April 2006
International Center for Research in Agroforestry (ICRAF): Experimental plot growing trees with crops, Kenya. Photo by CGIAR.
As living entities on this planet, we are at the mercy of the Earth’s living conditions just as any other species. Simply put, we are just another passenger on the ship called Earth. The difference, however, is that we are affecting life in a way that no other species has and altering our planet at a much faster rate. In particular, human activity and specifically habitat destruction have dramatically increased rates of biodiversity loss.
Biodiversity is:
- the total variety of living entities on this planet including all living organisms, as well as their surrounding habitats, or ecosystems, and the genetic material of which they are made1
- extremely important to maintain the proper functioning of ecosystems and society; it is the diversity of life that makes this planet extraordinary
- a provider many services and uses, as well as being a source of aesthetic joy: oil, coal, cement, and limestone are all part of the past biodiversity on which our economies depend and the majority of our medicines and agricultural crops come from the environment
- also important for providing ecosystem services such as pollination and pest control.
Even though humankind only uses a few species of crop plants, ensuring that agriculture is carried out in a manner compatible with biodiversity is crucial.
Agricultural systems and biodiversity
Traditional approaches to biodiversity conservation focus on protecting natural habitats in parks and reserves while ignoring the possibilities found within certain agricultural habitats.2 Preservation is necessary to maintain native habitats and those species reliant on pristine habitats, but very little land area is currently protected.
- The total amount of land in protected areas worldwide in 2004 was estimated at only 6.1 percent of the total land area.3
- In contrast, approximately 75 percent of arable lands are now cultivated in some sort of agriculture4 and in large part are contributing to the destruction of biodiversity.
Increasingly, however, conservation biologists are incorporating some agricultural systems into their management plans as important refuges for biodiversity. The general concept underlying incorporation of agricultural areas is that if a certain percentage of land is dedicated to environmentally friendly and sound agriculture, and another percentage of pristine habitat is protected, then preserving the two land uses in combination can contribute to the capacity of the planet to keep us afloat. In order to protect biodiversity, the thinking goes, we need to incorporate agricultural lands managed to protect the environment and therefore our own survival. In contrast, continuing with inappropriate land use will deplete our resources and destroy the remaining biodiversity at ever increased rates.
That we need to incorporate agriculture to save biodiversity may come as a surprise to those who envision agriculture as a wasteland. Yet some agricultural areas with trees may protect as much biodiversity as neighboring forests and provide other benefits necessary for proper ecosystem functioning.5,6 Agricultural systems that differ in number of crop plants and vegetation structure can be located along a gradient of intensification from areas where crops are grown under natural forest canopies (agroforests) to intensively managed areas with only one crop plant (monocultures).7
Monoculture was heavily promoted in the 1970s under the guise of the green revolution. Techniques included planting large expanses with a single crop to increase efficiency and yield and adding high inputs of fertilizers, pesticides, and herbicides. The green revolution promised to increase crop yields and feed a growing population. Many still argue that using such intensive techniques in smaller land areas will allow protection of more habitats for wildlife conservation.8
- Although such techniques may allow short-term yield increases, the costs of intensive and extensive agriculture continues to place a great toll on biodiversity and native habitats, and these negative effects are not limited to the particular agricultural habitats in which they are practiced.4,9
- Pest resistance to pesticides also became a problem causing widespread overuse of chemicals and subsequent pollution.
- Furthermore, intensive production systems have resulted in overproduction of many crops and as a consequence a price decline, making it harder for farmers to continue making a living from agriculture.10
Intensive agriculture also intensifies the problem of forest fragmentation. Even in large forest fragments, there are high levels of species extinction, especially when the fragments are isolated.11 Thus a strategy focusing on intensive agriculture with pesticide applications and intensive monocultures will further isolate those fragments and will prevent movement of individuals between fragments.5 In contrast, when forest fragments are embedded within high quality matrix that promotes interfragment movement, there is a high probability that forest populations can be maintained as metapopulations.12,13
Agroforestry systems, in contrast to intensive monocultures, may provide such high quality habitats important for biodiversity conservation. Agroforestry systems are broadly defined as agricultural systems where trees are grown together with annual crops and/or animals, resulting in enhanced complementary relations between components and increased multiple use.14 Agroforests often fall at the least intensive end of the agricultural spectrum; agroforestry systems such as coffee, cacao, or jungle rubber, where crops are grown under a diverse and dense canopy of trees, protect biodiversity and are a far cry from the images of agriculture as the enemy. Particularly among tropical agroforestry types, shaded coffee and cacao have received the most attention and study. Coffee and cacao were traditionally cultivated under a diverse, dense shade canopy, but recent production is characterized by increased management intensity: reducing shade tree density and diversity, shade tree pruning, and use of agrochemicals.15
Agroforests and maintenance of biodiversity
In general, scientists refer to two types of biodiversity:
- Planned biodiversity encompasses the variety of plants and animals chosen and placed in the system by the farmer. Examples generally include crop plants and livestock.
- Associated biodiversity comprises all other species attracted to or found within the system apart from the farmer’s plans. Examples include wildlife such as birds and mammals, soil organisms, and weeds.7
Agricultural intensification, moving along the gradient from agroforests grown with a diverse and dense shade canopy to those agroforests with only a few types of trees to monocultures with only one crop species or cattle pastures, by definition includes a reduction in the planned biodiversity. Agricultural intensification also results in a loss of associated biodiversity. The following key studies in a range of agroforest types and other agricultural systems document losses in associated biodiversity (relative to nearby forest fragments) with increasing agricultural intensification:
- In several agricultural and forest habitat types sampled in Chiapas, Mexico, bird diversity was highest in forest, but it was closely followed by woodlots and shaded coffee agroforests, each supporting approximately 105 species of birds. Other habitats sampled, such as cattle pastures, multigrain fields, arboreal pastures, and pine savannas, had many fewer species.16
In Panama, of 11 habitat types sampled for bird species, wooded habitats (including forests, shaded coffee agroforests, and residential areas) supported many more bird species than introduced pine plantations or sugar cane and at least twice as many species as active cattle pastures.17
Insects were sampled from trees in three coffee agroforest types in Costa Rica: traditional (with the highest diversity and density of trees), moderate, and unshaded. Ant diversity across the three types decreased from 22 species per tree in the traditional agroforest to 5 species in the moderate type to 0 species found in the unshaded plantation. Similarly, beetle diversity decreased from 118 in the traditional plantation to 48 in the moderate type to 0 in the unshaded type.18
In coffee agroforests in Chiapas, Mexico, samples of fruit-feeding butterflies revealed that diversity was similar in forest and coffee agroforests with high density and diversity of trees, but only a half to a third as many species were found in coffee agroforests with few trees.19
Although only a few studies are presented here, many more show similar patterns for other groups of animals and plants, including small mammals, bats, bees, and epiphytes, in a range of geographic locations.20 The pattern is clear: diverse agroforestry systems provide important habitat for biodiversity, especially when they are compared with other agricultural systems. Although less is known about the long-term maintenance of biodiversity within these systems, or the ability of species to reproduce in agroforests, much work has been carried out recently and is underway to determine what factors promote and maintain biodiversity in agroforests. Factors that are known to be important for the maintenance of biodiversity generally include high plant diversity, including any species necessary for reproduction or food, and a variety of microclimates and microhabitats to support diverse life forms and species.21,22
Overall benefits of agroforestry systems
Agroforests benefit biodiversity, which in turn can benefit the agroforest functioning and humans.
Agroforests
provide habitat for biodiversity to live and breed:
-predator species that protect crop plants from pest outbreaks.23
-pollinator species important for ensuring harvests of important crops.24-26act as buffers to protected areas, in other words, protecting them from the direct effects of more intensive agriculture and human settlements.
create a high quality matrix that increases the movement of animals from one protected area to another and increasing the overall connectivity of natural habitats.12,13
reduce soil erosion, increase carbon sequestration, increase water uptake and storage.27
provide economic benefits to farmers through the large diversity of crops produced.28
Agroforestry and sustainability
The problem of conserving biodiversity goes hand in hand with larger issues of social and economic development: Land use must be both ecologically and economically sustainable. If practiced in a sustainable manner, agroforests can contribute toward these goals. Sustainable use is defined generally as use of components of biological diversity in a way and at a rate that does not lead to the long-term decline of biological diversity, thereby maintaining its potential to meet the needs and aspirations of present and future generations.1 Ecologically, agricultural systems need to be oriented in such a manner that they can last longer than just a few years. Agroforests provide one significant way in which this can be done, but it is important they be managed organically. Organic agriculture—eliminating chemical use and promoting soil enriching practices—works hand in hand with agroforests in promoting sustainable agricultural systems.
Economically speaking, agroforests can provide incomes from many alternative sources as well as provide materials and foods used by the farmers. In contrast to sun crops, frequently grown solely for export and with often volatile markets, diverse agroforests produce many crops, buffering the ups and downs of international markets. Diverse systems with high levels of biodiversity also have better ecosystem services, increasing local functions of pest control23 and pollination,24-26 often with high economic returns. In sum, agroforestry systems are not only ecologically sustainable ways to grow crops but also provide income and resources to farmers and protect biodiversity.
The future of agroforestry
To protect biodiversity and other benefits that biodiversity provides, agroforestry systems should continue and expand, especially in buffer zones near protected areas. There are two basic ways to encourage agroforestry systems:
- Provide incentives to the farmers that already have agroforestry systems.
- Restore vegetation in monoculture systems with perennials and tree crops.
Although growing crops under existing forest canopies would easily create very diverse agroforests, it is problematic if farmers turn to existing forests and especially forest reserves to do so.29 However, it is important to provide farmers who already have agroforestry systems with economic incentives to prevent them from turning agroforests into intensive monocultural systems.30 Creating agroforestry systems by restoring degraded agricultural habitats is one way in which to avoid the problem of forest conversion while still maintaining and promoting recovery of biodiversity. In such a restoration process, sun coffee, for example, slowly transformed to shade coffee. Furthermore, areas with sun grown crops like vegetables can be planted with sun tolerant trees, such as cinnamon, allspice, and fruit trees, which eventually grow into shade tolerant hardwoods. This process leads to recovery of forested areas with diverse canopies from which people still can retrieve economic benefits.
Conclusion
Agroforests provide important habitats for biodiversity, ecologically sustainable buffer zones for protected areas, a high quality matrix that promotes movement between forest fragments, and ecosystem services such as pest control, pollination, and erosion control. Furthermore, agroforests produce important sources of income for local people. It is possible to see agriculture as a diverse system and treat it as an extension of natural habitats that can be guided to grow our needs.
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