Conservation and Landscape Transformation in ...

Journal of the Southwest 39:371-398

Conservation and Landscape Transformation in Sonora, Mkxico ALBERTO BURQUEZ and A N G E L I N A M ARTINEZ -Y RIZAR Resumen: Conservation y Transformacibn del Ambiente Natural en Sonora. Se presenta una sintesis de la historia y estado actual de las areas protegidas, o en vias de proteccion oficial, en el estado de Sonora, MCxico, y se identifican 10s principales agentes de cambio antropog h i c o . Las primeras reservas naturales en Sonora aparecieron muy tardiamente. N o fut sino hasta mediados de 10s aiios 30s que se declararon algunas areas protegidas en esta region, y para 1994 ninguna de las areas declaradas alcanzaba aun estandares internacionales de manejo. Entre 1936 y 1939 se declararon cinco areas protegidas con el proposito de asegurar el funcionamiento de las principales cuencas hidrologicas, propiciar un mejor entorno citadino y salvaguardar la fauna silvestre. Estas reservas n o fueron formalmente delimitadas. Sin embargo, estimaciones actuales indican un area aproximada de 260,000 ha. La siguiente reserva n o se decreto sino hasta 1963 con la declar a c i h de refugio de las islas del Golfo de California. El siguiente impulse a la proteccih del medio natural Sonorense se dib en 10s 90s con las declaraciones de las reservas de la biosfera de "El Pinacate y Gran Desierto de Altar," y del "Alto Golfo de California y Delta del N o Colorado," ademas de la "Zona Protectora Forestal y de la Fauna Silvestre y Acuatica Sierra de Alamos y Arroyo Cuchujaqui," y el decreto estatal "Presas Abelardo L. Kodriguez y El Molinito." Actualmente, aproximadamente un 8 por ciento (1,500,000 ha) de la superficie terrestre del estado de Sonora presenta a l g h grado de protecci6n oficial. De esta area, la mayoria es desierto. Sin embargo, una gran proporcion de las areas decretadas esta aun sin proteccibn real, presenta escaso monitoreo y carece de planes de manejo. Existen ademas numerosas areas en Sonora listadas con atencion prioritaria para su proteccion a nivel national. Para algunas de estas se tiene informacibn preliminar, mientras que en otras existen grandes vacios de conocimiento. Recientemente Sonora ha presentado un increment0 en poblacion y actividades productivas sin regulacibn estricta que han impactado profundamente el ambiente natural. El impacto es m b aparente en las planicies y la faja costera, mientras que en las regiones montaiiosas hacia el este

del estado, con la reciente apertura de nuevas vias de comunicacion, se esta produciendo un cambio extensive en en 10s ecosistemas serranos. Los prilicipales agentes de cambio ambiental han sido la agricultura, la ganaderia, la extraccibn forestal, la mineria, y el desarrollo ~ ~ r b a n o , mientras quc en 10s ambientes marinos, la sobrepesca y la contamination han crcado cambios ambientales considerables. En Ios ambientcs terrestres destaca como el principal agente de cambio, la ganadcria de bovinos, la cual 113 sido una industria tradicional c11 la region. Las elevadas cargas animales, que en algunos casos rebasan el 200 por ciento, han deteriorado notablemente 10s pastizales destrticos, y el desierto propiamente. Ademas, 10s aclareos para scmbrar LIII pasto africano, el zacate buffel, hail iniciado un ciclo de pasto-fuego que ha alterado notablemente la diilimica de muchas comunidades deskrticas. z de la superficie estatal. Este pasto esta ahora naturalizado en m h del y En el medio marino, la sobrepesca, la disniinucion de 10s caudales de los rios por la construcci6n dc presas, y la contaminaciGn con agroquimicos ha modificado profundamcnte 10s ecosistemas del Golfo de California.

INTRODUCTION

Mtxico is considered a "mega-diversity" center. It is among the ten richest areas in the world in terms of biodiversity (Flores and Gerkz 1988). The efforts t o set up nature reserves in Mtxico have been uneven over time, with three major peaks during its recent history: one at the turn of the century, another during the mid-1930s, and the last one, still ongoing, beginning in the 1980s. These early attempts were oriented toward protecting dense, tall temperate forests, disregarding tropical lowlands and drylands, which were considered more abundant and less valuable ecosystems. However, despite the great emphasis given to temperate and more recently t o wet lowland tropical forests, the largest share of the richness of Mexican species is in the drylands, which cover about 7 0 percent of the country and have a high degree of endemism (Rzedowski 1991a). Yet, drylands have received little recognition as areas of high biodiversity, and until recently little effort was made to protect them (Flores and GerCz 1988; Janzen 1988). Mexico's protection of the environment began at about the same time as in the United States. In 1876, the first protected area, Desierto de 10s Leones, was decreed t o safeguard a small portion of the extensive

Consevvation in Sonora, Mexico

I 1

+

373

pine and fir forests of central Mexico. Since then, the Mexican government has tried t o develop a copy of the National Park Service of the United States. The main emphasis was on isolating small areas considered as national monuments. These were set aside mainly for recreation, rather than for management or protection of biological diversity. This first reserve system added only nine more areas in the next fiftyeight years following the creation of the Desierto de 10s Leones Kesenre (Anaya et al. 1992). In the late 1930s there was intense lobbying by the Director of Flora and Fauna, Miguel Angel de Quevedo, t o protect natural areas in Mkxico. Again, the main emphasis was the protection of natural resources of immediate use, such as forests, the protection of upriver basins from crosion, and the establishment of large wildlife game ranges. As a result of this early effort, about 800,000 ha distributed over seventeen Mexican states were protected (Colosio 1993). However, these rcserve areas have not been part of the development strategy of the country and have never been allocated adequate management or clear boundaries. In many cases serious problems of land ownership have arisen within reserves. Also, nature reserves have repeatedly been used simply as land banks for future development (Anaya et al. 1992; INE/CONABIO 1995). Despite the long history of reserve designation in Mkxico, until 1936 there was not a single protected area in Sonora, and in 1994 there was still n o area in the state attaining minimum international management standards of protection. In this paper we briefly describe the present extent and history of the reserves in the state of Sonora and discuss the major threats that the Sonoran natural environment is facing.

EXTANT RESERVES I N S O N O R A

Like most of the Southwestern U.S.A. and northern Mkxico, the state of Sonora has been associated with the desert. However, the Sonoran landscape encompasses a wide range of biomes including most of the major Mexican vegetation types (Gentry 1942; White 1948; Marshall 1957; Rzedowski 1978; Brown and L o w 1980; Brown 1982; Burquez, Martinez, and Martin 1992). Sonora is a major reservoir of what kedowski (1991a, b ) called the "genuinely Mexican speciesnspecies that have differentiated mainly in the arid and semi-arid zones of northern Mkxico. Current estimates are that the Sonoran flora in-

cludes as many as 4,500 species (Rzedowski 1991a; Felger and Johnson 1995). This figure represents about 20 percent of the Mexican flora in an area less than 10 percent of the country (following Rzedowslci's [1991a] estimate of a national total of 22,000 known species). As reviewed by Fclger and Wilson (1995), even more striking figures are found for the fauna, \vhich encompasses a rich, poorly lcno\\m asscniblage of species with Neotropical and Neoarctic affinities for almost ever), major taxon. The Soiloran Desert, as defined by Shreve ( 195 l ) , covers a wide rangc of environments, from extremely xeric to relatively mesic. The former are readily recog~iizcdas deserts by the scant plant cover and extreme climatic constraints. Howe\w, much of the desert o\fres its diversity to diffuse transitions into varied thornscrub and tropical deciduous forests (Burquez 1997; Burquez et al. in press; Felgcr and I,o\ire 1976; Martinez-Yrizar, Bi~rquez,and Maass in press). The desert also has oases that arise from seeps and artesian \veils fed by permanent aquifers originating in the mountains, as \veil as forests that flourish in tlie great desert river deltas (Yetman and Biirqucz 1996; Felger in press). Toward the east, tlie great Sierra Madre Occidental harbors at different elevations tropical deciduous forests, oak noodlands, pineoak forests, and conifer forests (Rurquez, Martinez, and Martin 1992; Felger and Johnson 1996), while on the shores of the Gulf of California, wetlands add to the overall diversity of Sonora, including the northernmost mangrovc stands in the continent (Felger and Moser 1985). The great diversity of Iregetation, habitats, and plant and animal species found in Sonora is primarily caused by tlie une\.en distribution of precipitation, surface nrater, and climate, along mith the estraordinary variation in topography, geological substrates, and soils (Bro\vn 1982; Burquez et al. in press). In spite of the biological and habitat richness of the state of Sonora, no nature reserves were decreed until 1936-1939. During these years, five areas were established: ( 1) Arroyo Los Nogales, near Nogales; (2) Cajbn del Diablo, southvest of Hermosillo; ( 3 ) Zona Protectora Ciudad de Her~nosillo;( 4 ) Sierras Los Ajos, Buenos Aires, y La Phica, southeast of Cananea; and ( 5 ) Reserva Forestal Nacional y Kefugio dc Fauna Silvestre Ravispe, near the town of Bavispe (figure 1). Bavispe comprised five areas, of which Los Ajos, Ruenos hires, y La Purica were included as allotment number 4. Some of these reserves were not strictly delimited, but present estimates indicate a total area of about 260,000 ha (table 1 ). These actions protected about 1.4 percent of the

I

1

Ftqure I . Schematic location of reserves almzdy decreed with some form of ~nanapevuzent (shaded in p a y ) , decreed but without manapement Cqraded inpi,ay), and proposed as hi&-priority m,eas for consernation in the state cj'Sonosa (dotted). Bavispe includes the j?)lloa&p mnLqes:El T&e, Jiwiquipa/ Pilares, Las I&sitas, Los Ajos, fhenos Aires y La Wrica. and La Madem. The reserves of'Barw M. Goldwater Air Foiace Kanpe, Cabeza 1'~ietaWildlife Kefupe, awd Oyman Pipe Cactus National Monument (diaponal lines) in the U.S.A. aire indicated t o show the possible extent of the US.-Mtxico International Sonovan Desert Reserl~e.Data derivedfiom the ofJcinl deci)eesin Dial& Qficial de La Nacidn, Benitez and Lon (1996) and unpublished reportsfiom Centro Ecolupico de Sonora.

state of Sonora, and included mainly pine and oak forests. Only two of these reserves, Cajon del Diablo and Zona Protectora Ciudad de Hermosillo, included portions of the Sonoran Desert. Twentyfour years later, in 1963, a sixth reserve was established: Isla Tiburon. O n paper the protected areas amounted t o about 380,000 ha (2.1 percent of the state), but by then the edicts protecting the environs of Nogales and Hermosillo were long ignored or forgotten. These early protected areas near major towns were cannibalized by city growth and dcvelopInent. The remaining reserves were protected by their isolation. However, they did not escape logging (the high sierras close t o Cananea and Bavispe), cattle ranching (all reserves, excluding the Gulf of California islands), the introduction of large wild herbivores, and the overexploitation of fisheries (Gulf of California islands). A new edict, including an increase in area and designation change, was decreed for the islands of the central Gulf of California in 1978. This act increased the protected area by nearly 30,000 ha. About ten years later a small area in the magnificent Sierra El Pinacate was protected as a wildlife refuge (table 1; Bi~rquezand Castillo 1994). At about the same time, in 1988, a prominent desert reserve in Raja California was established: Reserva de la Biosfera del Vizcaino. The last addition was a major step in conservation of Mexican drylands because it gave protection t o the largest area of Sonoran Desert so far decreed-more than 2.5 million ha (Ortega and Arriaga 1991). Work by Centro de Ecologia, Universidad Nacional AutGnoma de Mexico (now Instituto de Ecologia, UNAM) and Centro Ecologico de Sonora led to the decree of the Mexican Biosphere Reserve "El Pinacate y Gran Desierto de Altar" in 1993 (table 1). Along with the biosphere reserve of El Vizcaino the area of El Pinacate includes the most pristine areas of the Sonoran Desert. At the same time, another Sonoran biosphere reserve, proposed by a consortium of Mexican institutions, was decreed: Reserva de la Biosfera del Alto Golfo de California y Delta del Rio Colorado. The latter reserve is mainly marine, with approximately 70 percent of its area in the sea (i.e., not accounted as land). Other recent additions of protected land in Sonora are the Presas Abelardo L. Rodriguez and El Molinito (Del Castillo 1994), and the Zona Protectora Forestal j 7 de la Fauna Silvestre v Acuitica Sierra de Alamos y Arroyo Cuchujaqui. The former includes large stretches of standing freshwater in two artificial impoundments, and the highly disturbed riverside of the Kio Sonora near Hermosillo, ~vhilethe latter

Conservation in Sorzom, Mexico

+

379

provides a measure of protection for highly diverse tropical deciduous and oak forests in southern Sonora (table 1). Today about 8 percent (1,500,000 ha) of the land in the state of Sonora is under some form of protection. This figure does not include reserves lost to city development and marine areas. So far, only fbur areas are operating formally as rcsen7es, with a permanent staff and headquarters: the two biosphere reserves, the area of Sierra 1,os Ajos, and the protected area in Alamos. All other rrservcs haw no protection, except for that given b!, the edicts and the natural protection offered by their isolation.

P L A N N E D RESERVES

,

In addition t o the government environmental agency (Secretaria del Medio Ambiente, Recursos Natiirales y Pesca: SEMAlWAP), several institutions are presently selecting and studying high-priority areas for protection in Mtxico. These organizations include academic institutions such as the National University of Mexico (UNAM) and state universities, the Comisihn Nacional para el Conocimiento y Uso de la Biodiversidad (CONABIO), several state-le\d agencies, and many nongovernmental organizations (NGOs). For Sonora, the Centro Ecologico de Sonora and CONABIO (Benitez and Loa 1996), with the aid of many NGOs and academic institutions, have proposed high-priority areas for conser\.ation in various parts of the state (table 2 ) . If approved, decreed, and provided ~vithan endo\vn~entand proper regulations, in~portantportions of the Sonoran L h e r t , Gulf of California marine environment, and the sky islands in the Sierra Madre Occidental will receive protection and more adequate management. Proposed reserves such as Sierra San Luis, Sierra La Mariquita, Sierra El Tigre, Mesa El Campanero, and Sierra Mazatan are sky islands a,ith pine, oak, and pine-oak forests. They also include portions of desert grasslands and small areas of foothills thornscrub, as d o the alread!: established reser\.es at Sierras Los Ajos, Buenos Aires y La Purica, Bavispe, and Sierra Mazatan (for S. Mazatan see Navarro 1985). Coastal environments are considered in the proposed reserves of Las Bocas, Bahia de Lobos, Estero El Soldado, Cajon del Diablo y Caii611 El Nacapule, and Sierra Bacha. These reserves also include considerable portions of Sonoran Desert or, as in the cases of Bahia de Lobos and

Table 2. Proposed nature reserves in Sonora, Mkxico. Location gives the municipio (county) where most of the reserve is located. Locations are illustrated in figure 1. Keser~reName

Location

Main Biomes

RESERVES STILL NOT D E C R E E D

Las Bocas

Coastal thornscrub Coastal wetlands

h h i a de Lobos

Coastal thornscrub Coastal wetlands

Estero El Soldado

Guaymas Coastal wetlands

Sonoran Desert

Sierra Bacha

Pitiquito

Sonorm Desert

Bahia San Jorgc

Caborca Coastal wetlands

Sonoran Desert

Sierra El Viejo

Caborca Foothills thornscrub

Trincheras

Trincheras

Sonoran Dcscrt

Sierra Libre

La Colorada/Hermosillo/ Guavmas

Sonoran Desert Foothills thornscrub Tropical deciduous forest Sonoran Desert Foothills thornscrub Tropical deciduous forest

Sierra Bacatete

San Javier/YCcora/Onavas Tropical deciduous forest Foothills thornscrub

Riparian

Soyopa/Sahuaripa Tropical deciduous forest

Foothills thornscrub

Mazocahui/ Puerta dcl Sol

Ures/Baviacora

hparian Foothills thornscrub Tropical deciduous forest

Sierra San Luis

Agua Prieta

Desert grasslands Oak-pine forests

San Javicr/Tepoca

Mesa El Campanero y Arroyo El Reparo

Pine-oak forests Tropical deciduous forest Highland wetlands

Conservation in Sonova, Mexico

+

38 1

Table 2. (Cmtz'nucd) lieserve Name

Location

Main Biomcs

Bavispe/Los Ajos/El Tigre/Bucnos Ares/ La I'i~rica

Bavispe/Nacozari/ Moctezun~a/othcrs

Oak-pine forests l k s e r t grasslands Highland wetlands

Sierra La ~Mariquitay Kio San Pedro

Cananea/Kaco/ Santa Cruz

Oak-pine forests Desert grasslands Highland vetl lands

El Carrizo

Sonoran Desert Desert grasslands

Sierra Mazatan

Mazatin/Ures

Sonoran Dcscrt Foothills thornscrub Oak woodlands

Ccrro Agualurca/Ccntro Ecolbgico de Sonora

Hermosillo

Sonoran Desert Foothills thornscrub

Santuario del Aguila Calva

Sahuaripa/Soyopa

Foothills thornscrub Tropical deciduous forest

RESERVES ALREADY DECREED, BUT W I T H A NEW PROPOSAL I N PROGRESS

Sonoran Desert Foothills thornscrub Tropical deciduous forest Marine Islas Tiburhn y San Esteban

Gulf of Califorma

Sonoran Desert Foothills thor~lscrub Marine

Sierras 1.0s Ajos, Buenos Ares y La Purica

K E Sonora

Oak-pine forest Desert grasslands Highland wetlands

Las Bocas, coastal thornscrub (Friedman 1996; Biirquez et al. in press). These reserves protect a gradient along the coast harboring notable transitions in vegetation. Marine environments and coastal wetlands have already attained some protection by the edicts creating the reserves of the islands of the Gulf of California and Alto Golfo de California y Delta del Rio Colorado. Mesa El Campanero y Arroyo El Keparo, San Javier/Tepoca, and Soyopa/Sahuaripa are proposed re-

serves protecting riparian habitats, foothills thornscrub, and the most northern tropical deciduous forests o n the continent. The last vegetation type is one of the least protected in Mtxico (see Florcs and Gertz 1988) and is among the most endangered tropical major ecosystem (Janzen 1988). The areas of Soyopa and Sahuaripa have also been recognized as important breeding areas of the endangered bald eagle (G. Morales, personal communication). Planned reserves i~icludingsubstantial portions of Sonoran Desert are located at Caj6n dcl Diablo y Cali611 El Nacapule, Sierra Libre, Sierra Bacatete, Sierra El Viejo, Sierra Bacha, Kancho El Carrizo, Sierra Mazatan, Trincheras, and Cerro Agualurca. The area of Cajbn dcl lliablo v Catibn El Nacapulc, located in Gentry's Guaymas Monadnock (Gentry 1949), has a wealth of endemics and disjunct tropical taxa shared with Baja California and the Sierra Madre foothills along the Pacific coast of Mtxico (Turner, Bo\vers, and Burgess 1995; Biirquez et 31. in press; Felger in press). Similar cases are those of Sierra Libre, l'uerta del Sol/Mazocali~ii, and Sierra Racatete, virtually unexplored but known to have a rich disjunct tropical flora and archeological remains of Seri, Opata, and Yaqui cultures (see Yetnian and Bi~rquez 1996). Trincheras is a rich area of Sonoran Desert famous for its agricultural terraces and many archeological remains. Sierra El Viejo near Caborca, in addition to having renlarkable transitions of several Sonoran Desert subdivisions, is probably the north\vesternniost extension of foothills thornscrub (Broum and Lo\ve 1980; Bro~vn1982). Here, large herds of endangered desert bighorn sheep (Ovis cnnndensis) are still found. The only population of cirio, or boojum tree (Foztquieria colz~tnnnris),o n the mainland is located in the Sierra Bacha, along the coast south of Pucrto Libertad (Hastings and Turner 1965). In this pristine area of the Central Gulf Coast subdivision of the Sonoran Desert there are also populations of desert bighorn and archaeological remains of the Seri culture. South of Benjamin Hill in the northern desert grasslands of the Plains of Sonora subdivision is l b n c h o El ebrott, and Haro 1992). Rancho El Carrizo supports fine exanlples of the desert grassland-shrubland continuum of the Plains of Sonora (Burquez et al. 1998). These areas, ifdecreed, will add distinct units of the Sonoran Lksert t o the existing reserves of El Pinacate y Gran Desierto de Altar (Burquez and Castillo 1994) and Alto Golfo de Califor-

Consevvation in Sonora, Mexico

O

383

nia y Delta del KOColorado, both in northwestern Sonora, as well as t o the special microcosms of the protected Islas del Golfo de California and C a j h del Diablo.

H U M A N IMPACTS I N T H E S O N O R A N DESERT R E G I O N

The extinctions of the megafauna following the arrival of man in the Americas in prehistoric times (Martin 1984) most likely generated the first large-scale human impact in the region. Paradoxically, the arrival of Europeans in the sixteenth century restored, in part, this long-lost large herbivore guild. Nevertheless, cows, horses, goats, and burros are only a small sample of the missing megafauna, and have been stocked at densities far higher than can be sustained, altering again the status of the Mexican vegetation. Europeans also brought new crops and their associated weeds, as ~vellas other animals in the form of crop pests, pets, and useful fauna such as the honeybee. Some of these are now naturalized at the expense of habitat loss to native species. Recently, the failing of large-scale agriculture, the effects of extensive and intensive cattle ranching, the damming and silting of rivers, the indiscriminate logging, and over-exploitation of fisheries have highlighted the everincreasing change in the landscape and resources o f t h e region.

Until recently, most of the settlements in the state of Sonora were on the western edge of the Sierra Madre and in the Madrean foothills. Pre-Columbian agriculture was common along the river margins, sometimes with sophisticated irrigation systems (Camou 1991; Moreno 1992). Although agricultural development in the desert was relatively small in scale and confined to areas with a shallow water table, nonetheless it had seriously affected riparian habitats by the end of the nineteenth century (Bahre 1991; Camou 1991). After many years of low rates of population increase, growth in Sonora has accelerated rapidly. The coastal plain did not play a significant role in development until the appropriation of the vast underground aquifers in the Rio Concepci6n, Rio Sonora, and Kio Mitape basins in the late 1940s. The deltas of the Rio Mayo and Kio Yaqui

were not extensively altered ~ ~ n tthe i l construction of dams upriver in the 1940s, 1950s, and 1960s. These reservoirs opened the way t o further growth through the generation of electricity and the stim~dusto rapid expansion of agriculture. Ry the late 1970s the deltas were almost entirely converted t o agriculture. Within a few years of intensive developnicnt, huge cxpanses of natural vegetation had been cleared. T h e vast mesquite forests o f the Llanos de San Juan Bautista, in the delta of the Kio Sonora, disappeared \vith the colonization of the Costa de Hermosillo irrigation district (Felger and Loive 1976). The progressive salinization of the aquifer and the increasing cost of water extraction have caused a decrease in the land devoted t o crop production. From the original 150,000 ha cleared for agriculture, only about 70,000 ha remain operative. The rest are derelict agricultural fields with allnost no vegetation cover. The non-sustainable operation of this district, the ever-lowering \vatu table (dropping up to 1 nl yr-I), and the faltering local economy are documented by Moreno (1994). In the Rio Yaq~liand No Mayo deltas, more than 1,000,000 ha of mesquite, cottonwood, and willovr riparian forests and coastal thornscrub disappeared once dams uprijrr started t o operate. These rivers followed the same path of \!egetation eradication as the delta of the Colorado after the construction of the Hoover Dam (Felgcr 1993; Glenn e t al. 1992, 1996). As with the Colorado, both irrigation districts now face serious environmental problems because of poor drainage, salinization, and toxic levels of pesticide and fertilizer contamination (Celis 1992).

Cattle Ranchin. Cattle raising has had a relatively brief history in the dr!Lu~ds of North America when compared to agriculture. However, it is no\v recognized as a hallmark of the South\\wt. By the sixteenth century, preColumbian agricultural towns along the fertile foothills of the Sierra Madre and along major dcsert rivers, as well as hunter-gatherer societies in the desert, had attained a precarious equilibrium \vith their use of resources. This equilibrium was broken by the arrival of Europeans, who introduced cattle as a new form of land use. Cattle created a major source of conflict between the new pastoralists and the agriculturistscattle did not respect boundaries, particularly those of unfenced crops (Camou 1991; Moreno 1992; Doode and Pkrez 1994). I t also affected hunter-gatherer societies because, from the natives' point of vie\{,, cat

Consevvation i n Sonora, Mexico

+

385

tle was a ne\v element of the ecosystem-a rich source of animal protein roaming in the common land, ready and easy t o harvest (Felger and Moser 1985; Thompson 1989). Until the last century, only localized areas of desertscrub \vere used for cattle raising. Because of constant raids by Native Americans, cattle were confined t o small areas, which were in most cases overgrazed. At the turn of the century, the organization of the large "haciendas" allo\ved the extensive exploitation of the arid lands. Large cattle herds transformed the natural balance between desert grasslands and desertscrub, contributing to the so-called invasion of mesquite and thornscrub (Johnston 1963; Hasti n g ~and Turner 1965; Bahre 1991; Archer 1989, 1994; Burquez et al. 1998). During the Mexican Kevolution cattle stocks diminished drastically (Machado 198 1 ), allo\ving some recovery of the rangelands. However, the cattle industry regained momentum mainly in northern MCxico, transforming, at least in part, large expanses of semi-arid and arid lands (Barral 1988; Ezcurra and Montana 1988; Bi~rquezet al. 1998). The introduction about thirty years ago of the African buffelgrass (Peuzennisetuvn ciliavis) from the United States Soil Conservation Service (Cox et al. 1988; Johnson and Navarro 1992), has led to alteration beyond recognition of large expanses of Sonoran drylands. Buffelgrass has been planted for cattle forage by stripping clear the desertscrub and thornscrub. It increases the range productivity of cattle forage by about three times (Hanselka and Johnson 1991). Unfortunately, it does so at the expense of the eradication of desert perennials that provide winter and spring forage when buffclgrass is dormant. The replacement of perennials coupled with cattle overstocking has led to a perceived higher occurrence of drought, despite the fact that rainfall patterns have not changed appreciably in this century I t is paradoxical that the desert has historically been devoted t o cattle raising, the most waterdemanding way of using the land. l'roducing 1 kg of beef in the desert requires 100,000- 200,000 kg of water, while most crops can yield the same amount of energy ~ v i t honly 500-2,000 kg of\vater. Broiler chickens, a source of high-quality protein, necd only about 4 percent of the water used by beef per gram of protein produced (Pimentel et al. 1997). years of management, buffelgrass Ranchers linou. that after a productivity decreases and needs prescribed fires t o increase fertility and t o stop the return of some desert and thornscrub species. Also, some time after the deliberate establishment of buffidgrass there is

enough fuel accumulation in the form of indigestible stubble t o allow extensive natural burning. Since Sonoran Desert plant species are not fire-adapted, a cycle of decreasing biodiversity begins-converting the rich desert into a species-poor grassland (Burquez et al. 1998; Miller, Biirquez, and Martinez-Yrizar in review). Paired samples on neighboring plots with and without introduced buffelgrass show an order of magnitude decrease in species numbers, and a fourfold decrease in standing crop biomass (maximum of 5 Mg ha-' above-ground standing crop in buffelgrass \7ersus20 Mg ha-1 in natural desertscrub, Burquez et al. 1998). Central Sonora, particularly the Plains of Sonora desert subdivision, has been the most severely affected area, with about 1,000,000 ha already cleared for pastures (Johnson and Navarro 1992; A. Burquez unpublished data), and the government has called for as much as 6,000,000 additional ha. Adventive buffelgrass is now expanding its range through repeated natural burning of the desertscrub, and is present throughout Sonora at elevations below 1,000 m (Cox et al. 1988; Burquez and Quintana 1994; Felger 1990, 1993; Yetman and Burquez 1994; Miller, Bi~rquez,and Martinez-Yrizar in review). Given the government subsidies to establish exotic introduced grasslands, to maintain large cattle herds, and t o support marginal cattle ranching, the desert and thornscrub in Sonora will probably be replaced in the near term by ecosystems with significantly lower species diversity and reduced structural complexit)., unless control measures are implemented. Mesquite Ltcqivg, Chavcoal Production, and Clearivgs The largest production of legal hardwood legumes for charcoal is in the districts of Herrnosillo, Guaymas, Puerto Pefiasco, Sonoyta, and San Luis Rio Colorado. These districts have historically accounted for more than half of the mesquite extraction in Sonora (for example, in 1985, 74,700 out of a total of 135,300 m3; INEGI 1990). Former mesquite t'orests have disappeared at an alarming rate due t o demand for charcoal in Sonoran and North American markets. The establishment of clearings for buffelgrass is closely related t o charcoal production. The woody remains of clearings are piled in long chorizos (sausages), or strips of dead vegetation, that are later sorted for file1 lvood and charcoal. Populations of ironwood (Olneya tesota), among the oldest plants in the Sonoran Desert, have experienced shifts in their population structure toward smaller sizes because of illegal logging for charcoal, deswzontes (clearings), and the gathering of wood for carv-

Conservation i n Sorzora, Mexico O 387 ings (Burquez and Quintana 1994; Nabhan and Plotkin, 1994). Oldgrowth ironwood is a major co~nniunitystructuring element of the southern Sonoran Desert, allowing tlie persistence of many species and forming true islands of diversity under its canopy (Bi~rquezand Quintana 1994; Tewksbury and Petrovich 1994). Although removal of this old growth does not endanger ironwood populations in the short term, it does affect the regeneration niche of many species that grow under their canopies. Any reduction in this particular habitat can endanger populations with low numbers, affecting such plants as the queen of the night cactus (Peniocewzzs stviatz