the vineyard interstices establishing a set of trophic interactions leading to a natural enemy enrichment, which led to lower numbers of leafhoppers and thrips on vines located up to 30-40 m from the corridor.

All of the above examples constitute forms of habitat diversification that provide resources and environmental conditions suitable for natural enemies. The challenge is to identify the type of biodiversity that is desirable to maintain and/or enhance in order to carry out ecological services of pest control, and then to determine the best practices that will encourage such desired biodiversity components. A few guidelines need to be considered when implementing habitat management strategies (Landis et al 2000):

    1. Selection of the most appropriate plant species

    2. The spatial and temporal arrangement of such plants, within and/or around the fields

    3. The spatial scale over which the habitat enhancement operates at the field or landscape level

    4. The predator/parasitoid behavioral mechanisms which are influenced by the habitat manipulation

    5. Potential conflicts that may emerge when adding new plants to the agroecosystem (i.e. in California, Rubus blackberries around vineyards increase populations of Anagrus wasps, a parasitoid of the grape leafhopper, but can also enhance abundance of the sharpshooter which serves as a vector of Pierce’s disease).

    6. Develop ways in which added plants do not upset other agronomic management practices, and select plants that preferentially have multiple-effects such as improving pest regulation but at the same time improve soil fertility and weed suppression.

 

Conclusions

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 1994). 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. 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 restoring the shattered elements of the community homeostasis through the addition or enhancement of biodiversity (Gliessman 1999, Altieri 1999) can repair this breakdown.

A key strategy in sustainable agriculture is to reincorporate diversity into the agricultural landscape through various cropping designs. Emergent ecological properties develop in diversified agroecosystems that allow the system to function in ways that maintain soil fertility, crop production, and pest regulation.The main approach in ecologically based pest management is to use management methods that increase agroecosystem diversity and complexity as a foundation for establishing beneficial interactions that keep pest populations in check (Altieri and Nicholls 2000). This is particularly important in underdeveloped countries where sophisticated inputs are either not available or may not be economically or environmentally advisable, especially in the case of resource -poor farmers.

As argued in this chapter, agroecosystems that mimic the structure and functional complexity of nature confer an important degree of pest protection. However, diverse and complex agroecosystems are hard to manage and their implementation may run counter to current economic forces that promote farm specialization. Nevertheless, new agroecosystems are urgently needed worldwide in an era of deteriorating environmental quality, biodiversity reduction, heavy reliance on non-renewable resources and escalating input costs. This approach to agriculture will only be practical if it is economically sensible and can be carried out within the constraints of a fairly normal agricultural management system. However, given the trend toward large-scale, monoculture production units throughout the world (USDA l973), objectively there is not much room left for a fair implementation of a regional insect-habitat management program. Emerging biotechnological approaches such as transgenic crops deployed in more than 40 million hectares in the year 2000 are leading agriculture towards further specialization, and the potential effects of transgenic crops on non-target beneficial organisms is of concern to biological control practitioners (Rissller and Mellon 1996, Altieri 2000, Marvier 2001). Regardless, habitat management may not always demand a radical change in farming as illustrated by the relative ease with which beetle banks, flowering strips or corridors can be introduced into cropping systems, and thus bringing biological control benefits to farmers (Landis et al 2000).

When properly implemented, habitat management leads to establishment of the desired type of plant biodiversity and the ecological infrastructure necessary for attaining optimal natural enemy diversity and abundance. Such diversity may not always warrant total pest regulation, therefore at times the action of such enemies might have to be complemented using augmentative releases of predators or parasites and/or application of entomopathogens. This may be especially true in the initial stages of the conversion from conventionally managed systems to agroecological management (Vandermeer, 1995 Landis et al., 2000).

Long-term maintenance of diversity requires a management strategy that considers regional biogeography and landscape patterns, as well as design of environmentally sound agroecosystems above purely economic concerns. This is why several authors have repeatedly questioned whether the pest problems of modern agriculture can be ecologically alleviated within the context of the present capital-intensive structure of agriculture. Many problems of modern agriculture are rooted within that structure and thus require the consideration of major social change, land reform, redesign of machinery, research, and extension reorientation in the agricultural sector to increase the possibilities of improved pest control through vegetation management. Whether the potential and spread of ecologically based pest management is realized will depend on policies, attitude changes on the part of researchers and policy makers, existence of markets for organic produce, and also the organization of farmer and consumer movements that demand a more healthy and viable agriculture and food system.

 

References

Adams, M.W., A.H. Ellingbae and E.C. Rossineau. 1971. Biological uniformity and disease epidemics. BioScience 21: 1067-1070.

Altieri, M.A. and D.K. Letourneau. 1982. Vegetation management and biological control in agroecosystems. Crop Protection. 1: 405-430.

Altieri, M.A. and D.K. Letourneau. 1984. Vegetation diversity and outbreaks of insect pests. CRC, Critical Reviews in Plant Sciences. 2:131-169.

Altieri, M.A. 1994. Biodiversity and pest management in agroecosystems. Haworth Press, New York.

Altieri, M.A. 1999. The ecological role of biodiversity in agroecosystems. Agric. Ecosyst. Env. 74:19-31.

Altieri, M.A. 2000. The ecological impacts of transgenic crops on agroecosystem health. Ecosystem Health. 6:13-23.

Altieri, M.A. and C.I. Nicholls. 2000. Applying agroecological concepts to development of ecologically based pest management systems. In: Proc of a Workshop "Professional societies and ecological based pest management systems." Pp. 14-19. National Research Council. Washington D.C.

Andow, D.A. 1991 Vegetational diversity and arthropod population response. Ann.Rev. Entomol. 36: 561-586.

Barbosa, P. 1998. Conservation biological control. Academic Press, San Diego. 396 pp.

Conway, G.R. and J.N. Pretty l99l. Unwelcome harvest: agriculture and pollution. Earthscan Pub., London.

Ewel, J.J. 1986. Designing agricultural ecosystems for the humid tropics. Amm. Rev. Ecol. Syst. 17: 245-71.

Ewel, J.J. 1999. Natural systems as models for the design of sustainable systems of land use. Agroforestry Systems. 45: 1-21.

Gliessman, S.R. 1999. Agroecology: ecological processes in agriculture. Ann Arbor Press, Michigan.

Khan, Z.R., J.A. Pickett, J. van der Berg and C.M. Woodcock. 2000. Exploiting chemical ecology and species diversity: stemborer and Striga control for maize in Africa. Pest Management Science. 56: 1-6.

Landis, D.A., S.D. Wratten and G.A. Gurr 2000. Habitat management to conserve natural enemies of arthropod pests in agriculture. Annual Review of Entemology. 45: 175-201.

Marvier, M . 2001. Ecology of transgenic crops. American Scientist 89: 160-167. National Academy of Sciences. 1972. Genetic vulnerability of major crops. NAS, Washington, DC.

Nicholls, C.I, M.P. Parrella and M.A. Altieri 2000. Reducing the abundance of leafhoppers and thrips in a northern California organic vineyard through maintenance of full season floral diversity with summer cover crops. Agricultural and Forest Entomology 2: 107-113

Nicholls, C.I., M.P. Parrella and M.A. Altieri 2001 The effects of a vegetational corridor on the abundance and dispersal of insect biodiversity within a northern California organic vineyard. Landscape ecology 16: 133-146.

Pickett, C.H. and R.L. Bugg. 1998. Enhancing biological control: habitat management to promote natural enemies of agricultural pests. University of California Press, Berkeley. 422 pp.

Power, A.G. 1999 Linking ecological sustainability and world food needs. Environment, Development and Sustainability 1: 185-196.

Reganold, J.P., J.D. Glover, P.K. Andrews and H.R. Hinman 2001 Sustainability of three apple production systems. Nature 410: 926-930.

Risch, S.J., D. Andow, and M.A. Altieri. 1983. Agroecosystem diversity and pest control: data, tentative conclusions, and new research directions. Environmental Entomology. 12:625-629.

Rissler, J. and M. Mellon. 1996. The ecological risks of engineered crops. MIT Press, Cambridge.

Robinson, R.A. 1996. Return to resistance: breeding crops to reduce pesticide resistance. Ag. Access, Davis, CA.

Smith, H.A. and R. McSorley 2000. Intercropping and pest management: a review of major concepts. American Entemologist. 46: 154-161.

Soule, J.D. and J.K. Piper 1992 Farming in nature’s image. Island Press, Washington D.C.

Staver, C., F. Guharay, D. Monterroso and R.G. Muschler 2001 Designing pest-suppressive multistrata perennial crop systems: shade grown cod=ffe in Central America. Agroforestry Systems (in press).

Thies, C. and T. Tscharntke. 1999. Landscape structure and biological control in agroecosystems. Science. 285:893-895.

U.S. Department of Agriculture (USDA). 1973. Monocultures in agriculture, causes and problems. Report of the Task Force on Spatial Heterogeneity in agricultural landscapes and enterprises. U.S. Government Printing Office, Washington, DC.

Van Driesche, R.G. and T.S. Bellows, Jr. 1996. Biological Control. Chapman and Hall. New York.

Vandermeer, J. 1995 The ecological basis of alternative agriculture. Ann. Rev. Ecol. Syst. 26: 210-224.

Vandermeer, J and I. Perefecto. l995 Breakfast of biodiversity. Food First Books, Oakland, California.

VanEmden, H.F. and G.F. Williams. 1974. Insect stability and diversity in agroecosystems. Annual Review Entemology. 19: 455-75.

Wolfe, M. 2000. Crop strength through diversity. Nature. 406: 681-682.

Zhu, Y., H. Fen, Y. Wang, Y. Li, J. Chen, L.Hu and C.C. Mundt. 3000. Genetic diversity and disease control in rice. Nature. 406: 718-772.

 

 

 

Agroecosystem

Management

 

 

Planned

Biodiversity

Promotes

 

 

 

Create conditions Ecosystem Function

that promotes e.g., pest regulation,

nutrient cycling, etc

 

Associated

Biodiversity Promotes

 

Biodiversity of Surrounding

Environment

 

 

 

 

Figure 1. The relationship between planned biodiversity (that which the farmer detrmines, based on management of the agroecosystem) and associated biodiversity and how the two promote ecosystem function. (Modified from Vandermeer and Perfecto, 1995)