November 17, 2017

Why crop diversity is so important

Traditional agriculture involved planting a wide diversity of different plants. This had considerable advantages. Among other things it was a method of insurance. The farmer who grows a single crop runs the risk that conditions in a particular year might not be appropriate for it. The weather may not be right or his crop might be subject to pest infestations. As Professor Pimentel of Cornell University notes:

“no single organism can make use of all forms of energy and nutrient resources, attack all host genotypes, survive in all temperature and moisture conditions, or itself resist all parasitism and predation although certain organisms would be able to tolerate a slightly wider array of environmental conditions than others”.

This means that the more different crops the farmer grows, the lower must be the risk, since at least some of his crops are likely to tolerate the weather conditions, and the pest outbreak is extremely unlikely to affect each of his crops.

Traditional peoples had to assure that food would be available regardless of the vagaries of climate. Thus, as Steven Feierman tells us, the Shamba farmer of Tanzania:

“had to sow a great variety of crops with a great variety of characteristics in order to survive, no matter what the climatic variations: so that he would not be, in effect, wiped out. By taking a single ecological zone, understanding its complexity with a thoroughness incomprehensible to even a rural Westerner, developing a rich and subtle language with a profusion of terms for the understanding of local ecology, planting dozens of crops to which the environment was peculiarly suited, the farmer sought to defeat famine, to cheat death”.

This was also true of the traditional European peasant. As James C. Scott notes:

“he seeks to avoid the failure that will ruin him rather than attempting a big, but risky, killing. In decision making parlance, his behaviour is risk-averse: he minimises the subjective possibility of maximum loss”.

Scott show how peasant agriculture only makes sense if one understands this principle. “It is this ‘safety-first’ principle” he writes,

“which lies behind many of the technical, social and moral arrangements of a pre-capitalist agrarian order. The use of more than one seed variety, the European traditional farming on scattered strips, to mention only two, are classical techniques for avoiding undue risks often at the cost of a reduction in average return”.

Of course the modern market-oriented farmer in the third world is not concerned with minimising risks. He seeks only to maximise profits, so very few varieties are now planted; basically those that are most profitable, and what is more, those that are particularly receptive to artificial fertilisers, pesticides and irrigation water. There are two reasons for this, the first is that the new strains of wheat and rice that have been introduced with the “Green Revolution”, the so called high-yielding varieties (or HYVs) are particularly receptive to fertiliser, pesticides and irrigation water and, if enough of these inputs are applied, will indeed provide very high yields.

The second associated reason is that these inputs have been massively subsidised throughout the world making it particularly profitable to grow these varieties that are designed to make maximum use of them. As a result, twenty years after their introduction, a very considerable proportion of the traditional strains of wheat and rice and indeed of many other crops have virtually disappeared. In Sri Lanka, for instance, there were once some 400 different varieties of rice, today only about 20 remain. In India, there were once between 30-70,000 different varieties and they are fast disappearing too.


Traditional farmers when planting large numbers of different crops, did not just plant them at random. Particular combinations of crops were desirable for different reasons. In a well-planned intercropping system, early established plants will reduce soil temperature and produce the appropriate micro-climate for other plants. In the multi-storeyed Javanese gardens and in similar systems of agro-forestry such as those found in the Indian state of Karnataka and in the humid zone of Sri Lanka, the closed canopy trees (often jack fruits, coconut palms, mangoes, bread fruits, etc.) insulate the shrubs growing crops such as cinnamon, cardamom, tea, coffee and those growing at ground level such as cassava and mountain rice from the elements providing them with the appropriate micro-climate.

Plants also complement each other with regards to 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 which have deeper roots permitting a better use of nutrients and of soil moisture and on whose root-nodules live species of bacteria specialised in fixing nitrogen. Maize and sorghum for instance can advantageously be grown together with legumes such as cow peas and rye grass.

Phosphorus uptake is also greatly increased by including plants with mycorrhizal associations (special fungi that live on their roots and that play a big role in making soil nutrients available to them).

Obviously too, by growing a vast mix of crops, one increases the total biomass and hence the amount of organic matter that can be returned to the soil or fed to animals whose dung is then removed for manure.

In Senegal, a crop—livestock—tree (acacia albida) system was found capable of supporting 50—60 people per hectare on a sustainable basis, which is several times the number of people that can be sustained with a crop monoculture in that area. One of the reasons is that the acacia sheds its leaves in the rainy season and the leaf litter adds to the organic as well as to the nitrogen contents of the soil. In the dry season the leaves and pods provide fodder for the animals which than provide dung which further improves the soil. The trees also provide the shade which the animals badly need during the dry season.

Intercropping minimises the disturbance to the agro-system as the ground is largely covered by the vegetation and thereby protected from the elements. Among Swidden agriculturalists, cropping of some sort continues throughout the year and because of the diversity of crops grown, the ground is protected for most of the year and erosion is reduced to a minimum. Indeed the Sarawak agricultural department recently carried out erosion trials on swidden farms on which crops were cultivated on very steep land with a more than 25 degree gradient. Soil erosion was found to be insignificant. However when this land was put under permanent production on inadequately terraced slopes, there was serious erosion lending to development of gullies.

It now seems quite certain that intercropping with complementary plants, a practice known as “companion planting”, can increase yields quite substantially. The anthropologist Paul Richards showed that this is true in the Plateau State of Nigeria, where maize inter-cropped with sorghum in a traditional farm—making use of fertilisers—is nearly four times greater than when these crops are grown separately in similar conditions. In Mexico it has been found too that a mixture of maize, beans and squash produces 70% more food than maize grown by itself.

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Crop Disease

As already noted, crop diversity is a means of reducing vulnerability to pests. Dr Altieri of Berkeley University in California, considers that “monocultures are almost invariably prone to disease”. In a diversified ecosystem on the other hand there is but a small niche for pests that are usually specific to particular crops and they are thereby unlikely to proliferate. What is more, the presence of specific plants reduces the vulnerability to certain pests. Thus in Colombia it has been found that beans when grown together with rice had 25% less leaf-hopper adults than beans grown separately. Populations of another pest, the leaf beetle, were also lower when the beans and corn were grown together then when grown separately. Army-worm populations were also found to be 23 times lower in corn polycultures then in corn monocultures.

By planting the companion plants 10 to 20 days before the crop that is susceptible to leaf hoppers and army worms permitted a further reduction in the incidence of these pests. According to Altieri, it has also been shown that increasing weed diversity by the use of weed borders and alternative rows of weed and crops or by providing weeds in certain periods of crop growth, also reduced pest infestations. Intercropping systems of corn, mung beans and sweet potato are also known to inhibit competition from weeds.

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Many weeds, Altieri considers, should be regarded as resources, as indeed they are by traditional cultivators, instead of being systematically eliminated as in modern agriculture. In an agro-ecosystem with high diversity, individual weeds are likely to be at low density and offer little competition to crops—on the contrary they can serve all sorts of useful functions as demonstrated by a study of non-crop plants in traditional rice fields in India.

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Crop Rotation

Traditional farmers also sought to maintain plant diversity in time as well as in space and thereby preventing the development of a permanent niche for potential pests. Thus they avoided growing the same strains of crops year after year. Instead different strains were often grown each year. Usually, traditional rice growers avoid growing more than one crop of rice each year even if the irrigation water is available. One reason for this is to avoid damage by the rice stem borer. For the rest of the year the land may remain fallow or be grazed by domestic animals, which also maintains the fertility of the land as does the ploughing in of weeds and stubble.

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Natural Immunity

The maintenance of a diversity of insects, fungi and other potential pests may also be an advantage. It is becoming increasingly clear that it is often advantageous to allow mild pest infestations rather than to eliminate the pests with chemicals. This again is a question of insurance. A mild pest infestation can remain relatively stable whereas to try and eliminate the pest can lead to major pest infestations largely because the pesticides will also kill the pest predators. Richards, whom we have already referred to, notes:

“that a low level of deliberate infestation of spider mites of the genus tetranychus stimulates as ‘immune response’ in cotton, thereby reducing the cotton’s vulnerability to serious infestations”.

Professor Pimentel, of Cornell University, considers that epidemics can be avoided by maintaining the genetic diversity of the crops grown.

In natural conditions, resistance is assured by the combined action of a large number of different genes. In Pimentel’s words “resistance is polygenic and hence stable”. This explains why in the fertile crescent of the Middle East where the progenitors of wheat, barley and oats originated and grow naturally the,

“boom and bust cycles that typify pathogenic parasites in host populations do not occur, no crop epidemics in fact, have been reported. The broad polygenic resistance that exists in the host grain population apparently stabilises the parasite—host system”.

It is now claimed that, with genetic engineering it will be possible to breed strains that are vulnerable to any new pathogens. But this is unlikely to be so. Pimentel notes that genetic engineering will tend to achieve resistance to pathogens largely by changing single genes. Resistance to pathogens is however, only temporary when achieved in this way. Long-term resistance to pathogens may require changing 10 to 20 genes which is at present almost impossible. As Pimentel warns, it would “require major reorganisation and reintegration of the genome and a significant quantity of the organism’s resources and this is not readily achievable”.

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Institutional Misadventure

In spite of this, we are totally committed to agricultural policies that could only further increase the amount of land under mono-culture and further intensify the use of agricultural chemicals, all of which can only further reduce the diversity of our agricultural ecosystems. Even those who promote this disastrous policy—such as United Nations agencies and National Governments, have expressed their concern for the increased reduction in the diversity of agro-ecosystems.

The World Bank has gone so far as to draw up a Biological Diversity Action Plan to be administered by a World Bank Task Force on Bio-diversity. However the plan does not address the true causes of the problem. It does not seek to reduce the destructiveness of modern agricultural processes. Instead it talks of setting aside reserves in which bio-diversity would be carefully preserved. Indeed John Spiers at one time head of Forestry at the World Bank actually recommended the further intensification of mono-cultural practices in agriculture and forestry as a means of “preserving biological diversity”.

The idea behind this is that, by further increasing yields on the existing agricultural lands, it will be unnecessary to extend agriculture into remaining wildernesses—an argument that is constantly used by the FAO that has spearheaded the intensification of agriculture throughout the Third World. It is of course an untenable argument because agricultural intensification is the main cause of the present massive rate of soil degradation, as a result of which more and more degraded agricultural land is being taken out of production every year.

To further increase the intensive agricultural practices can only further increase the rate at which land is taken out of agriculture and hence the need for ploughing up more wildernesses.

In any case, the world’s diversity cannot be preserved in small reserves which are very vulnerable to official or unofficial development initiatives. Indeed the future of existing natural parks and nature reserves throughout the world is already very gloomy. They are being encroached upon by developers and also by squatters everywhere.

Further more, biological diversity is not something that can be merely set aside, in case it is required on some future occasion—it is required right now, since it is an essential ingredient of the sort of agriculture that is alone capable of feeding the world’s population on anything approaching a sustainable basis.

The argument that biotechnology can give rise to new forms of genetic diversity is equally misconceived. John Duesing of Ciba-Gigy tells us that patent protection together with free-trade will serve to stimulate the development of genetic diversity—making available all sorts of “genetic solutions” to our food problem. However as Vandana Shiva notes:

“the diversity of corporate strategies and the diversity of life forms on this planet are not the same thing, and corporate competition can hardly by treated as a substitute for nature’s evolution in the creation of genetic diversity”.

Indeed it is not just any diversity that is required. The ecosphere (or Gaia) as Jim Lovelock has shown, is a natural system. As such, it is an organisation of living things and this organisation has a critical structure that must be maintained in order to assure its stability in the face of external or internal challenge (i.e. its homeostasis). The genetically engineered forms of life developed by Genetech and other such corporations are not going to contribute to the maintenance of the ecosphere’s critical structure, on the contrary, they are random to it just as is the rabbit to Australian ecosystems or the gypsy moth, the fire ant, the spruce bud worm and the other imported pests to the ecosystems of north America.

As such, rather than contribute to its stability they could easily have the opposite effect—with disastrous consequences for agriculture and hence for food availability.


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