Oldfield, Sara (comp.)(1997). Cactus and Succulent Plants - Status Survey and Conservation Action Plan. IUCN/SSC Cactus and Succulent Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK. 10 + 212 pp.
Chapter 1. Taxonomic groups
Nigel P. Taylor
The Cactus family (Cactaceae Juss.) is characterised botanically by the presence of stems bearing specialised, felted short-shoots termed areoles, which usually develop the spines (modified leaves) that represent the familiar hallmark of this plant group. Except for one species of Rhipsalis (R. baccifera - Neotropics eastwards to Sri Lanka), the family is endemic to the New World, although various economically important and weedy species have been introduced and become naturalised in warmer parts of the Old World. In the Americas they range from southern Canada to Patagonia, being most frequent in the dry climatic zones between 35° N and S latitude and outside the moist equatorial (Amazonian) region. The family has very considerable value in terms of horticultural production, with millions of artificially propagated specimens traded across international borders each year.
Taken as a whole the Cactus family has a surprisingly wide climatic and ecological spectrum, encompassing almost rainless desert such as parts of the western Atacama in northern Chile at one extreme to tropical rain forest receiving more than 2000 mm of rain per year at the other. They range from sea level to a reported 5200 m altitude in the Andes and vary considerably in their resistance to frost. None is parasitic, but epiphytes, lithophytes, "cactus geophytes", mound-forming caespitose, decumbent, scandent and erect, free-standing tree and shrub species are frequent. Apart from the "cactus geophytes", amongst the most remarkable are dwarf species restricted to cliffs, such as Aztekium, Strombocactus, and Blossfeldia, which have dustlike strophiolate seeds. In contrast, Opuntia (Brasiliopuntia) brasiliensis is adapted to life in high, dense, dry to very humid forest, where it forms a tree to 20 m, with a cylindrical trunk and flattened leaflike ultimate stem-segments (southern Neotropics). At least four Mexican species appear to be restricted to gypsum, and isolated outcrops of particular rock types account for a significant part of the narrow endemism seen in the family. Some species are able to grow on, or are restricted to, extremely oligotrophic substrates, for example, Uebelmannia and Discocactus spp. Seed dispersal is effected by various means including mammals, birds, lizards, insects (especially ants), fish (Amazonia), wind (Eriosyce spp., Pterocactus), and water (Frailea). Bats are a particularly important group of associated organisms (Dobat and Peikert-Holle 1985) on whose conservation the survival of some cactus species may well depend. Nurse plants of various shapes and sizes are often important for establishment of delicate cactus seedlings (Cactoideae).
Although amateur horticulturists continue to play a significant role in describing and classifying cacti, the past 20 years have seen the family subjected to fairly intense morphological and taxonomic scrutiny by professional botanists and it is now probably one of the better understood major succulent plant groups. The Cactaceae comprises four subfamilies, namely the primitively leaf-bearing Maihuenioideae and Pereskioideae (2 genera / c. 19 species), the highly derived Opuntioideae (5 or more genera / 185-265 spp.) with barbed spines and glochids, and the leafless, morphologically complex Cactoideae (c. 97 genera / > 1000-2000 spp.). The last of these, containing the bulk of the family, is divisible into at least eight tribes, each of which has a characteristic geographical range.
Collaborative study of the family's classification at generic level by a broad group of botanists and knowledgeable amateurs (under the auspices of the IOS) has resulted in two published reports listing the genera accepted by a consensus of systematists (Hunt and Taylor 1986, 1990). Following these reports, a checklist of species names and their synonyms in current usage has been compiled for the family on a consultative basis. This includes distribution data at country level and country by country species lists with details of endemism (Hunt 1992). This checklist, prepared at the behest of the CITES Secretariat, distinguishes between species fully accepted as such according to certain primary sources (botanical monographs, floristic treatments, and in the opinions of authors of a few unpublished taxonomic revisions) and those only provisionally accepted, which include taxa whose status awaits expert botanical study and also those treated as subspecies or geographical varieties in the primary sources consulted. In total, the Cactus family comprises about 105 genera and, following the CITES Checklist, 1208 accepted and a further 1300 provisionally accepted species. If infraspecific taxa are excluded from the latter figure, leaving those whose taxonomic status is least understood, then it becomes clear that the greatest concentrations of taxonomically doubtful species are found in Peru and Bolivia. It is this region of the central Andes where competent field study is most needed to resolve questions of taxonomic and conservation status. Nevertheless, it is probable that a significant proportion of such taxa will prove to be "good" endemic species and so their numbers should be taken into consideration when analysing this Andean region from a conservation perspective.
Centres of diversity
In spite of the above-mentioned uncertainties, it is a relatively simple matter to identify the geographical centres of diversity and of endemism for cacti. In first place by far comes Mexico and the immediately adjacent south-western United States (southern California, Arizona, New Mexico, and south-west Texas) where some 27 per cent of all cactus genera are endemic and at least 570 accepted species are native, 430 being endemic to Mexico alone (27 cactus species have been reported sympatric in a single locality in north-east Mexico, Fitz Maurice, pers. comm.). The region is notable for the high number of endemic tree-like or columnar species belonging to tribe Pachycereeae, which are characteristic of the dry tropical forest and Sonoran Desert floras, where they sometimes dominate or are so conspicuous as to be worthy of description as forming "cactus forest". Many are of local economic importance for their edible fruits, woody skeletons used in house construction, and for planting as impenetrable living fences. Bats are known to be important pollinators and seed dispersers of these columnar cacti. Mexico and the bordering Rio Grande valley region of the USA (especially the Chihuahuan Desert and its margins) have many remarkable small or monotypic genera from tribe Cacteae, which exhibit unique and sometimes extraordinary morphology, for example, Ariocarpus (7 spp.), Astrophytum (4 spp.), Aztekium (2 spp.), Epithelantha (2 spp.), Geohintonia (1 sp.), Leuchtenbergia (1 sp.), Lophophora (2 spp.), Mamyrcilloydia (1 sp.), Obregonia (1 sp.), Ortegocactus (1 sp.), Pelecyphora (2 spp.), Stenocactus (8 spp.), and Turbinicarpus (14 spp.). Most of these include species which are either of very restricted distribution or found only in very unusual habitats, and some are known to be under threat from collecting for horticultural purposes, which has required their protection from commercial over-exploitation by inclusion in Appendix I of CITES. Other low-growing genera, which are particularly diverse and species rich in the south-west USA and Mexico, include Mammillaria ( > 150 spp.), Coryphantha ( > 50 spp.), and Echinocereus ( > 50 spp.), each with threatened taxa, some of which are included in Appendix I of CITES in view of horticultural demand. Apart from collecting pressure the most threatened taxa are those "cactus geophytes" that inhabit fertile land undergoing agricultural development (e.g. Echinocereus pulchellus). Opuntioideae, i.e. Opuntia spp., make up a very important component of this cactus flora and some are of considerable local economic importance for their edible fruits and stem-segments. Southern Mexico has a significant number of species belonging to the primarily epiphytic tribe Hylocereeae (e.g. Disocactus s.l.), inhabiting moister tropical forest and mountainous regions, but in comparison with tribes Cacteae and Pachycereeae, a smaller proportion of these is endemic, with ranges frequently extending into the adjacent countries of Central America, where they represent the most important component of the cactus flora.
The second most diverse region for cacti is that of the central Andes comprising the countries of Peru and Bolivia, with the addition of southern Ecuador, north-east Chile, and north-west Argentina. About 18 per cent of all cactus genera are endemic to this region and there are about 420 taxa named as species endemic to Peru and Bolivia, but most of these are only provisionally accepted by the CITES Checklist. The cacti of southern Ecuador, north-east Chile, and north-west Argentina are better known and comprise about 115 accepted species. As in the case of Mexico, cactus forest vegetation with large shrubs and columnar or tree-like forms occurs, but highly specialised and unique growth habits are less common, exceptions being represented by Blossfeldia (1 sp.) and an unusually diverse array of Opuntia spp. However, there are numerous species of globose habit from tribes Notocacteae and Trichocereeae. Genera containing many species include Parodia, Rebutia, Gymnocalycium, Echinopsis, Cleistocactus, Oreocereus, and Matucana. The commonest pollination syndromes in this region are those for bees, hawkmoths, and hummingbirds.
The third most diverse region is that here loosely described as eastern Brazil, comprising the segment of that great country east of a line drawn between the states of Maranhão and São Paulo (north-east and south-east Brazil plus eastern Goiás and eastern Tocantins). About 80 per cent of its cactus flora is endemic, including 11 per cent of all cactus genera and having a total of approximately 145 taxonomically acceptable native species (plus numerous taxa recognisable as subspecies). Its cacti are perhaps the best understood after those of the USA and northern Mexico (Chihuahuan and Sonoran Deserts). Columnar, shrubby and sometimes tree-like forms (tribe Cereeae and Pereskia) and epiphytic forms (tribe Rhipsalideae) predominate, the former being particularly diverse in the dry interiors of Bahia and Minas Gerais states, the latter characteristic of the coastal Mata Atlântica and mountains of Rio de Janeiro and São Paulo (and also the states of southern Brazil). Much of the Mata Atlântica has now been destroyed and with it the habitat of these epiphytic cacti. Rhipsalis pentaptera, for example, although common in cultivation, is probably Extinct in the Wild, since its only known habitat formerly oecurred in what is now a built-up area within the city of Rio de Janeiro. The east Brazilian cereoid cacti are notable for the frequent development of diverse kinds of woolly lateral cephalia, which in some cases are thought to be adaptations connected with pollination by bats, the commonest cactus pollen vector in the region. Hummingbird and hawkmoth pollination syndromes are also frequent. The fewer species of globose habit include the taxonomically isolated Uebelmannia (3 spp. restricted to a small area in Minas Gerais state) and Melocactus ( 15 spp.) and Discocactus (6 spp.), with flowers borne in peculiar inflorescence structures called terminal cephalia. Representatives of each of these three genera are regarded as severely threatened and are currently the only South American cacti listed in Appendix I of CITES (Braun and Esteves Pereira 1988; Taylor 1992). Members of Opuntioideae are diverse at generic level, but represented by very few species when compared with other areas within the family's range.
The geographical region that is fourth in order of importance comprises central-western and southern Brazil, Paraguay, Uruguay, and Argentina (excluding the north-west and southern parts). There are approximately 85 accepted species (about half of these endemic) and a greater number of endemic but only provisionally accepted taxa. Columnar, shrubby, or semi-scandent cacti are mainly represented by species from the genera Cereus, Harrisia, Opuntia, and Pereskia, most of which are endemic, but the greater representation of the family is in the form of low-growing or globular members of Parodia s.l. (incl. Notocactus etc.), Frailea, and Gymnocalycium, each with numerous named species and in need of taxonomic revision. Habitats of various of these smaller cacti are shrinking due to agricultural development and field studies are required to determine their conservation and taxonomic status. There is also a high number of epiphytic species from tribe Rhipsalideae, whose habitat is shrinking through deforestation, as in eastern Brazil.
Smaller, but nevertheless important, centres of endemism include the Caribbean region with northern South America (north Venezuela, north and west Colombia and Panama), where the most notable genera with endemic species are Melocactus with 9 and Cereus with 5, and central to north-western Chile (especially the western edges of the Atacama Desert), where an isolated flora has developed. The Caribbean islands have relatively small cactus floras, exceptions being Cuba and Hispaniola, but are notable for peculiar endemics such as the Consolea group of Opuntia, Leptocereus (including the giant Neoabbottia), Harrisia subg. Harrisia, the massive Dendrocereus (now included in Acanthocereus by some authors), and a unique group of dioecious Pereskia spp., some of which are very rare and of particular conservation concern (Leuenberger 1986).
If the north-easternmost corner of Chile is excluded (see above), the remainder of the country has an almost exclusively endemic cactus flora of about 60 accepted species. The coastal fringe of the Atacama Desert in northern Chile is home to many remarkable cacti whose existence depends to a large degree on moisture derived from fogs which roll in from the cold Pacific Ocean. Various remarkable life forms have evolved in this region, including extreme "cactus geophytes" and certain members of the endemic genus Copiapoa (about 25 spp.) with stems covered in dense white or grey wax. Excepting a few hummingbird-syndrome species, nearly all the endemic Chilean cacti are thought to have flowers adapted for pollination by hymenoptera. Cacti found in the regions of Santiago, Valparaíso, and southward include species under threat from urban and agricultural expansion, and at least one member of the largest Chilean genus, Eriosyce (27 spp. endemic), is now believed to be Extinct in the Wild (E. aspillagae; see Kattermann 1994).
Towards the extremities of the family's range, and near its equatorial centre, few species are represented due to obvious climatic constraints. In the north, in the USA, should be mentioned the Rare, narrowly endemic species of Pediocactus and Sclerocactus, some of which are protected by CITES Appendix I listing. To the south, in Argentinian Patagonia and adjacent Chile, there are peculiar representatives of the Maihuenioideae (Maihuenia, 2 spp.), Opuntioideae (Pterocactus spp.), and tribe Notocacteae (Austrocactus spp.), but none of these is regarded as particularly threatened at present. In the constantly humid Amazonian region there are only a few widespread epiphytic species, plus some poorly known endemics (Melocactus and Cereus spp.), the most notable cactus being Selenicereus wittii, which inhabits flooded forest (igapó), climbing up the trunks of trees by means of aerial roots (Mee 1988). Farther west, in the Galapagos archipelago, is a small but entirely endemic cactus flora comprising the monotypic genera Jasminocereus (a close relative of Armatocereus from the South American mainland) and Brachycereus, and various unusual Opuntia spp.
The major pressures affecting the conservation status of cacti are, in decreasing order of severity, (1) agricultural development and deforestation, (2) urbanisation and infrastructural development including road building/widening and hydroelectric dam projects, (3) collection for horticulture, (4) mining of stone for construction (especially threatening to species restricted to limestone or granitic rock outcrops, for example, in eastern Brazil and Mexico).