In a trivial sense the planned elements of an agroecosystem are all ecological engineers. The farmer, being the ultimate engineer (but not the focus of this chapter) decides to transform the original ecosystem, but the crops he or she chooses inevitably have a major effect on the environment that is constructed. And frequently those effects are not directly trophic, so may be considered as ecological engineering. Perhaps the first acknowledgement of these effects was in the mid-nineteenth century by Liebig, in recognizing that some crops had different nutrient requirements than others and that the nutrient mix in the soil would be at least partially determined by the type of crop planted. It is likely that European farmers generations before him understood this principle, which is what led them to their various complicated rotational systems. However, it was Liebig who elaborated the idea in what might be the first formal scientific framework of ecological engineering.
A half century later, Albert Howard, an Englishman working in India, devised his famous Indore composting system, which was far more than simply a composting system. Howard's approach, which we can presume came from extensive observations of and conversations with Indian farmers, was focused on the health of the soil, arguing that a "healthy" soil, by which he meant one that contained a well-balanced mixture of worms, fungi, and bacteria and other microorganisms, would produce healthy food, while a soil devoid of those healthy elements would not produce such healthy food. Indeed, the connection between ecological health on the farm and the health-promoting qualities of food produced there was a key element of the early organic agriculture movement (Conford 2001). Howard thus noticed the immense effect of ecological engineers (i.e., worms, fungi, and bacteria and other microorganisms in the soil) in creating what he referred to as a healthy soil.
Modern soil science completely accepts the basic ideas of Howard (1940), acknowledging that ecosystem engineers operate in a variety of ways to maintain (or denigrate) soil fertility. For example, the practice of composting (Epstein 1997, Dreyfus 1990) is clearly an example of eco system engineering. Organic matter, a vital component of both physical and chemical processes in soils, is dramatically altered by the addition of compost, which is created by active treatment of organic residues with decomposing engineers (Insam et al. 1996, 2002). And the addition of worms as the major engineer has become commonplace, with vermi-culture sometimes taking center stage in the production of compost (Frederickson et al. 1997, Berc et al. 2004).
In more conventional farming practices, the mechanical and chemical manipulation of soils is overwhelmingly dominant in forming the ecological base on which soil dynamics occur, and if we were to include the direct effects of Homo sapiens in this review, it would take center stage. However, in the spirit of nonhuman actors as engineers, we leave that topic to the already extensive literature easily accessible to the interested reader (Johnsen et al. 2001, Vandermeer 1995, Altieri 2000). Otherwise, excluding composting and related activities, most engineering effects on the soil are only indirect effects of the planned components, and we thus defer further discussion of them to the section on associated effects.
Crop rotations are ubiquitous in traditional farming systems, from the famous Norfolk rotation of industrializing England to corn-soybean rotations in the Midwestern United States today. Each crop in the cycle engineers some aspect of the environment for the other crops. Sometimes the critical factor is to eliminate a pest or pathogen that has built up (Altieri 1999, Liebman and Davis 2000, Johnson et al. 2001), other times to utilize a different mixture of nutrients from one year to the next (Douglas et al. 1998, Riedell et al. 1998). In either case, the crop in the one rotation clearly engineers part of the environment for the crop in subsequent rotations.
Intercropping systems are common in traditional tropical farming systems (Vandermeer 1989, 1995). Using the effect-response framework, it is easy to see how each of the crops may facilitate some aspect of environmental improvement for the other crops (Vandermeer 1984, Li 2003). The earlier example of corn and beans is only one in a large collection of examples that could be cited.
In many tropical agricultural systems trees are an integral part of a system that includes annual or semi-annual crops, in a system commonly referred to as agroforestry. Agroforestry systems must be regarded as just as paradigmatic for ecosystem engineering as trees in the forest. The question "What does a tree do in a forest?", the answer to which is thought to be a quintessential example of ecological engineering (Jones et al. 1997), applies equally to agroforestry systems. The literature on the effects of trees on the physical structure of microhabitats is huge as a glance at almost any issue of the journal Agroforestry Systems will attest.
Trees in agroforestry systems create windbreaks (Sturrock 1988; Bird 1991, 1998; Mayus et al. 1998), keep the understory cool (Rao et al. 1997, Rhoades 1996, Campanha et al. 2004), provision organic matter and nutrients (Palm 1995, Fassbender et al. 1991, Chander et al. 1998), provide refuge for natural enemies (Dix et al. 1995, Stamps and Linit 1998), disrupt the ability of pests to find crops (Altieri and Nicholls 2003, Nich-olls and Altieri 2004), sequester carbon (Montagnini and Nair 2004), and likely produce other effects that have escaped our attention. Add to this any general engineering effect already attributed to trees in unmanaged systems (e.g., the paradigmatic example of Jones et al. 1997), and agro-forestry emerges as a seemingly paradigmatic case of ecosystem engineering in general (Garcia-Barrios and Ong 2004).
In the modern industrial agricultural system, the planned ecosystem engineers have become less important as many of their functional effects have been taken over by the direct engineering activities of that one key species, Homo sapiens. Thus, for example, the important engineering activity of provisioning nitrogen, traditionally the job of legume engineers, has been replaced by the engineer located in the nitrate-manufacturing plant. The engineering activity of attracting parasitoids to control key pests has been taken over by the engineer located in the pesticide-manufacturing plant. Since we have chosen to ignore the direct effects of Homo sapiens, for the most part the conventional industrial system thus falls outside of the intended scope of this chapter.
Nevertheless, it is worth mentioning the overwhelming fact, that enormous elephant in the living room, that the human engineers in this case have had some major negative effects, not only on the practice of agriculture, but also on unintended environmental and health consequences (Raynolds 2004, Badgeley et al. 2007, Conway and Pretty 1991, Allen 1993, Lappé et al. 1998).
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