Kemurphy

Ecological effects[edit | edit source]

Agricultural ecosystems function effectively as self-regulating systems provided they have sufficient biodiversity of plants and animals. Apart from producing food, fuel, and fibre, agroecosystem functions include recycling nutrients, maintaining soil fertility, regulating microclimate, regulating water flow, controlling pests, and detoxification of waste products.

However, modern agriculture seriously reduces biodiversity. Traditional third world agriculture that is best at maintaining diversity with multiple cropping systems such as polyculture (including companion planting and intercropping) and agroforestry systems such as shifting cultivation. In Latin America, between 70% and 90% of beans are grown mixed with other crops such as maize and potatoes. Worldwide, multiple cropping provided up to 20% of all food in 1986. Traditional systems also maintain diversity within a crop species, such as in the Andes mountains where up to 50 varieties of potato are grown.

The effects of loss of biodiversity are especially noticeable in pest control. Herbivorous pest insects tend to become more abundant in monocultures. Stable agroecosystems are biodiverse and contain sufficient numbers of natural enemies of crop pests, such as parasitoidal wasps which are able to control aphids and lepidopteran caterpillar populations. These ecosystems continuously provide food and breeding sites for these beneficial species. Agroecosystems can achieve this equilibrium by arranging a suitable combination of crops in space and time, given the type of soil and surrounding environment; sufficient abundance of non-crop plants in and around fields; and appropriate management of vegetation. Suitable farming strategies include polyculture, crop rotation, mosaics of small scattered fields and uncultivated land. Additional strategies include perennial crops such as orchards (especially if a diverse floral undergrowth is permitted), tolerating specific weed species, and raising genetic diversity such as by planting mixtures of crop varieties.

Genetic diversity of crops can be used to help protect the environment. Crop varieties that are resistant to pests and diseases can reduce the need for application of harmful pesticides.While crops that are more vigorous can better compete with weeds which reduces the need for applying herbicides. This is shown in a case study completed at Aarhus University in Denmark which used more robust maize. Drought resistant plants can help save water and reduce the need for irrigation while deeper rooting varieties can help stabilize soils; and varieties that are more efficient in their use of nutrients require less fertilizer.

Economic impact[edit | edit source]

Agriculture is the economic foundation of most countries, and for developing countries a likely source of economic growth. Growth in agriculture can benefit the rural poor, though it does not always do so. Profits from crops can increase from higher value crops, better marketing, value-adding activities such as processing, or expanded access for the public to markets. Profits can also decrease through reduced demand or increased production. Crop diversity can protect against crop failure, and can also offer higher returns.

Disease threats[edit | edit source]

Loss of low-diversity crop to a single disease: the Irish Potato Famine, caused by the oomycete Phytophthora infestans. Starvation followed, as illustrated by James Mahony, 1847

One particular threat to mass-producing plants for harvest is their susceptibility to diseases. Wild species have a range of genetic variability that allows some individuals to survive a disturbance. In agriculture, resistance through variability is compromised, since genetically uniform seeds are planted under uniform conditions. Monocultural agriculture thus causes low crop diversity, especially when the seeds are mass-produced or when plants (such as grafted fruit trees and banana plants) are cloned. A single pest or disease could threaten a whole crop due to this uniformity. A well-known historic case was the Irish Potato Famine of 1845-1847, where a vital crop with low diversity was destroyed by a single fungus. Another example is when a disease caused by a fungus affected the monocultured 1970 US corn crop, causing a loss of over one billion dollars in production. Wheat stem rust is evolving new, virulent strains, threatening many low-diversity cultivars.

A danger to agriculture is wheat rust, a pathogenic fungus causing reddish patches, coloured by its spores. A virulent form of the wheat disease, stem rust, strain Ug99, spread from Africa across to the Arabian Peninsula by 2007. In field trials in Kenya, more than 85% of wheat samples, including major cultivars, were susceptible, implying that higher crop diversity was required. The Nobel laureate Norman Borlaug argued for action to ensure global food security.

Reports from Burundi and Angola warn of a threat to food security caused by the African Cassava Mosaic Virus (ACMD). ACMD is responsible for the loss of a million tons of cassava each year. CMD is 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. The FAO emergency relief and rehabilitation program is assisting vulnerable returnee populations in the African Great Lakes Region through mass propagation and distribution of CMD resistant or highly tolerant cassava. Low-diversity crop variety destroyed: the 'Gros Michel' banana was commercially destroyed by Panama disease, caused by the fungus Fusarium oxysporum (illustrated).

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 cultivar, growers and farmers began to use the Gros Michel banana almost exclusively. Genetically, these bananas are clones, and because of this lack of genetic diversity, are all susceptible to a single fungus, Fusarium oxysporum (Panama disease); large areas of the crop were destroyed by the fungus in the 1950s. 'Gros Michel' has been replaced by the current main banana on the market, the 'Cavendish', which in turn is (2015) at risk of total loss to a strain of the same fungus, 'Tropical Race 4'.

Such threats can be countered by strategies such as planting multi-line cultivars and cultivar mixes, in the hope that some of the cultivars will be resistant to any individual outbreak of disease.

Organizations and technologies[edit | edit source]

The implications of crop diversity are at both local and world levels. Global organizations that aim to support diversity include Bioversity International (formerly known as International Plant Genetic Resources Institute), the International Institute of Tropical Agriculture, the Borlaug Global Rust Initiative, and the International Network for Improvement of Banana and Plantain. 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. One such step taken in the action against the loss of biodiversity among crops is 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 off-site methods such as seed and sperm banks for further research and crop breeding. Six bean varieties at a gene bank

The Global Crop Diversity Trust is an independent international organisation which exists to ensure the conservation and availability of crop diversity for food security worldwide. It was established through a partnership between the United Nations Food and Agriculture Organisation (FAO) and the Consultative Group on International Agricultural Research (CGIAR) acting through Bioversity International. The CGIAR is a consortium of International Agriculture Research Centers (IARC) and others that each conduct research on and preserve germplasm from a particular crop or animal species. The CGIAR holds one of the world's largest off site collections of plant genetic resources in trust for the world community. It contains over 500,000 accessions of more than 3,000 crop, forage, and agroforestry species. The collection includes farmers' varieties and improved varieties and, in substantial measure, the wild species from which those varieties were created. National germplasm storage centers include the U.S. Department of Agriculture's National Center for Genetic Resources Preservation, India's National Bureau of Animal Genetic Resources, the Taiwan Livestock Research Institute, and the proposed Australian Network of Plant Genetic Resource Centers. Plants in the International Center for Tropical Agriculture's gene bank, Colombia

The World Resources Institute (WRI) and the World Conservation Union (IUCN) are non-profit organizations that provide funding and other support to off site and on site conservation efforts. The wise use of crop genetic diversity in plant breeding and genetic modification can also contribute significantly to protecting the biodiversity in crops. Crop varieties can be genetically modified to resist specific pests and diseases. For example, a gene from the soil bacterium Bacillus thuringiensis (Bt) produces a natural insecticide toxin. Genes from Bt can be inserted into crop plants to make them capable of producing an insecticidal toxin and therefore a resistance to certain pests. Bt corn (maize) can however adversely affect non-target insects closely related to the target pest, as with the monarch butterfly.