Monoculture refers to the growth of a single plant species over a large area of land. Examples of monocultures in farming include Russet potatoes, certain types of corn, and soybeans. Monoculture is also seen in lawns, ornamental plants, and even forest that are replanted after mining or other activity.
The major problem with monoculture is that the plants lack genetic diversity. In other words, every individual plant is nearly identical to every other plant. This poses a problem when that particular species of plant is susceptible to a certain pest or is subjected to environmental changes with which it cannot cope.
The problem is larger than that though. Clearing natural habitat to make room for monoculture eliminates a large variety of species in an ecosystem. Besides the direct extinction that this causes, it also decreases the health of surrounding ecosystems. Loss of diversity increases the prevalence of certain diseases as well and makes it easier for them to spread by removing natural barriers.
Monocultures are often genetically modified to make them less reliant on water and pesticide, but this poses a problem as well. When monocultures are stronger than their natural counterparts in certain conditions, they can supplant those natural variants and further decrease biodiversity. The problem with this is that when a disease or condition does arise that the monoculture cannot deal with the scale of devastation can be massive.
Finally, the Earth has a limited capacity to produce living organisms. The thin layer of the Earth that supports life is referred to as the biosphere. Planting large regions with a single crop, particularly if that crop requires large amounts of water and fertilizer, removes resources that could be used by other crops. This further weakens ecosystems.
The advantage of a monoculture is that it can be grown on an industrial scale, which is exactly what is needed if we are to produce enough biofuel to meet our needs and to keep prices low. Unfortunately, the ecologic costs of producing that much biofuel may be unsustainable. Thus, we are faced with a problem. Currently, the growth of algae in contained systems is the most viable. However, even these systems have been shown to require unsustainable levels of fertilizer and water. As it turns out, the biggest impediment to biofuel production may be biofuel feedstock itself.