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Polyculture is a form of agriculture in which more than one species is grown together in the same space and at the same time in imitation­ of natural ecosystems. Polyculture is the opposite of monoculture, a method of agriculture in which only members of one plant or animal species are cultivated together. Polyculture has traditionally been the most prevalent form of agriculture in most parts of the world and is growing in popularity today due to its environmental and health benefits. There are many types of polyculture including annual polycultures such as intercropping and cover cropping, permacult­ure, and integrated aquaculture. Polyculture is advantageous because of its ability to control pests, weeds, and disease without major chemical inputs. As such, polyculture is considered a sustainable form of agriculture. However, issues with crop yield and biological competition have caused many modern major industrial food producers to continue to rely on monoculture instead.

Historical and Modern Uses
Polyculture has traditionally been the most prevalent form of agriculture. A well-known example of historic polyculture is the intercropping of maize, bean, and squash plants in a group often referred to as "the three sisters". This crop mixture can be traced back several thousand years ago to civilizations in Latin America and Africa. Integrated aquaculture, or the growing of seafood and plants together has been common in parts of Eastern Asia for several thousand years as well. In China and Japan, for example, fish and shrimp have historically been grown in ponds with rice and seaweed. Other countries where polyculture has traditionally been a substantial part of agricultural and continues to be so today include those in the Himalayan region, Eastern Asia, South America, and Africa.

Because of the development of pesticides, herbicides, and fertilizers, monoculture became the predominant form of agriculture in the 1950's. The prevalence of polyculture declined greatly in popularity at that time in more economically developed countries. Polyculture farming has not disappeared entirely though as traditional polyculture systems continue to be an essential part of the food production system today. Around 15% to 20% of the world’s agriculture is estimated as relying on traditional polyculture systems. In fact, the majority of Latin American farmers continue to intercrop their maize, beans, and squash. Due to climate change, polyculture is returning in popularity in more developed countries as well as food producers seek to reduce their environmental and health impacts.

Common practices
The kinds of plants that are grown, their spatial distribution, and the time that they spend growing together determines the specific type of polyculture that is implemented. There is no limit for the types of plants or animals that can be grown together to form a polyculture. The time overlaps between plants can be asymmetrical as well, with one plant depending on the other for longer than is reciprocated, often due to differences in life spans.

Annual Polycultures
When more than two crops are grown in complete spatial and temporal overlap with each other, it is referred to as intercropping. Intercropping is particularly useful in plots with limited land availability. Legumes are one of the most commonly intercropped crops, specifically legume-cereal mixtures. Legumes fix atmospheric nitrogen into the soil so that it is available for consumption by other plants in a process known as nitrogen fixation. The presence of legumes consequently eliminates the need for man-made nitrogen fertilizers in intercrops.

When a crop is grown alongside another plant that is not a crop, the combination is referred to as cover cropping. If the non-crop plant is a weed, the combination is called a weedy culture. Grasses and legumes are the most common cover crops. Cover crops are greatly beneficial as they can help prevent soil erosion, physically suppress weeds, improve surface water retention, and, in the case of legumes, provide nitrogen compounds as well.

Strip cropping is a form of polyculture that involves growing different plants in alternating rows. While strip cropping does not involve the complete intermixing of plant species, it still provides many of the same benefits such as preventing soil erosion and aiding with nutrient cycling.

Permaculture
Polycultures of perennial plants are referred to as permacultures. Legume-grass mixtures and wildflower mixtures are both common forms of permaculture that are especially popular in Europe or more temperate climates. Permacultures most notably increase soil fertility through nitrogen fixation, decrease soil erosion, regulate water consumption, and decrease the need for tillage thereby conserving soil nutrients. Permacultures require even less human intervention than other forms of polyculture because of lower harvest and tillage rates.

In many Latin American countries, agroforestry is a popular form of permaculture as well where trees and crops are grown together. Trees provide shade for the crops alongside organic matter and nutrients when they shed their leaves or fruits. The elaborate root systems of trees also help prevent soil erosion and increase the presence of microbes in the soil. In addition to benefiting crops, trees act as commodities themselves for use in paper, medicine, firewood, etc. Growing coffee plants alongside other tree species in Mexico is a common practice of agroforestry.

Integrated Aquaculture
Integrated aquaculture is a form of aquaculture in which cultures of fish or shrimp are grown together with seaweed, shellfish, or micro-algae. Mono-species aquaculture, a form of aquaculture where only members of the species are grown together, poses several problems for farmers and the environment. The harvesting of seaweed crops in monocultures, for example, releases nitrates into the water and can lead to severe eutrophication as has occurred in the Venice Lagoon. In terms of seafood growth, the greatest problem in monocultures is the high cost of feed which accounts for about half of production costs. However, more than half of seafood feed is shown to go to waste and can lead to further problems with excess nitrogen release and eutrophication or algal blooms of freshwater. Many technological approaches to lowering these harmful environmental effects such as bacterial bio-filters have proved to consume high levels of energy and be economically costly.

As such, many farmers have transitioned towards integrated aquaculture. In integrated aquaculture farms, plants serve a dual purpose, acting as food for the sea animals and as a water filtration device for the surrounding environment by absorbing nitrates and excess oxygen. Nutrients are also able to be recycled between plants and animals, reducing the need for chemical nutrient supplements. Plants that are grown alongside seafood such as seaweed often hold significant commercial value by themselves and so incorporating them into already existing seafood monocultures increases economic value as well. Integrated aquaculture is the form of polyculture that is growing most in popularity today.

Pest Management
Pests are less predominant in polycultures than monocultures due to crop diversity. The lack of concentration of a single crop makes polycultures less appealing to pests who have a strong preference towards a specific crop. These specialized pests will often have more difficulty locating a favorable host plant inside of a polyculture than in a monoculture. If a pest has more generalized preferences, it will leave more quickly to other plants in the polyculture and as such have a lesser effect on any one plant. When pests are present in the nearby area, polycultures consequently experience lower yield loss than monocultures do in a theory known as the associational resistance hypothesis. Because polycultures mimic naturally diverse ecosystems, general pests are less likely to distinguish between polycultures and the surrounding environment as well. As such, pests travel more freely between the two environments, and have a relative smaller in presence in polycultures to begin with.

Because of the diversity of plants in a polyculture, natural enemies, or predators, of pests are often attracted to the polyculture alongside pests themselves as well. These natural enemies help suppress pest populations while doing no harm to the plants themselves.

Disease Control
Diseases are less predominant in polycultures than monocultures as well. The disease-diversity hypothesis states that a greater diversity of plants leads to a decreased severity of disease. Because different plants are susceptible to different diseases, if a disease negatively impacts one crop, it will not necessarily spread to another and so the overall impact is contained. However, the type of disease and the susceptibility of the specific plants inside the polyculture to a particular disease can greatly vary these effects as being affected by a more generalized disease or more having a mixture of extremely susceptible plants will create no noticeable change across a polyculture and monoculture (Mitchell, 2002).

Weed Management
Both the density of crops and the diversity affect weed growth in polycultures. Having a greater density of plants reduces the available water, sunlight, and nutrient concentrations in the environment. Such a reduction is heightened with greater crop diversity as more potential resources are fully utilized. This level of competition makes polycultures particularly inhospitable for other plant species to grow and weed growth is consequently mitigated.

When they do grow, weeds can, in fact, help polycultures. They can assist in pest management by attracting natural enemies of pests. They can also act as hosts to arthropods that are beneficial to other plants in the polyculture.

Sustainability
Because polycultures use methods of pest, disease, and weed control that do not rely on human intervention, harmful pesticides and herbicides used in modern agriculture are not released into the environment. Fertilizer use is averted as well as diverse plants more fully share and use all available soil and atmospheric nutrients. As such, environmental impacts related to the release of agricultural chemicals into the environment such as eutrophication of fresh water or the presence of excess atmospheric nitrogen are greatly reduced in polycultures.

Other negative impacts of modern agriculture are similarly reduced. Excessive tillage occurs in most modern agricultural practices, but removes essential microbes and nutrients from the soil that are conserved in polycultures, especially permacultures. Agricultural machinery not only uses fossil fuels and creates waste, but is more costly to farmers. Because polyculture relies on natural systems of crop maintenance these costs can be eliminated as well.

Modern agriculture is also dependent on a very limited number of plant species. Only around 70 plant species are estimated to be responsible for the majority of world agriculture. In the face of changing climate and biological patterns, this lack of biodiversity poses problems for food security, for if any crops become drastically affected by disease, pests, or changing climates, food availability greatly decreases. Because polycultures encourage crop diversity and provide a greater multitude of food products, they reduce such food security risks.

Agriculture is also responsible for the most land usage globally with about 40% of global land being used solely for agriculture. By growing multiple plants or animals together in the same space, agricultural land use can be conserved overall in polycultures as well.

Polyculture increases local biodiversity. Increasing crop diversity can increase pollination in nearby environments specifically as diverse plants attract a broader array of pollinators. This is one example of reconciliation ecology, or accommodating biodiversity within human landscapes. This may also form part of a biological pest control program.

Human Health
The chemicals used in monoculture food production can be directly harmful to human health when released into the environment. Nitrogen is a chemical found in especially high concentrations in fertilizers. Nitrates from these fertilizers often become integrated in water sources due to agricultural runoff. The consumption of nitrates at high doses has been shown to lead to methemoglobinemia in infants.

Many of the crops consumed today are calorie-rich crops as well which can lead to illnesses such as obesity, hypertension, and type II diabetes. Because it encourages plant diversity, polyculture can help increase diet diversity by incorporating non-traditional foods into agriculture and people's diets.

Effectiveness
The effects of interspecific competition and intraspecific competition can cause great damage to plants in certain polycultures. In order for a polyculture to be effective, the diverse species that are a part of it must have distinct biological needs such as absorbing different nutrients or requiring different amounts of sunlight as stated by the competitive exclusion principle. Due to the large number of plant species that are cultivated by humans, finding and testing combinations of plants where interspecific and intraspecific competition do not significantly negatively affect the individual plants is extremely difficult. As such, for crops where historic polycultures do not exist, such a multiplicity makes the creation of new polycultures a significant issue.

Crop yield is also an issue in polycultures. While a polyculture produces more biomass overall than a monoculture, individual crops inside of the polyculture are not as prevalent. When there is a focal crop whose cultivation is especially important for a society a lower yield for a certain crop may pose food availability issues.

Similarly, while diseases and pests affect a polyculture less as a group, they do not necessarily have a decreased effect on a focal crop. If targeted by a specialized pest or disease, a focal crop in a polyculture will likely experience the same yield loss as its monoculture counterpart.