Basic principles of agroecology a...
Basic Principles of Agroecology and Sustainable Agriculture V.G. THOMAS Department of Zoology College of Biological Science University of Guelph, Guelph, Ontario, Canada N1G 2Wl P.G. KEVAN Department of Environmental Biology Ontario Agricultural College University of Guelph Guelph, Ontario, Canada N1G 2Wl Abstract In the final analysis, sustainable agriculture must derive from applied ecology, especially the principle of the regulation of the abun- dance and distribution of species (and, secondarily, their activities) in space and time. Interspecific competition in natural ecosystems has its counterparts in agriculture, designed to divert greater amounts of energy, nutrients, and water into crops. Whereas natural ecosystems select for a diversity of species in communities, recent agriculture has minimized diversity in favour of vulnerable monocultures. Such systems show intrin- sically less stability and resilience to perturbations. Some kinds of crop rotation resemble ecological succession in that one crop prepares the land for successive crop production. Such rotations enhance soil organic processes such as decomposition and material cycling, build a nutrient capital to sustain later crop growth, and reduce the intensity of pest build- up. Species in natural communities occur at discrete points along the r - K continuum of reproductive maturity. Clearing forested land for agriculture, rotational burning practices, and replacing perennial grass- land communities by cereal monocultures moves the agricultural com- munity towards the r extreme. Plant breeders select for varieties which yield at an earlier age and lowerplant biomass, effectively moving a var- iety towards the r type. Features of more natural landscapes, such as hedgerows, may act as physical and biological adjuncts to agricultural production. They should exist as networks in agricultural lands to be most effective. Soil is of major importance in agroecosystems, and main- taining, deliberately, its vitality and resilience to agricultural perturba- tions is the very basis of sustainable land use. Keywords: agroecosystems, agriculture, ecology, sustainabihty, bio- diversity, competition, succession, culture. Journal of Agricultural and Environmental Ethics 1993
2 V.G. Thomas and P.G. Kevan Introduction Agroecosystems are the most intensively managed ecosystems known, comprising 30% of the Earth's land area and encompassing the most productive soils (Coleman and Hendrix, 1988). All forms of agriculture can be examined through applied ecology, i.e., the application of knowledge about organisms and their environment (Paul and Robertson, 1989). Thus the terms "agroecology" and "ecological agricul- ture" are partly misnomers. Their modern origin and ever-more frequent use derive as much from the growing environmental concerns of agriculturalists as from the realization that intensive, high chemical and energy input agriculture is neither always full of promise nor of profit (National Research Council, 1989, p. 90-93). The term "sustainable agriculture", likewise, has spawned numerous problematic definitions, perhaps because there appears to be no consensus on what constitutes unsustainable agriculture, practices which are argued pro and con to destroy the resource bases of the land, consume beyond replacement, and may have impacts which defy recovery. The global, historical, record shows that many forms of agricul- ture have rarely achieved sustainability. Over millennia, civilizations in different parts of the world have fallen because of unviable agricultural practices (Dale and Carter, 1955 Hyams, 1976 Agnew and Warren, 1990), and the same failures of agriculture are still occurring today (Postel, 1989). The quest for ecologically and economically sustainable agriculture remains a quest for the future and not just a reversion to old practices of animal husbandry and land management. Nevertheless, Conway (1985, 1987) has presented an approach in which sustaina- bility implies that the system is highly-productive, intrinsically viable, and can readjust from the perturbations caused, for example, by the growing of cereal crops or any other agricultural commodity (Helling 1986). Agriculture, according to that approach, then involves two simultaneous practices: one of producing a harvestable commodity and the other of maintaining the robust vitality of the soil and other agricultural land communities. Long-term sustainable agriculture is not the max- imizing of commodity production per hectare. Nor are agricultural practices deemed to be sustainable according to simple economic criteria alone. Sustainable agricul- ture implies land practices which operate at lower levels of purchased inputs than does intensified energy and chemically-dependent agriculture, and which embraces agroecology by putting a wide suite of natural processes to work directly and indirectly in commodity production, rather than paying to eclipse all but a few such processes (Altieri, 1987 Edwards, 1987 National Research Council, 1989, p. 3-23). Realizing this goal implies an intensification of human management effort by producers. Awareness of these ideas is not new and can be traced back to the classical and insightful writings of a variety of nineteenth and twentieth century authors (see Harwood (1990) for a review of this subject). Agriculture is generally sensitive to, and constrained by ecological principles, principles which are basic to sustainable agriculture (Odum, 1983). However, the succinct exposde of those principles and how they relate directly to agriculture often appear to have been overlooked (see, however, Gliessman (1989)), perhaps because it is assumed (erroneously) that such principles are widely known and accepted.
Agroecology and Sustainable Agriculture 3 This article attempts to overcome this deficiency by stating the principles of ecology, explaining how they permeate all forms of agriculture, and shows what should be recognized, or changed, if agriculture is to become sustainable in an ecological context. The Unifying Principle of Ecology In any ecosystem individuals of a species exist in one or more populations and the different species which interact biologically form an ecological community. Bio- diversity refers to all the species existing in an ecosystem, such as an agroecosystem (Pimentel et al., 1992). The number of species in an ecosystem defines the species richness. Species which occur ubiquitously throughout habitats have a higher equitability of distribution, or evenness, than species which occur sporadically, or in just a few habitat types. The two terms, "richness of species" and "evenness of distribution", define collectively the term "ecological diversity" (May, 1975 Pianka, 1983). Thus high diversity ecosystems have more species occurring more frequently among the habitats than ecosystems of low diversity, comprising fewer species clumped in fewer habitat types. This theory provides the basis of a fun- damental principle, namely that species in natural ecosystems are regulated in terms of their abundance (how many individuals) and distribution (where they occur) (Andrewartha and Birch, 1954). Necessarily, this is manifest through the activi- ties of organisms, including reproduction and dispersal. The studies of the mechanisms of regulation of distribution, abundance, and activi- ties are the domain of ecology. Thus applied, ecology runs the gamut from coloni- zation by, and the extinction of, organisms to the creation of monocultures and the conservation of natural communities. The distribution and abundance of a spe- cies is influenced by a group of processes that may act directly or indirectly on individual organisms (Harper, 1977, Chapter 10 Krebs, 1985). The abundance, the distribution, or both may be affected by any given process. Fundamental Processes and Components in Ecosystems Tolerances refer to the limits for environmental factors (living or non-living) which determine whether or not an organism can exist and reproduce in a given area. These are, in approximate order of importance, temperature, moisture, light con- ditions, chemical nature of the substrate, and the influence of other organisms (Harper, 1977). 1. Maintenance Processes Nutrient cycling of macronutrients (e.g., phosphates, nitrates, and potassium) and micronutrients encompasses the solubilization, uptake, retention, return to soil, and inevitable losses from the local system, both to drainage waters and atmosphere (Smith, 1974). It has both spatial and temporal dimensions. The water cycle is a fundamental regulator of plant growth and productivity in ecosystems, and is also implicated in determining climate, especially in tropical regions (Rosenzweig, 1968