Crop diversity

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 world's human population depends on a diminishing number of varieties of a diminishing number of crop species. Crops are increasingly grown in monoculture, meaning that if, as in the historic Irish Potato Famine, a single disease overcomes a variety's resistance, it may destroy an entire harvest, or as in the case of the 'Gros Michel' banana, may cause the commercial extinction of an entire variety. With the help of seed banks, international organizations are working to preserve crop diversity. 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. If current trends persist, as many as half of all plant species could face extinction. Among the many threatened species are wild relatives of our crops – the wild and weedy cousins of domesticated plants that possess valuable traits for crop breeding, such as pest and disease resistance. Some 6% of wild relatives of cereal crops such as wheat, maize, rice, and sorghum are under threat, as are 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. While 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 insufficiently represented in genebanks are distributed across almost all countries worldwide. Since 1961, human diets across the world have become more diverse in the consumption of major commodity staple crops, with 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 were 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, which 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%). Whereas nations used to consume greater proportions of locally or regionally important crops, 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. 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 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. 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. 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.