Monoculture

In crop monocultures, each plant in a field has the same standardized planting, maintenance and harvesting requirements resulting in greater yields and lower costs. When a crop is matched to its well-managed environment, a monoculture can produce higher yields than a polyculture.[9] In the last 40 years, modern practices such as monoculture planting and the use of synthesized fertilizers have reduced the amount of additional land needed to produce food,[10] called land sparing.

Diverse rotations of crop monocultures can minimize the risk of disease and pest outbreaks.[11] However, the shorter the rotation (fewer crops included) the higher the risk. There are examples of short, two-year rotations selecting for pests that are adapted to such rotations [12] Therefore, rotations of 3 or more crops are recommended for pest and disease management.

In Ireland, exclusive use of one variety of potato, the "lumper", led to the Great Famine of 1845–1849. Lumpers provided inexpensive food to feed the Irish masses. Potatoes were propagated vegetatively with little to no genetic variation. When Phytophthora infestans arrived in Ireland from the Americas in 1845, the lumper had no resistance to the disease, leading to the nearly complete failure of the potato crop across Ireland.

Until the 1950s, the Gros Michel cultivar of banana represented almost all bananas consumed in the United States because of their taste, small seeds, and efficiency to produce. Their small seeds, while more appealing than the large ones in other Asian cultivars, were not suitable for planting.[18] This meant that all new banana plants had to be grown from the cut suckers of another plant. As a result of this asexual form of planting, all bananas are grown had identical genetic makeups which gave them no traits for resistance to Fusarium wilt, a fungal disease that spread quickly throughout the Caribbean where they were being grown. By the beginning of the 1960s, growers had to switch to growing the Cavendish banana, a cultivar is grown in a similar way. This cultivar is under similar disease stress since all the bananas are clones of each other and could easily succumb as the Gros Michel did.[19]

Many of today's livestock production systems rely on just a handful of highly specialized breeds. Focusing heavily on a single trait (output) may come at the expense of other desirable traits - such as fertility, resistance to disease, vigor, and mothering instincts. In the early 1990s, a few Holstein calves were observed to grow poorly and died in the first 6 months of life. They were all found to be homozygous for a mutation in the gene that caused bovine leukocyte adhesion deficiency. This mutation was found at a high frequency in Holstein populations worldwide. (15% among bulls in the US, 10% in Germany, and 16% in Japan.) Researchers studying the pedigrees of affected and carrier animals tracked the source of the mutation to a single bull that was widely used in the industry. Note that in 1990 there were approximately 4 million Holstein cattle in the US, making the affected population around 600,000 animals.[20]

While having little to no variety in the genetics of an agricultural system can have drawbacks, increasing genetic diversity by introducing organisms with varying genes can divert them and make the system more sustainable. For example, by having crops with varying genetic traits for disease and pest resistance, there is a much lower chance of having those pests or diseases spread throughout the area. This is because if one crop becomes infected with a particular strain of disease or species of pest, there is a chance that the other plants around it will have genes that protect them from that strain/species.[21] This can help increase crop productivity while simultaneously lowering pesticide usage and risk of exposure.