EFFECT OF CASSAVA PRODUCTION ON BIODIVERSITY

Impact on the biodiversity of other native species

Cassava has been cultivated for centuries in the Americas, initially only for human consumption and more recently for production of dry chips used as animal feed and for starch. Since cassava was mostly a poorly commercialized crop in the past, area under cultivation has increased slowly, in pace with increases in the rural population. However, due to rapid urbanization in Latin America during the past 30 years, the cassava growing area has remained constant. Massive clearing of land for cassava cultivation has not occurred, or may have occurred only in isolated areas, mainly in the semi-arid region in northeast Brazil and in the eastern frontier regions of Paraguay. The effect of this expansion on the native biodiversity, if any, is poorly documented.

In Sub-Saharan Africa the cassava growing area has expanded in pace with a rapidly increasing population. Recent changes in government policies concerning the importation of other foods, like wheat, rice and maize, as well as improvements in cassava processing techniques, have markedly increased demand for cassava. This in turn has led to a very rapid expansion in cassava production areas, especially in Nigeria, partially at the expense of other crops, but also by deforestation in slash-and-burn agriculture. According to McNeely (1992), by 1986 Africa had already lost 65% of its wildlife habitats by conversion into farming land; this process must have accelerated even further during the past decade. This inevitably resulted in a serious loss of the biodiversity of the natural flora. Cassava production for human consumption clearly contributed to this process, but the large-scale cultivation of export crops, like groundnut, oil palm, coffee, cotton and cacao probably had a much greater impact on biodiversity than the small-scale production of cassava.

In Asia the situation is different. While cassava is still mostly a small farmer crop, it is also grown in some large plantations, mainly for starch extraction, in Sumatra island of Indonesia, and in Mindanao island of the Philippines. Owners of these starch factories acquired large tracts of land and quickly converted these to cassava plantations using heavy machinery for deforestation and subsequent land preparation. Locally this may have had a dramatic effect on the flora and fauna, but the areas involved were still too small to have major and long-lasting effects on the biodiversity of native species.

In Thailand, on the other hand, cassava has remained a small farmers' crop, but it is grown almost exclusively for export in the form of dry pellets for animal feed and of starch. From the mid 1970s to the late 1980s cassava production increased dramatically, from 10 million tonnes to 24 million tonnes annually, in response to a growing and very profitable animal feed market in western Europe. This increase in production was achieved mainly through area expansion, from about 460,000 ha in 1974/75 to 1.6 million ha in 1988/89. The area expansion through deforestation of frontier areas, mainly in the northeast, was not only permitted but actively promoted by the government as a way of combatting communist insurgents hiding in those forested areas at the time. The area has mainly poor soils and rather unpredictable rainfall, so cassava was the upland crop of choice for opening the area. The massive deforestation continued until the late 1980s, when most land within Thailand's borders, up to the borders with Laos and Cambodia, had already been cleared; at that time the government proclaimed a national logging ban to arrest the alarming rate of destruction of the native forest. The rapid area expansion for cassava production in northeastern Thailand from 1975 to 1989 must have resulted in a loss of biodiversity of many native forest species, including many plants used for traditional Thai herbal medicines, but the exact nature of this is not known.

Impact on Manihot biodiversity

Species inventory and ecogeography

The genus Manihot, estimated at 70 species, is exclusive to the New World. South America is home to 55 species, and Central and North America to the other 15. The two largest areas of diversity are Brazil, with close to 80% of the species, and Mexico with 15%. In Mexico, the genus is mostly restricted to xerophytic areas or dry thorny forest vegetation. In Brazil, many species occur in the Brazilian savanna known as the Cerrado, but important secondary centers of diversity exist in northeastern and southeastern regions, besides parts of the Amazon (Allem, 1994).

Effect of cassava production

Although cassava is more important in the food and industrial economies of Africa and Asia than in the Americas, there is potential for competition between the crop and wild Manihot species for arable land only in the latter. In general, the largest producing countries (e.g., Brazil, Colombia and Paraguay) have the most potential for cassava cultivation to displace wild species.

Cassava is of very minor economic importance in Mexico, and therefore expansion of production currently presents little risk to the rich diversity of species. In Central America and the Caribbean, cassava is locally quite important (e.g., Haiti, Dominican Republic, Cuba), but impact of its cultivation on wild species is minimal. In most of South America, the impact on the original vegetation of clearing land for cassava cultivation is likewise minimal. In Colombia and Paraguay, cassava cultivation may expand on virgin land, particularly in traditional rural communities, but there is negligible risk from a conservationist frame of reference. The picture changes in Brazil, where there is reason for concern about wild Manihot.

Brazil's southern (sub-tropical) and southeastern (cerrado, or savanna) regions have been grossly deforested over the last five decades for the cultivation of cash export crops, like soybeans, wheat, oranges, coffee and cocoa, besides pastures. The Cerrado remained virtually untouched until 1946, but 43% of this eco-region is now farmland. As a result, 10-12 wild Manihot species native of these areas were eliminated from their local habitats. Brazil's huge Cerrado savanna, home to tens of species of Manihot, covers 22% of the country's territory. Cassava is highly important in the south of Brazil, but less so in the Cerrado. Its participation in the displacement of local wild relatives has been minimal relative to the impact of the export crops mentioned above.

The Brazilian Amazon, home to an estimated 7 forest species of Manihot, has lost 15% of its original vegetation (Walker and Holmes, 1996). The importance of cassava in slash-and-burn agriculture in traditonal Amazonian communites is well-established (Salick et al., 1997; Emperaire et al., 1998). Thus, the threat of cassava cultivation per se to Manihot populations is not negligible.

Brazil's large semi-arid northeastern region is characterized by vegetation known as caatinga, spanning over 930,000 km² or 11% of the Brazilian land surface, and encompasses eight states. The region has lost much of its native vegetative cover to agriculture. The native caatinga vegetation decreased from a 64% cover to 47% in the period between 1984 and 1990 (Allem, 1997), and declined further to an alarming 41% cover by the year 1997.

The consumption of cassava in Brazil's semi-arid Northeast is among the world's highest, with a per capita consumption at just under 100 kg/year. Because cassava is an important staple, local species are more likely to experience the thinning out or eventual extinction of their populations.

The clearing of land to cultivate cassava in the Brazilian semi-arid region has been most prevalent in areas inhabited by seven wild Manihot species known as maniçobas. These species live in the thorny bushy vegetation called carrascos and in the harsh conditions of the innermost area called sertão. They are: M. caerulescens, M. diamantinensis, M. dichotoma, M. glaziovii, M. jacobinensis, M. janiphoides, and M. maracasensis. The clearing and burning of the vegetation in drought-plagued areas of the caatinga has the potential to hit hard local populations of Manihot, because they are usual components of the vegetation. About 100 municipalities of the area have annual rainfall rates below 500 mm, and not seldom between 0-250 mm. Few other crop species are adapted to the harsh conditions of the caatinga, and therefore intercropping of cassava is rather uncommon, especially in the harshest sites. As a result, staples like cassava may assume the form of monocrop plantations. These ecological and climatic conditions, along with a high human population density of the sertão, further compound the plight of Manihot populations in the area.

Effect of cassava breeding

Demand for wild Manihot species on the part of cassava breeders had a positive effect on their conservation and study. The plants became better known and this in turn raised interest in further collection and conservation efforts. In addition, the wild Manihot germplasm assembled in research institutions in Africa and Asia from the early 1930s through the late 1950s was multiplied for generations and made available to the community for other characterization studies.

During the 1970s the International Institute of Tropical Agriculture (IITA) in Nigeria remained the sole institution that continued to cross selected wild species, particularly M. glaziovii, with cassava with the aim of producing improved genotypes, especially resistance to African mosaic virus (Hahn et al., 1980a).

Most of the crossings involving cassava and wild species were carried out in Africa and Asia. In South America, breeders had access to a much broader range of genetic variability within the species Manihot esculenta, and did not resort to use of wild species for obtaining rare traits. African institutes crossed cassava and the Brazilian species M. dichotoma, M. catingae, and M.glaziovii, besides the Surinamese species M. saxicola and M. melanobasis (Nichols, 1947; Jennings, 1957).

Rationale for the conservation of wild Manihot

Historically a number of wild Manihot species were agronomically evaluated and had genes transferred to cassava, but these hybrids rarely reached commercialization. An exception is the species M. glaziovii, which was successfully crossed with cassava in Africa and provided high-yielding local commercial varieties with genes for resistance to the destructive African cassava mosaic virus (Hahn et al., 1980b).

Conservation policies and breeding goals can and should be complementary. For example, there are recommendations to conserve wild species for their potential contribution to elucidating the processes that led to the domestication and further evolution of the crop (Hershey, 1994), or because they are the very gene pools of the crop (Valle, 1991).

Blueprint for the long-term conservation of wild Manihot

Wild genetic resources of Manihot fall into the following five utilitarian categories (Allem et al., 1998): (1) agricultural value (agrobiodiversity - direct or indirect economic application either as food suppliers or as income generators); (2) scientific value (enlargement of knowledge); (3) social value (recreation); (4) cultural value (broader spectrum of interest to the community); and, (5) ecological value (keystone species or as participants in nature's food chains).

A first conservation target should be the group of species known as 'the cassava species complex' which is composed of the wild progenitor of the crop and four other closely related species from the Brazilian tropics. Study of this complex, from the perspectives of taxonomy, biosystematics and cladistics, will shed new light on the origin, phylogeny and evolutionary patterns of cassava. It was only through recent systematic studies of the biodiversity that the long-searched origin of cassava seems close to completion, a quest that lasted over a century (Allem, 1994, 1999; Olsen and Schaal, 1998).

A second conservation effort should elect as target species the so-called maniçobas from northeast Brazil's semi-arid caatinga region. Three of them in particular merit attention: M. caerulescens ("maniçoba do piauí", a source of cheap latex), M. dichotoma ("maniçoba de jequié", a minor supplier of latex and domestic utensils such as wooden spoons), and M. glaziovii ("maniçoba do ceará"). The latter is a most versatile species. It provides commercially useful latex and wood, and has been studied for potential use as feed for goats and cattle in Brazil's semi-arid Northeast (Barros et al., 1990). Most importantly, M. glaziovii supplied African cassava breeders with genes for resistance to the African cassava mosaic virus (Hahn et al., 1980a) and the cassava bacterial blight (Hahn et al., 1980b).

Collection and conservation of select stocks is timely, inter alia, because landraces of the crop growing in the Americas may be genetically very close to cassava's wild progenitor. A more far-reaching implication is the possibility that commercial cassava stocks worldwide share much of their genome with that of the wild progenitor. Wild materials related to the ancestry of the crop, and making up the wild primary genepool, should be in storage to enable comparative tests. Such materials enjoy scientific-cultural value for fields as diverse as agronomy, sociology, anthropology, archaeology and economic botany. A condensed summary of the species discussed in the text and assessments are given in Table 20.

Effect of breeding on the biodiversity of cassava

Breeding efforts to increase the yield potential and disease/pest resistance of cassava have in some cases reduced the number of commercially grown varieties and thus eliminated the use of certain farmers' landraces. However, extensive collections of Manihot esculenta varieties in the centers of origin in Latin America, and their conservation in field-grown germplasm banks, in vitro culture, of under cryopreservation, have helped to prevent the loss of biodiversity in Manihot esculenta. Presently, CIAT, in Cali, Colombia, maintains over 5500 cassava accessions. Moreover, the widespread distribution of sexual seed of Latin American origin, from CIAT to Africa and especially Asia, has greatly contributed to the increased biodiversity of cassava in those two continents.

Conclusions and recommendations

1. Cassava production has had a minimal effect on the biodiversity of other species, with a possible exception of Thailand, where a dramatic extension of cassava cultivated area during the late 1970s and early 1980s resulted in extensive deforestation in the northeastern part of the country. The actual loss of biodiversity in this process has not been well-documented.

2. Cassava production has had a minimal effect on the biodiversity of Manihot in the center of origin of the species, i.e. in Mexico and Brazil, with a possible exeption of the semi-arid northeast of Brazil where intensive monocropping of cassava may threaten the survival of seven Manihot species native to that area.

3. It is recommended to protect the wild Manihot species from extinction by conserving their natural habitat, mainly in the caatinga region of northeast Brazil and the cerrado of central west Brazil. Furthermore, wild Manihot species should be collected and conserved ex situ, especially the six species most closely related to Manihot esculenta, for possible future interspecific breeding to transfer favorable characteristics to the cassava plant.

4. Narrowing of the genetic base of commercial cassava varieties by the use of a small number of widely-adapted high-yielding varieties should be avoided, by the continued release of new varieties with a broad genetic background. This will reduce the risk of widespread crop failure, for example in case of adverse climatic conditions or the appearance of new diseases or pests.

Table 20. Species of Manihot of particular concern for humankind.

¹⁾1= Mexican species threatened because of development;
2= Brazilian species threatened because of development and cassava cultivation;
3= Species of maniçobas conomically valuable to dwellers of Brazil's NE semi-arid region;
4= Species involved in the ancestry of cassava and constituting the wild primary genepool of the crop;
5= The putative closest wild relatives of cassava and assumed to participate in the secondary genepool of the crop
²⁾ Includes species of column 3