Crop diversity is the variance in genetic and phenotypic characteristics of plants used in agriculture. Crops may vary in seed size, branching pattern, in height, flower color, fruiting time, or flavor. They may also vary in less obvious characteristics such as their response to heat, cold or drought, or their ability to resist specific diseases and pests. It is possible to discover variation in almost every conceivable trait, including nutritional qualities, preparation and cooking techniques, and of course how a crop tastes. And if a trait cannot be found in the crop itself, it can often be found in a wild relative of the crop; a plant that has similar species that have not been farmed or used in agriculture, but exist in the wild.[1]
Diversity in a crop can also result from different growing conditions: a crop growing in nutrient poor soil is likely to be shorter than a crop growing in more fertile soil. In addition, and perhaps most importantly, diversity of a harvested plant can be the result of genetic differences: a crop may have genes conferring early maturity or disease resistance.[2] It is these heritable traits that are of special interest as they are passed on from generation to generation and collectively determine a crop’s overall characteristics and future potential. Through combining genes for different traits in desired combinations, plant breeders are able to develop new crop varieties to meet specific conditions. A new variety might, for example, be higher yielding, more disease resistant and have a longer shelf life than the varieties from which it was bred. The practical use of crop diversity goes back to early agricultural methods of crop rotation and fallow fields, planting and harvesting one type of crop on a plot of land one year, and using a different crop the next based on differences in a plant's nutrient needs. Both farmers and scientists must continually draw on the irreplaceable resource of genetic diversity to ensure productive harvests, as genetic variability provides farmers resilience to pests and diseases and allows scientists access to a more diverse genetic bank. Diversification of harvests and maintaining wild biodiversity in crop relatives influence many aspects of human and global interaction, being important for environmental and species sustainability.[3]
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The loss of biodiversity is considered one of today’s most serious environmental concerns by the Food and Agriculture Organization of the United Nations.[4] According to some estimates, if current trends persist, as many as half of all plant species could face extinction.[5] Among the many threatened species are wild relatives of our crops – species that could contribute invaluable traits to future crop varieties. It has been estimated that 6% of wild relatives of cereal crops (wheat, maize, rice, sorghum etc.) are under threat as are 18% of legume species (the wild relatives of beans, peas and lentils) and 13% of species within the botanical family that includes potato, tomato, eggplant, and pepper.[6] The wise use of crop genetic diversity in plant breeding can contribute significantly to protecting the environment. Crop varieties that are resistant to pests and diseases can reduce the need for application of harmful pesticides more vigorous varieties can better compete with weeds;[7] reducing the need for applying herbicides as in the case study at Aarhus University in Denmark using more robust maize;[8] drought resistant plants can help save water through reducing the need for irrigation;[9] deeper rooting varieties can help stabilize soils; and varieties that are more efficient in their use of nutrients require less fertilizer.[10] Most importantly, perhaps, productive agricultural systems reduce or eliminate the need to cut down forest or clear fragile lands to create more farmland for food production.
Agriculture is the economic foundation of most countries, and for developing countries the most likely source of economic growth. Growth is most rapid where agricultural productivity has risen the most, and the reverse is also true. Growth in agriculture, although beneficial for the wider economy, benefits the poor most, and by providing affordable food these benefits extend beyond the 70% of the world’s poorest people who live in rural areas and for whose livelihoods agriculture remains central.[11] Ensuring agriculture is able to play this fundamental role requires a range of improvements including: the growing of higher value crops, promoting value-adding activities through, for example, improved processing, expanding access to markets, and lowering food prices through increasing production, processing and marketing efficiency, particularly for subsistence and very low income farming families.[12] Fundamental to all of these potential solutions is crop diversity – the diversity that enables farmers and plant breeders to develop higher yielding, more productive varieties having improved quality characteristics required by farmers and desired by consumers. They can breed varieties better suited to particular processing methods or that store longer or that can be transported with less loss.[13] They can produce varieties that resist pests and diseases and are drought tolerant, providing more protection against crop failure and better insulating poor farmers from risk.[14] Agriculture’s part in fighting poverty is complex, but without the genetic diversity found within crops, it cannot fulfill its potential.
One particular threat to mass producing plants for harvest is their susceptibility to diseases. Generally speaking, a species has a range of genetic variability that allows for individuals and/or populations within that species to survive should a stressor or disturbance occur. In the case of agriculture, this is a tricky business to ensure, as seeds are planted under uniform conditions. For example, monocultural agriculture potentially elicits low crop diversity (especially if the seeds were mass produced or cloned). It is possible that a single pest or disease could wipe out entire areas of a crop due to this uniformity.[15] One of the more historically known examples of harvests that suffered from low crop diversity was the Irish Potato Famine of 1845-1847.
One growing danger to present day agriculture is something called wheat rust: the name given from the reddish spores, it is a fungus that attaches to plants and breaks them down for food. A new form of the wheat disease - stem rust, strain Ug99 - has spread from Africa across to the Arabian Peninsula. This development was summarized on the January 16th 2007 by the international research centers Borlaug Global Rust Initiative and the Agricultural Research Service of the United States Department of Agriculture over 2 years of observation after its initial outbreak.[16] The Ug99 stem rust has recently proven to be even more virulent than other forms. Observations from field trials in Kenya showed that more than 85% of wheat samples, including cultivars from the major wheat producing regions in the world, have succumbed to the pest.[16] This is a serious pest alert for one of the major food crops of the world. The key to overcome the threat is genetic resistance found in certain wheat varieties. As Nobel laureate Norman Borlaug puts it: “We know what to do and how to do it. All we need are the financial resources, scientific cooperation and political will to contain this threat to world food security.”
Reports from Burundi and Angola warn of another looming food crisis partly caused by outbreaks of the African Cassava Mosaic Virus (ACMD).[17] Creating a “mosaic” of decay on the plants leaves, ACMD is responsible for the loss of a million tons of food each year. The Famine Early Warning Network of USAID reports from Angola that pockets of food insecurity exist in a number of districts partly due to the impacts of mosaic virus on the cassava crop. Likewise FAO (Food and Agriculture Organization of the United Nations) has warned about food insecurity in north and east central Burundi and one of the factors causing the precarious situation is declining yam harvests and the losses of cassava crops to the mosaic virus.[18] CMD also affects people already exposed to malnutrition and with limited coping mechanisms. CMD continues to be prevalent in all the main cassava-growing areas in the Great Lakes region of east Africa, causing between 20 and 90 percent crop losses in the Congo.[19] Breeders and relief agencies work together to fight the disease, and the FAO emergency relief and rehabilitation program is engaged in a project to assist vulnerable returnee populations in the African Great Lakes Region through mass propagation and distribution of CMD resistant or highly tolerant cassava planting materials.
A well known occurrence of disease susceptibility in crops lacking diversity concerns the Gros Michel, a seedless banana that saw world marketing in the 1940s. As the market demand became high for this particular species, growers and farmers of the Gros Michel banana began to use this species almost exclusively. Genetically, these bananas are duplicates of every other in their species due to its self-pollinating reproductive style,[20] and because of this lack of genetic diversity, are now virtually extinct due to a single fungus; Panama Disease. This fungus (also known as Fusarium wilt), which infected Gros Michel banana crops in the 1950s, completely wiped out the Gros Michel as the predecessor to the current, and most popular, banana on the market: the Cavendish.
The implications of crop diversity are at both the local and world level, and numerous organizations are emerging with great global backing in response to this ideology. International Plant Genetic Resources Institute (IPGRI – now known as Bioversity International), the International Institute of Tropical Agriculture (ITTA), the Borlaug Global Rust Initiative, and the International Network for Improvement of Banana and Plantain (INIBAP) are a few of the most prominent. Members of the United Nations, at the World Summit on Sustainable Development 2002 at Johannesburg, said that crop diversity is in danger of being lost if measures are not taken.[21] One such step taken in the action against the loss of biodiversity among crops is called gene banking. There are a number of organizations that enlist teams of local farmers to grow native varieties, particularly those that are threatened by extinction due to lack of modern-day use. There are also local, national and international efforts to preserve agricultural genetic resources through ex situ (off-site) methods such as seed and sperm banks for further research and/or crop breeding. Some of the major germplasm storage efforts include: