IN THE CHIHUAHUAN DESERT CONSERVATION OF CACTI OF NORTHERN MEXICO Over the last decade, a team of Mexican botanists have been studying the floristics, taxonomy and biogeography of the many cactus species that grow in the Chihuahuan Desert. They have revealed the pattern of diversity and distribution of these plants to a level of detail rarely dry, inter-montane valleys, canyons and achieved but vital for plant conservation. HÉCTOR M. depressions that contain an important part of the region’s biodiversity; these desert HERNÁNDEZ, leader of the team, reports on the research fragments are home of a considerable undertaken and how it has provided vital tools for conservation. number of cactus species and even contain endemic genera, of which Aztekium, Geohintonia and Obregonia are examples. ore than half of Mexico is arid The hottest, driest and lowest areas of and semi-arid. There are three this desert are in the Rio Grande and Mmain desert regions in Mexico Trans-Pecos regions of Texas (600 m ele- – the Chihuahuan Desert, the Sonoran vation at the valley bottom), in the Desert and the Tehuacán Valley – each Bolsón de Cuatro Ciénegas, Coahuila of which is distinctive but in different (740 m), and in the Bolsón de Mapimí, ways. In these regions, plants and ani- Durango (1075 m). As elevation increases mals have undergone a profound evolu- so too does the rainfall. The northern- tionary process, which has led to a wide most areas receive some snow during the variety of specialized life forms adapted winter months but, as the elevation to desert conditions. decreases and as one moves south, average temperatures rise and the seasons without With an estimated flora of 3,000 species frost become longer. and an interesting assemblage of animals, the Chihuahuan Desert in northern An amazing diversity of cacti Mexico is one of the most extraordinary ᮤ I.dumortieri H-2420 deserts in the world. The region is bor- The cactus family (Cactaceae) occurs dered by two large mountain ranges, the throughout the Americas, mainly in dry ᮡᮡ Caption . ? Sierra Madre Occidental and the Sierra or highly seasonal regions in Mexico, Madre Oriental. Some half a million km2 southern United States, eastern Brazil, ᮡ Mammilleria pottsil in size, it is the largest desert in North and parts of Bolivia, Peru and Argentina. ᮡ Caption . Top:C-1375 America, extending from central Mexico It comprises about 100 genera and 1500 Opuntia engelmannii north into parts of southeastern Arizona, species worldwide. Its highest diversity is C-191 N conoidea and southern New Mexico and Texas. in Mexico, where about 50 genera and Adjacent to the main part of the 550 species have been recorded. A reflec- Chihuahuan Desert are several disjunct, tion of their uniqueness and representa- 24 PLANTTALK 29 PLANTTALK 29 25 tivity is that 79% of these species are proved revealing. The study was in a mined, we can ask where the geographi- that have affected plant distributions in endemic, i.e. they are not found any- square-shaped polygon of 2855 km2, at cally rarest species are located. If we con- the past. However, during the last centu- where else outside Mexico. the southern extreme of the Chihuahuan sider only the 71 species restricted to one ry, human activities have caused these Desert. This relatively small area is state, we find that the Mexican states of plants to become even more rare. Cacti are among the most characteristic remarkable due to its unparalleled diversi- Coahuila, San Luis Potosí, Nuevo León plants of the Mexican flora, dominating ty of Cactaceae. With 75 species recorded, and Tamaulipas are the richest, with 14, The conservation challenge the landscape in many arid and semi-arid El Huizache has for its size the highest 13, 10, and 9 endemic species respective- parts of the country. However, they are concentration of cactus species in the ly. Thus, these four Mexican states are In Mexico, as well as in several other coun- not restricted to deserts – a considerable American continent. To give an idea of not only the richest in cactus diversity, tries, cacti are a highly endangered group of number of species inhabit wetter, more the extraordinary richness of Cactaceae in but also have the most endemic species. plants. It has been estimated that at least tropical parts of the country; some even El Huizache, in a single area measuring 6° one third of the Mexican species are threat- grow in tropical wet forests. latitude by 6° longitude (c. 114 km2), 41 If we repeat the analysis on a smaller ened species. In the Chihuahuan Desert, cactus species were recorded; in fact, with- scale, we discover that some cactus species natural populations of a number of species The region with the highest diversity of in this same area, 32 species were found in have extremely small areas of distribu- have been considerably affected by the col- cacti, not just in Mexico but in the whole a single locality. Moreover, 63% of the tion, which we can map with a high lection of plants for use as ornamentals or of the Americas, is the Chihuahuan Desert. cactus species found in El Huizache are degree of precision. The most extreme as collectors’ items. Although Mexican In a recent checklist, botanists have record- endemic to the Chihuahuan Desert. examples are of species that are known environmental authorities have developed a ed 315 cactus species there, some 57% of Among these, ten species are outstanding only from one or a few localities, such as legal framework to prevent illegal collecting the total number of cactus species in the for being endemic or nearly endemic to Ariocarpus bravoanus, A. scaphirostris, of flora and fauna, there is evidence that country. The species in the desert belong to this relatively small area. With all these Aztekium hintonii, Echinocereus waldeisii, plants and seeds of Mexican cacti are still 37 genera, 17 of which are endemic. elements in mind, it is not difficult to see Mammillaria carmenae, Opuntia chaffeyi being illegally collected in the field. An Interestingly, almost two thirds of the that El Huizache is a focal point of global and Turbinicarpus subterraneus. additional factor affecting the conservation species are concentrated in four genera – importance for cactus conservation. status of Chihuahuan Desert cacti is the Mammillaria (73 spp.), Opuntia sensu lato Another form of rarity is when the popula- deterioration of the habitat. Agricultural (59 spp.), Coryphantha (38 spp.), and tion numbers of a species are low. development, goat and cattle raising, min- Echinocereus (30 spp.). Conversely, there How rare are rare cacti? Turbinicarpus subterraneus, for example, is ing, road construction, dam building, and are as many as 23 genera each with only known only from two populations in other human activities have modified large one or two cactus species in the region. Rarity is a relatively common phenome- southern Nuevo León; in one of the popu- parts of this desert, in cases dramatically. ᮡ Cuatro Ciénegas (C/H-3901) non among living organisms. Accepting lations only a handful of individual plants A few of the cactus species in the the most simple definition, it is when a have been located, whereas in the second These forms of disturbance have had a ᮢ Caption. Chihuahuan Desert are large, arborescent species has a very small distribution range no more than a few hundred individuals tremendous impact on cactus populations or barrel-like species, such as Myrtillocactus and/or when individuals in the popula- have been found. An even more extreme because these plants usually have slow geometrizans, Isolatocereus dumortieri, tions occur in low densities. Obviously, example is that of Opuntia chaffeyi, also growth rates, long life cycles, and low Echinocactus platyacanthus, Ferocactus pilo- rare species are more vulnerable to extinc- known only from two disjunct localities in recruitment rates from seed germination. sus, F. histrix, and several species of prickly- tion than common ones. For this reason, Zacatecas. Extensive searching at the two Furthermore, as mentioned previously, pears. In contrast, most are small, usually the study of rarity is extremely relevant to known populations of this species has cacti frequently have narrow distribution globose or shortly-cylindrical plants, like conservation biology. And Chihuahuan revealed no more than 20 individuals in ranges and are presented in low popula- members of the genera Mammillaria, cacti are unusually rare. total. Examples such as these portray cactus tion numbers. All of these factors make Echinocereus, Turbinicarpus, Aztekium and species that are both geographically and Chihuahuan Desert cacti extremely vul- Ariocarpus. They are highly cryptic, mean- An analysis of the geographical distribution ecologically rare. Such examples are not nerable to disturbance. ᮡᮡ Caption . Mammillaria (H-2345) ing they are extremely hard to see, as they of each of the cacti in the Chihuahuan uncommon in the Chihuahuan Desert. ᮡ C-404 often look identical to the small stones Desert shows that almost 70% of the The entire cactus family is included in that make up the desert surface. species are strictly endemic to the region. If A different form of rarity in cacti is exem- Appendix II of the Convention of ᮢ Fercocactus histrix (H-1594) we examine the species distribution in plified by Sclerocactus uncinatus, a wide- International Trade of Endangered Analysis of their distribution pattern more detail, we find that the great majority spread species in the Chihuahuan Desert, Species (CITES). Specifically, 33 of the shows that species diversity is not distrib- are restricted to one or a few, usually con- with numerous populations recorded species in the Chihuahuan Desert are on uted evenly. Diversity is concentrated tiguous, states. In fact, more than one third from central Mexico to southern Texas. Appendix I.