In a cereal monoculture, plant biodiversity is extremely low - if weed control is successful there may be only one species present, a highly specialized and genetically homogeneous wheat variety. This is not common in natural ecosystems, although it can occur in extreme environments. As mentioned above, this means that a pest can have a field day if it can reproduce in the field (or migrate into the field at a large scale).
However, agricultural monocultures still are common and continue to produce good yields. There are several reasons for this. First, there is no simple relation between biodiversity, productivity, or ecosystem stability. A plant monoculture that is well adapted, grows under good conditions, and has a reasonable resistance to pests and diseases can survive and produce well. This is exactly what a highly productive agricultural field is - a well-adapted monoculture. The crop variety has been selected for high production under a number of years with different weather (and on different soils) in a region. A variety that would demand intensive treatment with herbicides, pesticides, and fungicides will not be economical and will be rejected.
Second, the low plant diversity reduces animal diversity in the stand, but perhaps less than one would expect. In a cereal monoculture stand, there can be hundreds of species of insects, mites, springtails, snails, slugs, etc. In the soil under a monoculture the biodiversity is almost always extremely high, though usually lower than in natural systems. Thousands, perhaps millions of bacterial species, tens to hundreds of species of earthworms, enchy-traeids, soil insects, springtails, mites, spiders, millipedes, flagellates, amoebae, blue-green algae, etc. can be found. There are no consistent indications that soil functions such as organic matter decomposition is hampered by a low biodiversity under monocultures - a given plant residue will decompose at the same rate under a monoculture as under mixed plants, if soil temperature and moisture are the same.
Third, the last line of defense is the crop protection measures that the farmer takes. For example, in several countries there is a sophisticated monitoring and prediction system for aphid outbreaks. Aphids suck the sap from the crop leaves, but they are also vectors for crop diseases. Therefore their hibernating stages are enumerated, weather is monitored, and if the conditions are 'right' the farmers are recommended to spray the fields with an insecticide (or a more specific aphicide) with dose x at date y. In less technically developed regions, experience and skill is a substitute for the model projections, but the principles are the same. It should also be mentioned that in spite of these defenses, pest insects, pathogens, and weeds still reduce worldwide crop yields considerably, and there is a great potential for improvements.
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