Cactaceae, Opuntia spp., prickly pear, raiketa, de France (Mauritius) in view of planting the seaside of the fort and thus provide an impregnable barrier (Decary rakaita, raketa 1947). The cactus was imported in 1769 from either Réunion or Mauritius and by 1819 the plant was Pierre Binggeli observed to be protecting fields and gardens from animals and thieves in the south of the island. In 1865 A. (2003) In Goodman S.M. and J.P. Benstead (Eds) The Grandidier walked about 40 km inland near Cap natural history of Madagascar, pp. 335-339. University Sainte-Marie along a track lined with prickly pears of Chicago Press, Chicago. (François 1938). Although the plant was first introduced into the relatively humid conditions of Antanosy, it was American cactus species of the genus Opuntia have been rapidly dispersed by native people and managed to widely introduced around the world chiefly as a source of spread rapidly into the sub-desertic region of Androy food for herbivores and humans as well as a hedge plant during the latter part of the 18th century (Decary 1947). (Russell & Felker 1987; Le Houérou 1996). A number of species have subsequently become invasive in many arid The area south of the line Tsiombe-Montovositra- regions including one species in the southwest of Beloha-Ampotaka was indicated on an early 20th-century Madagascar. The case of Opuntia monacantha Haw. map as the 'region cactée' where the vegetation was (Cactaceae) in Madagascar has been largely forgotten but dominated by the prickly pear. It is also roughly the it is a perfect example illustrating the appearance, boundary between the calcareous and crystalline rock dominance, and eventual disappearance of an invasive formations. In the latter zone the cactus was present but plant. Because the plant had been widely used by some did not appear to spread much and constituted up to 10- native ethnic groups and their livestock, but had also 20% of the vegetation cover. Elsewhere in the south it wasted much agricultural land and seriously hindered represented 40-50% of the vegetation. The densest cactus people's mobility, its disappearance from the island's formations (up to 100% cover), where all native bushy southern landscape, caused by the introduction of a scale plant species had vanished, were reported around insect (Dactylopius), generated much controversy. Nifondo and Ambalanosy to the south of Beloha (Decary 1921, 1930a; the 1921 paper refers to his unpublished The taxonomic nature of the invasive Opuntia in Opuntia distribution map). Madagascar was for a long time unclear and the species was variously called O. monacantha, O. vulgaris, and O. In the Toliara region the species was only found on dillenii. According to Chevalier (1947) the invasive alluvial soils along the Fiherenana River and on the species was O. monacantha (often called O. vulgaris coastal dunes as it was not mentioned by Poisson (1921) Miller in the literature) whilst O. dillenii did not appear in his description of the inland vegetation made during to have been introduced to Madagascar. Due to the lack his trip to Betioky. Thus, in the southwest prickly pear of collected material, Aymonin (1983) could not properly favored areas of low rainfall, sandy and alluvial soils, assess the status of the genus, but he also listed O. and soils overlaying calcareous rock formations. cochenillifera as cultivated and suspected that other Although it had become a 'pest', dominating many of the species had probably been introduced to Madagascar. It modified vegetation types of the region, Perrier de la is now recognized that the name O. vulgaris should refer Bâthie (1932) noted that the species was poorly adapted to a different, North American, species and the correct to the climate of the Toliara region. Its fruits hardly ever name is O. monacantha, although it must be noted that ripened and generally ended up changing into cladodes, no neotype has been designated (Leuenberger 1993). O. whereas in the slightly wetter Androy it produced large monacantha is a succulent shrub up to 5 to 6 m high quantities of fruit. which green branches consist in flattened stem units (cladodes). It bears large (7-9 cm in diameter) yellow Elsewhere in Madagascar Opuntia monacantha used to flowers, and the 7 cm long pear-shaped fleshy fruit, a be common around villages on the western side of the berry, is red containing many seeds. The plant is covered central highlands, between 1500-1800, but its spread was with sharp, 3-4 cm long. Undefined varieties, especially limited and it did not form vast impenetrable stands. In spineless, of O. ficus-indica, a morphologically similar the humid eastern regions and in the warmer zone of the species, were introduced during the early part of the northwest and west, Opuntia only grew with difficulty 20th-century. and eventually disappeared (François 1930). The arid southwestern region of Madagascar was During the 18th-century the French re-established inhabited by two pastoral ethnic groups, the Antandroy military positions along the east coast of Madagascar, and Mahafaly, each consisting of many clans, which had including one at Fort-Dauphin (Tolagnaro). Various forts been pushed to this inhospitable part of the island by had been built since 1643 but were often attacked and more powerful tribes. Especially in the case of the destroyed (Deschamps 1972). On 12 December 1768 Antandroy, the chief purpose of life was to amass as Governor Moldave wrote in his diary that a request had many zebu cattle as possible. During funerals most of the been made for the supply of Opuntia cladodes from Ile deceased's animals were slaughtered and eaten and in Binggeli, P. (2003) In Goodman S.M. and J.P. Benstead (Eds) The natural history of Madagascar, pp. 335-339. 6 effect these cattle provided a relatively small proportion suggested that it could be planted to protect forest edges of the Antandroy's diet. The zebus were a sign of wealth, from fires. essential to obtain a bride for instance, and were readily stolen. To these people the spread of the dense Opuntia In view of the drawbacks of Opuntia monacantha (e.g. stands appeared to be highly beneficial providing hiding spines, invasiveness, and health hazards) a thornless places from enemies for themselves and for their cattle. replacement was sought. Such a plant, then commonly In areas where agriculture was practiced it provided a called O. inermis (probably cultivars of O. ficus-indica), living fence against roaming cattle. During the early existed around Antananarivo around 1900 and by 1903 decades of colonial rule these stands also allowed the the military had it planted in the close vicinity of its indigenous people to escape conquest for a few years and Androy's fortified posts as a feed for their oxen (Decary then hide from the French authorities and the tax 1921 reported that in the Androy the plants exhibited authorities in particular. During dry months and short hard spines). Since 1912 Perrier de la Bâthie (1928) especially during periods of drought the cacti provided had promoted the establishment of spineless forms of food and water for cattle and humans. Three varieties of prickly pears and was in favor of the eradication of O. fruits were recognized and during the dry seasons were monacantha. He argued that in so doing the benefits of often the only foods on sale at local markets. The the invasive cacti would remain whereas its serious Antandroy considered the fruit of O. monacantha drawbacks would be eliminated. In 1919 an administrator superior to that of the thornless cultivars (Decary 1928, prescribed the planting of 1 ha plots throughout the 1930b, 1933; François 1930; Perrier de la Bâthie 1934). region. Most of the military plantations were abandoned but in the 1920s relics were known to exist in a few The main means of dispersal of Opuntia was by human locations whereas the fate of the 1 ha plots was unknown agency. New fields were established in scattered (Perrier de la Bâthie 1928). vegetation and delimited with cactus cladodes laid on the ground 5-6 m apart (Decary 1930a). Once abandoned the Due to the taxonomic difficulties in identifying species of cactus would rapidly overrun a field, as every part of the the scale insect genus Dactylopius, much confusion has plant broken off, including fruits, could root and form a occurred as to the nature of the form introduced to new plant. At the end of the rainy season the plant, Madagascar, as indeed elsewhere in the world. Species of gorged with water, would become brittle and break in genus Dactylopius feed on specific taxa of Opuntia. windy conditions (Decary 1921). New foci of invasion Some of Dactylopius have had much economic were also initiated via seeds dispersed by cattle (Decary importance as they provided the cochineal crimson dye 1925a). Bird dispersal (Corvus spp.) has been reported in whilst others have been used in some of the first highly South Africa (Dean & Milton 2000) and was probably an successful biological control programs. What happened agent of spread in Madagascar too. in Madagascar appears to be a continuation of the haphazard and confused efforts to use introduced The spiny and impenetrable nature of Opuntia thickets Dactylopius species for biological control or for dye had a number of serious drawbacks but these were production in many parts of the tropics and subtropics somewhat different in nature to indigenous and colonial where Opuntia spp. had become major weeds. In people. To both groups the wind-blown fruits' hairs southern India and Sri Lanka, according to Beeson caused lung problems and conjunctivitis. Both thorns and (1934), O. monacantha was almost destroyed by the D.