As will be developed further in the chapter by Fellows and Scofield, there is a fundamental trade-off for fitness purposes between the production of primary and secondary metabolites within plant species. Secondary metabolites were long-recognised but little understood because the evolutionary benefits from a non-productive chemical substance were not appreciated. The explanation that has been given is that coevolution between species within a predator-prey system generates the usefulness of such substances. Primary productivity can aid survivability but only to the extent that it makes the organism a better competitor within its environment. The capacity of the species to function within a system is actually the more fundamental criterion for success; primary productivity is only useful to the extent that it contributes relative to this framework. Secondary metabolites fulfil this purpose because they are effective precisely by reason of their attunement to their environments.
Secondary metabolites are usually effective by means of attracting responses from other organisms that might enhance its relative prospects for survival or by repelling those responses that might diminish its chances. Fruits for example, promote the response of other organisms that serves the purpose of seed dispersal. Other chemicals are bad-tasting or toxic in order to provoke the desired response from predators. The category of substances within plants that have these effects on animals are known as 'alkaloids'. By their definition the alkaloid group of chemicals are biologically active because they exist in order to provoke responses from animals. The known biological activity of these substances is the information that is useful to human societies; it eliminates the need to conduct trials or to develop scientific methods for the identification of such chemical activity. Obviously, for a chemical to be a potentially useful medicinal substance for human use, it must first be found to have some activity within the species.
It is not happenstance that plant and other communities have developed such substances; it is their manner of communicating between species. Secondary metabolites establish communication between plant and animal communities by generating the desired response from the particular organism. Animals have developed a wider range of interaction primarily on account of their greater mobility. Plants must perform these same functions through chemical production: 'plants produce, animals act'. Plants communicate to animals in order to elicit the response that aids their survivability by means of specific chemical production. In this manner plants follow strategies that allow them to adapt to their specific environment (that is, the predators and others within it). This 'dual' primary/secondary strategy of plants is successful because the production of biologically active ingredients creates a complex web of interaction within the particular system of which the organism is part.
Human societies also form complex webs of interaction (see Wood Sheldon and Balick, this volume). The primary distinction is that because of our evolved capacities for communication, most of that interaction is focused on other humans and a small number of closely associated species. Our comparative advantages in communication and mobility have led us to be unmindful of the need to adapt to the local environment as it is presented (the approach of the relatively immobile and uncommunicative plants). Instead we focus on the few organisms with which we have established a cooperative relationship, and we ignore the communicative and cooperative potentials of most others.
We are thus a species that focuses on generally applicable primary productivity to the exclusion of most other forms of interaction. This has an obvious cost in terms of foregone values; for example all of the potential communications from plant-animal relationships coevolved over hundreds of thousands of years are lost with the conversion of a forest in pursuit of an increase in agricultural commodity yields.
Why should human society learn to appreciate and incorporate heterogeneity within its systems of thought, production and cooperation? This is what the human species should learn from the continued existence of secondary metabolites within plant communities. An exclusive focus on a few primary characteristics that contribute to success may not be a good guide to ultimate survivability. Sustainability requires not only the pursuit of general characteristics of primary productivity but also the incorporation of specific characteristics conducive to environmental adaptation.
How can adaptation be incorporated as a criterion within societal decision making? The pursuit of primary productivity by human society goes hand-in-hand with a bias toward homogeneity within human systems, cultural and institutional. It is this broad-based pursuit of the primary to the neglect of the secondary that makes considerations of adaptability complex. The incorporation of a criterion of adaptation will require the incorporation of diversity across all of these systems simultaneously.
The lesson to be derived from the analysis of plant communities is that a strategy that combines both primary and secondary values is best for survivability. Ironically, it is our unwillingness to learn from these communities that has prevented us from recognising this point, and has led to a broad-based underappreciation of these organisms and has also led to the threat of their extinction.
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