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More than half of Mexico is aridand semi-arid. There are threemain desert regions in Mexico

– the Chihuahuan Desert, the SonoranDesert and the Tehuacán Valley – eachof which is distinctive but in differentways. In these regions, plants and ani-mals have undergone a profound evolu-tionary process, which has led to a widevariety of specialized life forms adaptedto desert conditions.

With an estimated flora of 3,000 speciesand an interesting assemblage of animals,the Chihuahuan Desert in northernMexico is one of the most extraordinarydeserts in the world. The region is bor-dered by two large mountain ranges, theSierra Madre Occidental and the SierraMadre Oriental. Some half a million km2

in size, it is the largest desert in NorthAmerica, extending from central Mexiconorth into parts of southeastern Arizona,and southern New Mexico and Texas.Adjacent to the main part of theChihuahuan Desert are several disjunct,

dry, inter-montane valleys, canyons anddepressions that contain an important partof the region’s biodiversity; these desertfragments are home of a considerablenumber of cactus species and even containendemic genera, of which Aztekium,Geohintonia and Obregonia are examples.

The hottest, driest and lowest areas ofthis desert are in the Rio Grande andTrans-Pecos regions of Texas (600 m ele-vation at the valley bottom), in theBolsón de Cuatro Ciénegas, Coahuila(740 m), and in the Bolsón de Mapimí,Durango (1075 m). As elevation increasesso too does the rainfall. The northern-most areas receive some snow during thewinter months but, as the elevationdecreases and as one moves south, averagetemperatures rise and the seasons withoutfrost become longer.

An amazing diversity of cacti

The cactus family (Cactaceae) occursthroughout the Americas, mainly in dryor highly seasonal regions in Mexico,southern United States, eastern Brazil,and parts of Bolivia, Peru and Argentina.It comprises about 100 genera and 1500species worldwide. Its highest diversity isin Mexico, where about 50 genera and550 species have been recorded. A reflec-tion of their uniqueness and representa-

Over the last decade, a team of Mexican botanists have been studyingthe floristics, taxonomy and biogeography of the many cactus speciesthat grow in the Chihuahuan Desert. They have revealed the patternof diversity and distribution of these plants to a level of detail rarely

achieved but vital for plant conservation. HÉCTOR M.HERNÁNDEZ, leader of the team, reports on the research

undertaken and how it has provided vital tools for conservation.

CONSERVATION OF CACTI IN THE CHIHUAHUAN DESERTOF NORTHERN MEXICO

� Caption . . . Top:C-1375Opuntia engelmannii

C-191 N conoidea

� I.dumortieri H-2420

�� Caption . . . ?

� Mammilleria pottsil

that have affected plant distributions inthe past. However, during the last centu-ry, human activities have caused theseplants to become even more rare.

The conservation challenge

In Mexico, as well as in several other coun-tries, cacti are a highly endangered group ofplants. It has been estimated that at leastone third of the Mexican species are threat-ened species. In the Chihuahuan Desert,natural populations of a number of specieshave been considerably affected by the col-lection of plants for use as ornamentals oras collectors’ items. Although Mexicanenvironmental authorities have developed alegal framework to prevent illegal collectingof flora and fauna, there is evidence thatplants and seeds of Mexican cacti are stillbeing illegally collected in the field. Anadditional factor affecting the conservationstatus of Chihuahuan Desert cacti is thedeterioration of the habitat. Agriculturaldevelopment, goat and cattle raising, min-ing, road construction, dam building, andother human activities have modified largeparts of this desert, in cases dramatically.

These forms of disturbance have had atremendous impact on cactus populationsbecause these plants usually have slowgrowth rates, long life cycles, and lowrecruitment rates from seed germination.Furthermore, as mentioned previously,cacti frequently have narrow distributionranges and are presented in low popula-tion numbers. All of these factors makeChihuahuan Desert cacti extremely vul-nerable to disturbance.

The entire cactus family is included inAppendix II of the Convention ofInternational Trade of EndangeredSpecies (CITES). Specifically, 33 of thespecies in the Chihuahuan Desert are onAppendix I. Similarly, 257 MexicanCactaceae are included in Mexico’s offi-cial list of threatened species, of which116 are found in this desert. Manyspecies of Cactaceae are currently beingevaluated using the new IUCN Red ListCategories and Criteria.

Conservation strategies

Cacti are perhaps the group of organismsthat most justify conservation activitieswithin the Chihuahuan Desert. However,

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mined, we can ask where the geographi-cally rarest species are located. If we con-sider only the 71 species restricted to onestate, we find that the Mexican states ofCoahuila, San Luis Potosí, Nuevo Leónand Tamaulipas are the richest, with 14,13, 10, and 9 endemic species respective-ly. Thus, these four Mexican states arenot only the richest in cactus diversity,but also have the most endemic species.

If we repeat the analysis on a smallerscale, we discover that some cactus specieshave extremely small areas of distribu-tion, which we can map with a highdegree of precision. The most extremeexamples are of species that are knownonly from one or a few localities, such asAriocarpus bravoanus, A. scaphirostris,Aztekium hintonii, Echinocereus waldeisii,Mammillaria carmenae, Opuntia chaffeyiand Turbinicarpus subterraneus.

Another form of rarity is when the popula-tion numbers of a species are low.Turbinicarpus subterraneus, for example, isknown only from two populations insouthern Nuevo León; in one of the popu-lations only a handful of individual plantshave been located, whereas in the secondno more than a few hundred individualshave been found. An even more extremeexample is that of Opuntia chaffeyi, alsoknown only from two disjunct localities inZacatecas. Extensive searching at the twoknown populations of this species hasrevealed no more than 20 individuals intotal. Examples such as these portray cactusspecies that are both geographically andecologically rare. Such examples are notuncommon in the Chihuahuan Desert.

A different form of rarity in cacti is exem-plified by Sclerocactus uncinatus, a wide-spread species in the Chihuahuan Desert,with numerous populations recordedfrom central Mexico to southern Texas.We have made over 100 counts of thisspecies along 2–3 km transects, and inmost of the samplings we found that thisspecies is extremely rare in terms of num-bers of individuals. Species such as thisone – geographically widespread butlocally rare – abound in the ChihuahuanDesert. Indeed, it is likely that rarity is anatural phenomenon among cacti in theChihuahuan Desert. It may be a result ofpast ecological factors, such as climaticchanges, including to minimum andmaximum temperatures and to rainfall,

tivity is that 79% of these species areendemic, i.e. they are not found any-where else outside Mexico.

Cacti are among the most characteristicplants of the Mexican flora, dominatingthe landscape in many arid and semi-aridparts of the country. However, they arenot restricted to deserts – a considerablenumber of species inhabit wetter, moretropical parts of the country; some evengrow in tropical wet forests.

The region with the highest diversity ofcacti, not just in Mexico but in the wholeof the Americas, is the Chihuahuan Desert.In a recent checklist, botanists have record-ed 315 cactus species there, some 57% ofthe total number of cactus species in thecountry. The species in the desert belong to37 genera, 17 of which are endemic.Interestingly, almost two thirds of thespecies are concentrated in four genera –Mammillaria (73 spp.), Opuntia sensu lato(59 spp.), Coryphantha (38 spp.), andEchinocereus (30 spp.). Conversely, thereare as many as 23 genera each with onlyone or two cactus species in the region.

A few of the cactus species in theChihuahuan Desert are large, arborescentor barrel-like species, such as Myrtillocactusgeometrizans, Isolatocereus dumortieri,Echinocactus platyacanthus, Ferocactus pilo-sus, F. histrix, and several species of prickly-pears. In contrast, most are small, usuallyglobose or shortly-cylindrical plants, likemembers of the genera Mammillaria,Echinocereus, Turbinicarpus, Aztekium andAriocarpus. They are highly cryptic, mean-ing they are extremely hard to see, as theyoften look identical to the small stonesthat make up the desert surface.

Analysis of their distribution patternshows that species diversity is not distrib-uted evenly. Diversity is concentratedtowards the eastern and southeastern frag-ments of the region, in the Mexican statesof San Luis Potosí, Coahuila, Nuevo Leónand Tamaulipas, each of which has over100 cactus species. It is in this portion ofthe Chihuahuan Desert that most of thebest known, cactus-rich localities arelocated; these include El Huizache, Mier yNoriega, Doctor Arroyo, Matehuala, Tula,Jaumave and Cuatro Ciénegas.

A recent study of cactus diversity in ElHuizache, an area of San Luis Potosi,

proved revealing. The study was in asquare-shaped polygon of 2855 km2, atthe southern extreme of the ChihuahuanDesert. This relatively small area isremarkable due to its unparalleled diversi-ty of Cactaceae. With 75 species recorded,El Huizache has for its size the highestconcentration of cactus species in theAmerican continent. To give an idea ofthe extraordinary richness of Cactaceae inEl Huizache, in a single area measuring 6°latitude by 6° longitude (c. 114 km2), 41cactus species were recorded; in fact, with-in this same area, 32 species were found ina single locality. Moreover, 63% of thecactus species found in El Huizache areendemic to the Chihuahuan Desert.Among these, ten species are outstandingfor being endemic or nearly endemic tothis relatively small area. With all theseelements in mind, it is not difficult to seethat El Huizache is a focal point of globalimportance for cactus conservation.

How rare are rare cacti?

Rarity is a relatively common phenome-non among living organisms. Acceptingthe most simple definition, it is when aspecies has a very small distribution rangeand/or when individuals in the popula-tions occur in low densities. Obviously,rare species are more vulnerable to extinc-tion than common ones. For this reason,the study of rarity is extremely relevant toconservation biology. And Chihuahuancacti are unusually rare.

An analysis of the geographical distributionof each of the cacti in the ChihuahuanDesert shows that almost 70% of thespecies are strictly endemic to the region. Ifwe examine the species distribution inmore detail, we find that the great majorityare restricted to one or a few, usually con-tiguous, states. In fact, more than one third(39.6%) of the species are restricted to oneor two states, and two thirds (67.6%)occur in five or less states. Conversely, onlya few of the species are really widespread,the more extreme examples being twospecies of prickly-pears, Opuntia engelman-nii and O. imbricata, which have beenrecorded throughout the ChihuahuanDesert, including Arizona, New Mexicoand Texas, and elsewhere well beyond theregion.

Once the general pattern has been deter-

�� Caption . . . Mammillaria (H-2345)� C-404

� Fercocactus histrix (H-1594)

� Cuatro Ciénegas (C/H-3901)

� Caption. . .

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Itravel widely in Europe and elsewhere,often with tour groups in search of flow-ers. It was on my return to England

from one such trip to Greece, where Ihad been waxing lyrical to a group ofBritish naturalists about the superbdisplays and varieties of flowers, that Iwent down in May to west Dorset tophotograph the roadsides. It struckme quite forcibly that these roadsides,in their own way, are just as impressiveas those one sees in Greece – and thatperhaps we too often undervalue our ownflora. It also struck me that no-one had pro-duced an illustrated book to show just howattractive our native (and some introduced!) wildflowers can be en masse; and that most people in Britainare probably entirely unaware of what is there to be seen andenjoyed, often on the doorstep.

This was the genesis of the book Flowers at my feet*, produced incollaboration with, and partly in aid of, the UK charity Plantlife,to try to raise awareness of how much floral richness we still havein Britain, and how important it is for all of us. The conservationmovement needs good photographs to help convey the message ofhow valuable and beautiful our botanical heritage is, whether itcomprises rarities, endemics or commoner flowers. For most peo-ple, it is the displays of common flowers that they can best relate

to and appreciate, and I see this book as a way ofdrawing people in, for them to better under-

stand why we need to conserve our flora.

There is no denying that the native floraof the British Isles is not a rich one.Thanks to the destruction of our floraduring last Ice Age, and the re-floodingof the North Sea and English Channelbefore full re-colonization had been

achieved, we have never recovered thefull complement of plant species that

other European countries possess. So ourflora is now poorer than that of equivalent

areas of northern Europe. We have few endem-ic species either, perhaps due to the short time of

isolation, and a cool climate which lengthens generationtimes. Many high profile British ‘rarities’, such as the orchid RedHelleborine (Cephelanthera rubra), or the sage Meadow Clary(Salvia pratensis), to which considerable conservation efforts areunderstandably devoted, are common, even abundant, not faraway on mainland Europe.

Yet there is more to wild plants, and their conservation, than bio-diversity, endemism and rarity. Britain, despite an impoverishedflora, vast areas of intensively managed agricultural land, and ahigh human population density, still retains some wonderful dis-plays of flowers in superb and varied scenery, and this is one of

use that would conserve the flora and besustainable. In countries such as inMexico, the degree of knowledge of thesebasic elements is very low compared tothat in developed countries.

A significant addition to the previouslyexisting protected areas within theChihuahuan Desert was the creation ofthe ‘Real de Guadalcázar ProtectedNatural Area’. Based on knowledge fromour studies on the Cactaceae of ElHuizache, a few years ago, theGovernment of San Luis Potosí passed adecree creating this protected area.Covering 188,758 hectares, it wasdesigned not only to preserve the diversityof cactus species from El Huizache andsurrounding areas, but also the range ofvegetation types found in the area, andseveral other plant and animal groupswhose diversity and degree of endemism iscomparatively greater there than in otherparts of the Chihuahuan Desert. Withoutdoubt, the creation of this protected area isa good example of the marriage betweenscience and conservation practice.

Despite this and other conservationachievements, many of the critical areas ofhigh cactus species richness and endemismare not yet protected. The number of areasunder formal protection in theChihuahuan Desert definitely needs to beincreased. The selection of these should bebased on a thorough analysis of variablessuch as species richness, degree ofendemism, taxonomic uniqueness, andhabitat diversity. Undoubtedly, conserva-tion efforts in this region should be inten-sified, through the consolidation of thealready existing natural protected areas andthe creation of new ones.

Dr. Héctor M. Hernández is SeniorResearcher and current Director of the

Institute of Biology at the NationalUniversity of Mexico, and Chairman of

the Cactus and Succulent PlantSpecialist Group of the Species Survival

Commission, IUCN. E-mail: hmhm@servidor.unam.mx

For more information see ‘A preliminary analysis ofthe diversity and endemism of Cactaceae in theChihuahuan Desert region’, by H.M. Hernándezand C. Gómez-Hinostrosa. In J.L. Cartron, R.Felger and G. Ceballos (eds.), Biodiversity, ecosys-tems, and conservation in northern Mexico. OxfordUniversity Press (in press, due out 2002).

this is a difficult task due to the extraordi-narily high beta-diversity in this region.In order to cover the complex distribu-tion patterns of the numerous narrowendemics, a network of scattered, relative-ly small, reserves has to be created. Infact, the results of our studies have beenextremely useful for the identification ofpotential areas for conservation.

There are several important protectedareas in the Chihuahuan Desert. On theU.S. side the most important ones are LaJornada Experimental Range, Big BendNational Park and White Sands NationalMonument. During the last 30 years,several protected areas have been createdin Mexico and these have contributed invarious degrees to the conservation of theregion’s flora and fauna. Outstandingamong these are the Mapimí BiosphereReserve, the Cuatro Ciénegas WildlifeRefuge, the Maderas del Carmen WildlifeRefuge, and the Metztitlán and SierraGorda Biosphere Reserves. However,most of these still are exposed to negativepressures and some of those on theMexican side lack sufficiently trained per-sonnel and adequate budgets. Also, onlyabout 2.5% of the Chihuahuan Desertarea is under any kind of protection, fartoo low a proportion for such an impor-tant area of plant diversity. Moreover, dueto the lack of comprehensive knowledgeon these areas, regarding, for example,their flora and fauna, the physical envi-ronment (geology, soils, climate) and eco-logical interactions, we have not beenable to propose alternative forms of land

� The configuration of the ChihuahuanDesert is shown by the shaded areaand the position of El Huizache area isindicated by the solid square. The fig-ures are numbers of cactus species perstate / endemic species.

People living intemperate countries with

small floras all too frequentlyunder-appreciate their own

wildflowers. The native displays ofcolour on our own doorstep may,

however, be perhaps the bestadvertisement for the richness, variety

and fragility of plant life. BOBGIBBONS reveals the sheer

splendour of Britain’swildflowers.

BRITAIN in Flower