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Crop diversity is the variance in genetic and phenotypic characteristics of plants used in agriculture. Over the past 50 years, there has been a major decline in two components of crop diversity; genetic diversity within each crop and the number of species commonly grown.

Crop diversity loss threatens global food security. As the human population grows, there is a higher demand for certain crops. As a result, the variety of crop species grown is diminishing.

The loss of crop diversity and the demand for specific crops has led to the use of monoculture in farming practices. This technique leaves a species vulnerable to disease, which could lead to crop devastation resulting in events such as the Irish Potato Famine. In extreme cases, these vulnerabilities can lead to plant species extinction such as in the case of “Gros Michel” banana.

With the help of seed banks, international organizations are working to preserve crop diversity.

Biodiversity Loss

Crop diversity is an aspect of biodiversity important for food security. The loss of biodiversity is considered one of today’s most serious environmental concerns by the Food and Agriculture Organization. The tendency for improving crop diversity is to select varieties with traits that give the highest return.This is done by focusing on genetic strains that combine the most desirable traits to create a new variety. Such benefits would include plants with a higher immunity to disease or plants that need less water to grow. However, in many cases the traits selected are the ones that allow cash crops to grow more abundantly in any condition.

If current trends persist, as many as half of all plant species could face extinction. The wild relative plant species are one of the many plants currently under the threat of extinction. This plant species is particularly valuable but they possess important traits for crop breeding. Some 6% of wild relatives of cereal crops such as wheat, maize, rice, and sorghum are under threat, as well as 18% of legumes (Fabaceae), the wild relatives of beans, peas and lentils, and 13% of species within the botanical family (Solanaceae) that includes potato, tomato, eggplant (aubergine), and peppers (Capsicum). In 2016, 29% of wild relative plant species were completely missing from the world’s genebanks, with a further 24% represented by fewer than 10 samples. Over 70% of all crop wild relative species worldwide were in urgent need of further collecting to improve their representation in genebanks, and over 95% were insufficiently represented with regard to the full range of geographic and ecological variation in their native distributions. The most critical priorities for further collecting were found in the Mediterranean and Near East, Western and Southern Europe, Southeast and East Asia, and South America, crop wild relatives are still insufficiently represented in gene banks worldwide.

Since 1961, human diets across the world have drastically increased in the consumption of major commodity staple crops, while as the same time there is a corresponding decline in consumption of local or regionally important crops, and thus have become more homogeneous globally. The differences between the foods eaten in different countries have reduced by 68% between 1961 and 2009. The modern "global standard" diet contains an increasingly large percentage of a relatively small number of major staple commodity crops. These crops have increased substantially in the share of the total food energy (calories), protein, fat, and food weight that they provide to the world's human population, including wheat, rice, sugar, maize, soybean (by +284%), palm oil (by +173%), and sunflower (by +246%). Nations used to consume greater proportions of locally or regionally important crops, but now wheat has become a staple in over 97% of countries, with the other global staples showing similar dominance worldwide. Other crops have declined sharply over the same period, including rye, yam, sweet potato (by -45%), cassava (by -38%), coconut, sorghum (by -52%) and millets (by -45%).

Within-crop diversity, a specific crop can result from various growing conditions, for example a crop growing in nutrient-poor soil is likely to have stunted growth than a crop growing in more fertile soil. The availability of water, soil pH level, and temperature similarly influence crop growth.[3]

In addition, diversity of a harvested plant can be the result of genetic differences: a crop may have genes conferring early maturity or disease resistance.[3] Such traits collectively determine a crop's overall characteristics and their future potential. Diversity within a crop includes genetically-influenced attributes such as seed size, branching pattern, height, flower color, fruiting time, and flavor. Crops can also vary in less obvious characteristics such as their response to heat, cold, a drought, or their ability to resist specific diseases and pests.

Modern plant breeders develop new crop varieties to meet specific conditions. A new variety might, for example, be higher yielding, more disease resistant or 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, where planting and harvesting one type of crop on a plot of land one year, and planting a different crop on that same plot the next year. This takes advantage of differences in a plant's nutrient needs, but more importantly reduces the buildup of pathogens.[5]

Both farmers and scientists must continually draw on the irreplaceable resource of genetic diversity to ensure productive harvests. While genetic variability provides farmers with plants that have a higher resilience to pests and diseases and allows scientists access to a more diverse genome than can be found in highly selected crops.[5] The breeding of monocultural crops steadily reduces genetic diversity as desirable traits are selected, and undesirable traits are removed. Farmers can increase within-crop diversity to some extent by planting mixtures of crop varieties; they can further increase in-field diversity by polycultural practices such as intercropping and companion planting.[6]