By increasing its diversity a system increases the range of environmental challenges with which it is capable of dealing

" No single organism can make use of all forms of energy and nutrient resources, attack all host genotypes, survive in all temperatures and moisture conditions, or self resist all forms of predation."
   David Pimentel

"Monocultures are almost invariably prone to disease."
   Miguel Altieri

The term 'diversity' is used by mainstream ecologists to denote the number of different species that inhabit a complex ecosystem. Diversity is generally considered as having two components. The first is 'species richness', also referred to as 'species density', a measure of the total number of species present in an ecosystem. The second is 'species evenness', a measure of the relative abundance of the different species present and of the extent to which the most abundant ones dominate.

It is difficult to see what is to be gained by including these two different, and not necessarily connected, notions under the same heading of 'diversity'. How can such a concept help us to understand the role of natural systems within the ecosphere, let alone their role in maintaining its critical order?

Eugene Odum sees diversity as a measure of redundancy. There is considerable apparent redundancy in the natural world. Flies, ticks, herrings and many other forms of life produce a vast number of offspring, of which but a minute fraction will survive and reproduce. Only a very small proportion of the genes that make up the human genome are made use of in protein-synthesis. The others, often referred to as 'junk-genes', are regarded as redundant.

A human can lead an apparently normal life even when deprived of half of the neurons that make up his neo-cortex. The rest might also be regarded as redundant or 'junk neurons'. A population, too, can be reduced drastically without its ability to survive being impaired, at least in the short-term.

It would be presumptuous to suppose, however, that the systems involved have not been affected in some way by the reduction in the number of their constituents. Eugene Odum obviously thinks they have, for he considers that to increase an ecosystem's redundancy must also increase its stability, for there would then be more than one species capable of fulfilling a specific function, and hence if one were to become extinct, the others could take over.

However redundancy is a misleading term to use. One of the basic techniques used in biotechnology is cloning, which means growing large numbers of plants or other forms of life that are all genetically identical. However, for Pat Mooney genetic uniformity in crops amounts to an invitation for an epidemic to destroy that crop.

The world has already experienced the terrible consequences of an agricultural system's dependence on too narrow a genetic base. The best known example is the fate of Ireland's nineteenth century potato economy. The bulk of the population of that country lived off the potato, cereals being produced largely for export to England. Since the potatoes were of a single variety, it was only a matter of time before the whole crop would be struck by a disease. When this happened, some two million people died and several million more were forced to seek a new life in North America.

In 1970, the world was reminded of this danger when the USA lost 15 percent of its maize crop to corn-blight. Fortunately, the superpower did not depend on maize for its sustenance but in many Third World countries, such a blight would have been a catastrophe.

Genetic uniformity also reduces a population's adaptive capacity. Thus D. J. Merrell and J. C. Underhill failed to induce resistance to DDT among inbred strains of the fruit-fly Drosophila melanogaster after two and a half years exposure to that poison. [1] In contrast to this, resistance to DDT was induced in normal wild populations in less than six months. Waddington also found that genetic assimilation "fails completely, at least over periods of fairly small numbers of generations, in in-bred strains lacking genetic variability". [2]

Not surprisingly, 'uniform redundancy' does not occur in the natural world. It is not part of the strategy of nature. Natural redundancy is highly diversified and the ecosphere's capacity to create this diversity is stupendous. We all know just how many different words can be constructed by using an alphabet of 26 letters. Similarly, it would seem that an almost limitless number of instructions can be issued by using the genetic code of four basic nucleotides; and an almost limitless number of different forms of life can be generated from the 20 different amino acids required to make proteins. Not surprisingly, the ecosphere is not only capable of generating an incredible number of different species but also of different sub-species, varieties and individuals.

This organised diversity enables a natural system to deal with a very wide range of both internal and external challenges - not a random but a specific range - that which, in the light of the system's phylogenetic and ontogenetic experience, is most threatening and most likely to materialise. The more diversity a system displays, the more unthreatening and the more improbable the challenges it is capable of dealing with.

This means that the sub-systems must be able to act very much more on their own. Each must be specialised in dealing with different possible challenges. On the other hand, a complex system, displaying very little diversity and organised to adapt with great accuracy to very specific environmental conditions, would only be capable of dealing with the most threatening and the most probable challenges - unless it had developed the requisite cybernismic diversity, as in the case of man and other sophisticated mammals.

It might be useful to think of the role of diversity as one of insurance against discontinuities. Thus the most significant feature of tribal agriculture is the incredible diversity of the crops (and varieties of the same crop) that are grown. According to Jurion and Henry, among Central African farmers

"fields of sorghum and millet are commonly composed not only of different varieties, but even of different species of the same genera. As for rice, maize, groundnuts and voandzu, it is more a matter of mixtures of different types than of varieties distinguished one from another." [3]

In peasant societies the same principle applies. Peasants are imbued with what has been termed the 'subsistence ethic'. Their concern is not to maximise yields but to reduce vulnerability and hence discontinuities such as droughts, floods or plant epidemics. As James Scott writes,

"the local tradition of seed varieties, planting techniques and timing was designed over centuries of trial and error to produce the most stable and reliable yield possible under the circumstances ... Typically, the peasant seeks to avoid the failure that will ruin him rather than attempting a big, but risky, killing." [4]

Such prudence, however, is inimical to the development of the market and hence to the interests of industrialists and politicians. To satisfy them, production and consumption must be maximised and insurance sought via market mechanisms. This motivates farmers to adopt precisely those biologically, socially and ecologically disruptive methods that will maximise rather than minimise risk. The results can only be disastrous for, in the long-term, there is no effective substitute for diversity as an insurance against discontinuities.

It is often very difficult to determine which of the constituents of a natural system contribute to its complexity and which contribute to its diversity. The planting of a large number of different crops by traditional agriculturalists increases not only the diversity but also the complexity of the agricultural ecosystem. In a well-planned intercropping system, early-established plants tend to reduce soil temperature and produce the appropriate micro-climate for other plants.

Plants also complement each other in terms of nutrient cycling; thus deep-rooted plants can act as 'nutrient pumps', bringing up minerals from deep down in the sub-soil. Minerals released by the decomposition of annuals are taken up by perennials. The nutrient intensity of some plants is compensated for by the addition of organic matter to the soil by others. Thus cereals benefit by being grown in conjunction with legumes, that have deeper roots, permitting a better use of nutrients and soil moisture, and whose root nodules host bacteria specialised in fixing nitrogen.

All these crops thereby play a significant role in the metabolism of an agricultural ecosystem and hence contribute to its complexity. On the other hand, such crops retain their ability to react individually to many environmental challenges. They can thus fulfil two roles, contributing both to the ecosystem's complexity and to its diversity.

It is interesting to note that some natural systems that are organised to display great diversity can, if required to deal with new environmental changes, transform this diversity into complexity and vice versa. A typical example is the slime mould, a small amoeba-like creature which normally lives in colonies that display considerable diversity. When there is a shortage of Enterechia coli - the bacteria on which they feed - they undergo an impressive metamorphosis, joining together to form a highly differentiated multi-cellular organism displaying a corresponding degree of complexity. As soon as their food supply increases, this organism breaks down once more into its constituent parts, complexity yielding once more to diversity.

The same occurs among vernacular human communities. When faced with an external challenge, they organise themselves to form larger social groupings - as did the Bushmen in their struggle against the Bantu invaders and as occasionally did the Indians of North America in their struggle against the colonial invaders. There is every reason to suppose that had these native peoples won their struggle and kept out the invaders, they would afterwards have resumed their normal lifestyles in the traditional social groupings within which such lifestyles are best led - complexity once more giving way to diversity.

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