Land use impacts
Unfortunately, agriculture requires land, and good agricultural land is very limited in the islands. There may also be questions of land tenure which limit the possible land areas available for agricultural development projects. One of the most difficult questions in planning agricultural development is thus choosing where to locate the project.
If the land to be developed is forested or otherwise still in its natural state, then its conversion to agriculture will result in the loss of its natural values and perhaps affect the water regime. The ability of the soil to support long-term agricultural development may also not be established. In some places, land not developed in pre-European times was not used for some good reason, since traditional farmers had a good understanding of the environment.
If the land is being used for subsistence agriculture, then its conversion to cash crops may result in local food shortages as food gardens are pushed onto marginal lands. Agricultural development projects may thus produce indirect social and health effects, such as child malnutrition or a shift to imported foods of poor nutritional quality.
Sometimes land for development is taken from that normally left for a long time in fallow to allow the soil fertility to recover. The result is generally the shortening of the time the soil is allowed to rest on the remaining land, which will lead to a steady decline in agricultural productivity unless other techniques are introduced to maintain soil fertility and humus content. Development projects may thus require changes in traditional agriculture as well.
Given the limited land available on islands, agricultural development should be accompanied by overall land use planning to achieve the best balance of uses. Whenever possible the best soils should be reserved for agriculture, and village or urban development should be directed to poorer areas.
Impacts on subsistence agriculture
It should be evident from the above that big agricultural projects can have a significant negative effect on subsistence food production. This can come from the competition for good land, from shifts in the agricultural calendar imposed by the labour demands of the cash crops, from declining soil fertility as traditional fallow systems break down, or even from a change in diet motivated by the availability of cash incomes and more attractive if less nutritious imported foods.
If care is not taken, this can produce the irony of agricultural development reducing local food availability and increasing food imports. Therefore the balance between export crops and food self sufficiency should be respected whenever possible.
Impacts on soils
The soil is an essential agricultural resource and it should be used sustainably. The kinds of agricultural development proposed must be adapted to the requirements for soil conservation at the site to be developed. The risk of soil loss is often greater with large development projects, which attempt to achieve economies of scale through large cleared areas and the use of machines. While these technologies are highly successful in the more temperate conditions of the developed countries, they are not always as appropriate to tropical island conditions with fragile soils and frequent problems of erosion. In some cases, the use of poorly chosen machinery or techniques has badly damaged the soil.
The economic requirement for a continuing return on investments may not permit the periods of fallow which allowed island soils to regenerate. While a decline in fertility can be made up with chemical fertilizers, the rapid loss of humus in the tropics is more difficult to replace, and the soil structure and its ability to hold water may deteriorate. Techniques like composting and mulching which restore organic matter to the soil are seldom practised on a large scale.
Agricultural development projects need to be adapted to these constraints. Crop rotations and the use of legume crops can help to maintain the soil. It may be necessary to use mixed plantings of more than one crop, to use cover plants to protect the soil from heavy rain, or to use windbreaks and other protective plantings around the crop. The new techniques of agro-forestry in which trees and food or crop plants are mixed are showing promise in tropical areas and may well be appropriate on small islands.
Impacts of chemical use
Small island ecosystems are particularly vulnerable to damage by chemicals, and modern agriculture tends to require increasing quantities of chemicals. Most dangerous chemicals imported in large quantities into the islands today are for agricultural use. These chemicals include fertilizers, insecticides, fungicides, herbicides and other pesticides.
Fertilizers are added to the soil because they help plants to grow. The environmental problems with fertilizers come if they are washed out of the soil by the rain and go into the ground water, into lakes or streams, or into the lagoon where they contribute to pollution. The fertilizers in the water encourage algae and other plants to grow, and these may become so thick that they start to rot and smell. They may also crowd out or shade out other valuable forms of life like corals. Nitrate fertilizers are also dangerous if they get into drinking water, because they may be turned into nitrites which can cause cancer.
Pesticides are by their very nature poisons for at least some kinds of life, often including people. Pesticides can hurt the environment or poison people in many ways. They may be used without following the instructions very carefully, so that too much is used, or at the wrong time. They may be absorbed by the people who apply the pesticides, especially in the tropics where people do not know much about pesticides and do not like or even have protective clothing. They may be washed off the crop or field by the rain, or blown into villages or into the forest by the wind. They may be applied too close to harvest time and thus still be on the food when it is harvested and eaten. They may not be used the way they were intended (such as for poisoning fish), or be washed into water supplies when users wash their equipment carelessly. They may be eaten accidentally by children thinking they are something good, or by people who use pesticide containers for food or drink. They may be taken intentionally by people who want to commit suicide (if it is a poison with no known cure, even if they change their mind they still die a horrible death). They may spill or leak while they are being transported or while they are in storage.
Once they get into the environment where they are not wanted, they can be a great danger to people and to many useful forms of life. Pesticides should thus only be used when absolutely necessary, and with the greatest care. The increasing use of dangerous chemicals for agricultural development projects can thus have serious impacts on the island environment either directly through the project or indirectly through the risks associated with simply having such chemicals on the island.
Agricultural development can bring with it other risks for the island environment. The large quantities of seeds imported for some projects may contain a few weed seeds. Even one or two unwanted seeds can introduce a noxious weed that may become a serious problem when released in an island ecosystem with few competitors or enemies.
Seeds or planting stock that are not carefully inspected and subject to strict quarantine requirements may also introduce pests or diseases previously unknown on the island and which can ruin its agriculture.
Even some supposedly useful animals and plants introduced for agricultural purposes have turned into pests in the islands with serious effects on native species and even on agriculture itself. Guava, myna birds, cane toads and mongooses are obvious examples.
It should be clear from all of the above that agricultural development projects require extremely careful planning if they are not to have unexpected and often serious effects on people and the environment.