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Mexico, with a territory of close to 2 million km², is a country of major biodiversity - it is home to between 10% and 12% of all living organisms on the planet (cf. Gómez-Pompa et al. 1994). This is evident in groups such as flowering plants, mammals, birds, reptiles, butterflies and bees (Toledo and Ordóñez 1993). Some authors (e.g. Mittermeier 1988) rank Mexico third in biological richness after Brazil and Colombia, and ahead of Indonesia, Australia, Zaïre and Madagascar. This wealth of biodiversity results from the size of the territory and the great variety of natural habitats that result from its complex orography and geologic history, with its latitudinal location at the intersection of two biogeographic realms: the Neoarctic and the Neotropical.
Mexico is very mountainous, with over half of the territory higher than 900 m above sea-level. There is evidence of past volcanic activity throughout most of the area. The most spectacular volcanic feature is the great Trans-Mexican Volcanic Belt (17°30'-20°25'N), which crosses the entire country at about the latitude of Mexico City. The landscape is characterized by thousands of old cinder cones and dozens of tall volcanic peaks. Volcanism is continuing, with many active or merely dormant volcanoes. Earthquakes are common, mostly along the Pacific coast and Gulf of California; they are also frequent in the Trans-Mexican Volcanic Belt - often causing considerable damage in this heavily populated region.
Mexico can be divided into five general physical regions based on landforms: Baja California and the Buried Ranges of north-west Mexico; the Central Plateau and the bordering Sierras Madre; the Gulf Coast Plain and Yucatán Peninsula; the Trans-Mexican Volcanic Belt; and the Highlands of Southern Mexico.
1. Baja California continues the California Coast Range of the U.S.A. as a mountain-dominated peninsula about 1300 km long and 50-240 km wide. Geologically it has the form of a tilted fault block, with its crest close to the Gulf of California. The Buried Ranges of north-west Mexico rise 600-1500 m above sea-level and form a continuation of the Basin and Range region from southern Arizona, U.S.A. These rugged mountains have been partly buried by immense outwashes of debris westward from the much higher and wetter Sierra Madre Occidental. Geologically the Buried Ranges are very complex, with large deposits of minerals.
2. The Central Plateau or Altiplano extends southward from the U.S.A. to the latitude of Mexico City. Geologically it is a continuation of the Basin and Range region of the U.S. intermountain west, and consists of a series of basins separated by small scattered mountain ranges. It is the most heavily mineralized area in Mexico and one of the great mining zones of the world. The Sierra Madre Occidental and Sierra Madre Oriental form dissected borders respectively on the western and eastern edges of the plateau.
The broad crest of the Sierra Madre Occidental rises to well over 3000 m. The upper portion of the range is covered with thick layers of lava; the western slope forms rugged canyons and narrow ridges descending to the Pacific Coastal Plain. The Sierra Madre Oriental rises to a sharper crest on the eastern rim of the Central Plateau, with elevations to 4000 m. In the north this sierra is composed of several irregular ridges separated by basins descending gradually to the Gulf Coast Plain.
3. The Gulf Coast Plain and Yucatán Peninsula are the two largest lowland areas of the country and are quite distinct. The Gulf Coast Plain is located mostly in the states of Tamaulipas, Veracruz and Tabasco; the low land is very flat and bordered by offshore barrier beaches and lagoons. The Yucatán Peninsula is a limestone platform geologically similar to western Cuba and peninsular Florida, U.S.A. The surface is stony and pitted with sinkholes, and has little surface drainage.
4. The Trans-Mexican Volcanic Belt forms a major geological break with the Central Plateau. Hundreds of volcanic peaks, cinder cones, lava flows, hot springs and ash deposits provide evidence of past and present volcanic activity. The region is bordered on the north by a series of high basins; to the south, the land drops sharply into the Balsas Depression. Included in this belt are Mexico's highest and best known peaks: Pico de Orizaba or Citlaltépetl (c. 5700 m), Popocatépetl (5452 m), Ixtaccíhuatl (5286 m) and Nevado de Toluca or Zinantécatl (4392 m).
5. The Highlands of Southern Mexico are a geologically complex region separated into two sections by the Isthmus of Tehuantepec: the Sierra Madre del Sur in the west and the Chiapas Highlands in the east. The Sierra Madre del Sur is a much dissected mountain system with narrow valleys, a discontinuous Pacific Coastal Plain and a few highland basins. The south-eastern highlands are dominated by the Chiapas Highlands, a plateau rising to 2500 m.
The geological composition of Mexico is thus varied and complex. Nearly 7000 bibliographic references deal with Mexico's geology. The territory was divided into eleven morphotectonic provinces in the recent synthesis by Ferrusquía-Villafranca (1993). This make-up provides a heterogeneous physical-geographical stage for one of the world's most diverse biotas. The regional diversity of Mexico as a result of its complex topography, geography, soils and climate has produced complex mosaics of natural ecosystems and types of vegetation.
Probably the determining factors for the most significant features of Mexico's climatic diversity are the wide range of elevations (mostly 0-5000 m), its location straddling the Tropic of Cancer, and the oceanic influences due to the narrow breadth of its continental mass. The Tropic of Cancer is not only a significant thermal demarcation, it also approximately marks the transitional strip between arid and semi-arid climates of anticyclone high pressures to the north, and humid and semi-humid climates under the influence of trade winds and cyclones to the south.
The complex topography, together with the differences determined by latitude and altitude, result in a climatic mosaic with very many variations (García 1973, 1989).
Average annual precipitation below 500 mm is found north of the 20° parallel, except for a small enclave in the Tehuacán-Cuicatlán Valley of Puebla and Oaxaca with possibly a minimal area of Veracruz (CPD Site MA4). The Sierra Madre Occidental separates the two principal dry zones of Mexico: the Chihuahuan Desert to the east and the Sonoran Desert to the west. The Chihuahuan Desert corresponds to the major portion of the Central Plateau (Altiplano) from west of Hidalgo, north of Guanajuato and Aguascalientes, to the border with the U.S.A., extending somewhat to the north-east coastal plain in the extreme north of Tamaulipas and areas adjacent to Nuevo León. In this region the annual rainfall generally averages between 200-500 mm; a few small areas register less than 200 mm. In the Sonora Desert, comprising the Sonora Coastal Plain and the major part of Baja California, the precipitation is less, particularly on the peninsula, where almost throughout its length the average rainfall is less than 200 mm and in some areas less than 50 mm. For a phytogeographic review of the arid zones of Mexico, see Rzedowski (1973).
The distribution of rainfall during the year constitutes a factor of the greatest importance for the plants, particularly where moisture is scarce, which is the case for most of the country's territory. In general the months of June through September are the rainy season, and May and October can also be moist. On the Atlantic slope and in large areas of northern Mexico, 5-18% of the precipitation occurs as "winter" rainfall, as a consequence of the incursion of polar air masses. In contrast, on the Pacific slope from Sinaloa to Chiapas, the months of November to April usually are absolutely dry.
Winds and cyclones
Mexico is one of the most culturally diverse countries in the New World. In addition to Spanish, there are 54 indigenous languages spoken, in five main language groups: Náhuatl, Maya, Zapotec, Otomi and Mixtec. The population has grown steadily, from 28 million people in 1950 to over 81 million in 1990; more than 40% are under the age of 15. Population density is 73 people per km², with 72% living in urban areas. It is projected that the population will grow just 1.8% between 1995 and 2000, due to population control policies (World Resources Institute 1992).
Vegetation and ecological zones
Mexico has a great diversity of vegetation types, comparable only to India and Peru. Although from studies of physiognomy and floristic composition in Mexico up to 70 different units of vegetation have been distinguished, it is possible to differentiate several principal types of vegetation (Rzedowski 1978) that are conceptually equivalent to the biome category some authors distinguish. This allows for a synthetic profile of the vegetation, and differentiates large natural (terrestrial) habitats or ecological zones.
The zonal habitats in this Regional Overview are defined very broadly (rather than in the usual strict sense, using criteria to define a habitat as determined by the distinctive nature of the species under study). The terrestrial as well as marine territory of Mexico should be classified into units that make sense from a biological point of view and are valid for the broad spectrum of organisms that make up this biotic universe. Mexico may be subdivided relatively easily based on the distribution of vegetation and climate.
Figure 1 shows the distribution of the main vegetation types of Mexico as a function of two main climatic gradients: precipitation and temperature. The first gradient is represented by the average annual precipitation, which ranges from 0-5000 mm; the second gradient is represented by the altitudinal belts, which range from 0-5700 m. The use of certain biogeographic and ecological criteria in this bioclimatic synthesis of the vegetation determines the zonal habitat types or ecological zones in the Mexican territory. Six natural terrestrial habitats, defined by climate, biogeography, ecology and vegetation, are then recognized (Map 9). The detailed methodology used to define these six zonal habitats is given in Toledo (1995a) and Toledo and Ordóñez (1993). This subdivision of the territory coincides in general with those adopted by most authors studying biogeographic and bioclimatic aspects of Mexico (West 1964; Rzedowski 1978, 1993; Flores-Villela 1993; Flores-Villela and Geréz 1988; García 1989).
1. The first of these terrestrial zonal habitats is the humid (and warm) tropical zone, which is characterized by the highest thermal systems and precipitation, and by an original cover of medium and high tropical forests and (very rarely) savannas. This zone extends over nine southern and south-eastern states and covers 200,000 km².
2. The subhumid (and warm) tropical zone extends over 320,000 km² and 21 states. This zonal habitat covers important portions of western and southern Mexico including the Pacific Coastal Plain, the Yucatán Peninsula and as well central Veracruz and southern Tamaulipas. It is characterized by a hot climate with an annual dry period of five to nine months, and by tropical deciduous forests.
3. Cloud forests, with a floristic composition including both boreal and tropical elements, characterize the humid temperate zone. Located at 600-2500 m, this zonal habitat occupies very restricted sites, especially on slopes of the Gulf of Mexico where it extends from Tamaulipas to Chiapas. Distributed in 21 states, it covers 10,000 km².
4. The subhumid temperate zone covers the greatest portion of the mountainous areas of Mexico. This zonal habitat, which is represented by pine, oak and mixed forests, is distributed through 20 states and covers 330,000 km². This vegetation is most extensive in the states of Chihuahua, Durango, Michoacán and Oaxaca.
5. The arid and semi-arid zone is the most extensive zonal habitat, with an area almost equal to half the country 900,000 km². With a very low annual precipitation, this zonal habitat includes two large phytoclimatic zones: the arid zone with scrub vegetation, an annual precipitation of 400 mm or less and 8 to 12 dry months; and the semi-arid zone partly covered with temperate grasslands, with an annual precipitation of 400-700 mm and 6 to 8 dry months.
6. The cool (or alpine) zone, located above treeline at 4000 m on the 12 highest mountains of Mexico, represents the sixth zonal habitat. It is dominated by "zacatonales" or páramos, which are high-altitude grasslands.
7. In addition to these six zonal habitats, at least one more terrestrial ecological zone should be recognized: the wetlands zone, which includes both interior or continental water bodies (e.g. rivers, lakes, swamps) and coastal waters (lagoons).
The floristic knowledge of Mexico is still incomplete. However, since c. 1975 there has been strong development of Mexican botanical institutions, collections and researchers; a consortium of 40 institutions - the National Council of the Flora of Mexico - has been created to carry out the ambitious project to inventory the flora. As a result, almost 20 main regional Floras have either been completed or are in progress (Map 10), as well as numerous Florulas and checklists (Rzedowski 1991; Toledo 1985, 1995c). The collections have grown from 567,000 plant specimens and 18 herbaria in 1974 to 2,100,000 specimens and 40 herbaria in 1990 (Toledo and Sosa 1993).
It has long been known that Mexico together with Central America constitutes one of the regions of greatest plant diversity. However, attempts to quantify this diversity in Mexico have been hindered not only by the lack of a comprehensive inventory of the known species, but also because a significant number have yet to be discovered or described for science. Based on the total number of species (30,489) in six regional Mexican Floras with different - even contrasting - biogeographic and ecological conditions (Table 22), and considering the high proportions of regional endemism in Mexico, Toledo (1995c) estimated that the entire flora of Mexico might have over 30,000 species of vascular plants. (This rough approximation recognized that some species are found in several of these six regions, but that other Mexican species are in none of these particular regional floras.)
Calculations using different methods indicate a moderately rich flora. Dirzo and Gómez (1996) estimated the total vascular flora to be 19,500 by counting the number of species ascribed to Mexico in Index Kewensis (named 1753-1885) and in the Gray Herbarium Card Index (named 1886-1988). Rzedowski (1993) figured floristic richness based on the observation that in latitudes close to Mexico where Compositae are prominent, the ratio of the number of species to the number of genera (s/g coefficient) in this family is similar to the s/g ratio of the entire phanerogamic flora. As a result, Rzedowski extrapolated that there are c. 18,000 species of flowering plants in the country. If undescribed species are added, the possible total reaches c. 21,600, and adding the 1200 species of pteridophytes, brings the total vascular plant species to 22,800. Gómez-Pompa et al. (1994) used an estimate of 26,000 species, which placed Mexico fourth in ranking countries with the most species of vascular plants - after Brazil, Colombia and China.
Families with the largest number of species are listed in Table 23. Six families make up c. 40% of the flora; their relative importance varies from region to region. Compositae, Gramineae and Cactaceae are best represented in the northern and central parts of the country, whereas Orchidaceae and Rubiaceae are more diverse in the southern half. Leguminosae become more abundant in warmer climates. The Orchidaceae may prove to be richer than the Gramineae, as there are many orchid species in Mexico yet to be discovered or scientifically described (Soto-Arenas 1988), whereas the grasses are relatively well known (Valdés-Reyna and Cabral-Cordero 1993).
Map 10 also shows what is known or estimated regarding floristic richness of various states and regions in Mexico. The states with the highest concentrations of species are Chiapas, Oaxaca and Veracruz. A significantly lower number of species occurs on the Yucatán Peninsula, which includes the states of Yucatán, Quintana Roo and Campeche. It is interesting to compare the floristic richness of some areas to their size. For instance, the Baja California Peninsula (two states), with 73,475 km², has 2705 species, whereas the State of Tabasco, which is one-third the size, has one and a half times to perhaps twice as many species.
The flora of Mexico is notable for its richness and also its large number of endemics. Ramamoorthy and Lorence (1987) compiled a list of 283 endemic genera of vascular plants, which represents 12% of the presently estimated total of 2410 genera - one of the highest percentages of generic endemism in the world. Rzedowski (1993) estimated 52% endemism at the species level in flowering plants. Thus with a flora of 20,000 to 30,000 species, 10,000 to 15,000 may live only in Mexico. Villa-Lobos (in prep.) has a database with over 3200 of the species considered to be Mexican endemics. According to Rzedowski (1991), plant endemism at the species level is particularly high in temperate and subhumid montane highlands (70%) and arid and semi-arid areas (60%), moderately high (40%) in subhumid tropical areas, moderate (30%) in humid temperate areas and low (5%) in humid tropical lowlands.
Table 24 shows the richness and endemism of the six terrestrial zonal habitats in Mexico. The reason for this significant wealth of endemic organisms is the existence of a number of regions that function as ecological islands and peninsulas (some extending over large portions of the country), and events and environmental conditions of the geologic past. Particularly, during much of the Cenozoic no terrestrial connection existed with South America. Mexico was a peninsula, much like South Africa today, which experienced sharply varying climatic conditions in contrast with those prevailing on the wider part of the continent.
A remarkable correlation can be observed between high endemism at the levels of genera and species and strong climatic aridity. At the species level, temperate and semi-humid areas are equally endowed with endemics. However in warm humid regions, endemism is poor. In general, endemic genera are better represented in the northern half of the country, and endemic species are more numerous on the Pacific slopes than the Atlantic slopes.
A large assemblage of palaeoendemics can be distinguished, partly concentrated in areas that acted as refugia during epochs of changing climates in the Tertiary and Quaternary. Gypsophytes, which bear a long evolutionary history, stand out among edaphic endemics. An important proportion of very local and/or rare endemic species can be recognized; however, the majority of endemics do not belong to this group - many of the most common and characteristic plants of Mexico are endemic taxa, including a large number of weeds and some cultivars. A rough estimate indicates that Mexican endemism is highest among shrubs and perennial terrestrial herbs, whereas lianas and aquatic plants show the lowest incidence. Among the larger families, Cactaceae, Rubiaceae and Compositae stand out with an average species endemism of 69%, whereas Orchidaceae and Gramineae have respectively 35% and 30% (Table 25).
Evolution of plant lineages in Mexico
The grand profusion and high degree of endemism of Mexico's flora, associated with its remarkable diversity, indicate that the country has been the place of origin and/or development for a great number of plant groups and life forms (Rzedowski 1993).
For example the Cactaceae, albeit of South American origin, have reached their maximum diversity, abundance and importance in Mexico - with c. 900 species, over 95% restricted to Mexico (or sometimes including nearby areas of the U.S.A., i.e. Mega-Mexico 1 of Rzedowski 1991, 1993). It has been known for a century or more that Mexico harbours more genera and species of Compositae than any other country (Turner and Nesom 1993). Within the Mexican Gramineae, after Muhlenbergia (see below), Bouteloua is particularly outstanding. This genus is now widely distributed on the American continent, but its diversity (c. 40 species) is concentrated almost entirely in Mexico and its origin, like that of eight derived satellite genera, is probably in Mexico (Rzedowski 1993; Stebbins 1975). Similar cases are presented for example by Cucurbita (Cucurbitaceae), Dalea and Marina (Leguminosae), Lopezia (Onagraceae), Karwinskia (Rhamnaceae), Lamourouxia (Scrophulariaceae), Achimenes (Gesneriaceae) and Bouvardia (Rubiaceae).
The Fouquieriaceae, endemic to Mexico and some portions of south-western U.S.A. and probably having originated in that region, is notable for its growth forms, which are unusual even among xerophytes. The variations offered by the species of Agave, which at present is not restricted to Mexico, are no less remarkable. The genus has diversified morphologically and taxonomically in Mexico and in all probability originated there. Yucca, Dasylirion, Nolina, Krameria and other taxa present similar examples. In the hot parts of the Pacific drainage area the outstanding feature is the diversity and importance of the some 60 species of Bursera, almost all endemic to Mexico and some portion of Central America, followed closely by many endemics in Acacia, Euphorbia, Ipomoea and the Malvaceae.
Other zones of the country also have been active centres of speciation. The montane cool semi-humid areas deserve special emphasis, as a surprisingly rich flora has evolved, not only of herbaceous plants but also shrubs and trees. Examples of this abundance are Pinus (c. 48 species), Quercus (c. 165 species), Sedum (c. 60 species), Eryngium (c. 50 species), Salvia (c. 300 species - Ramamoorthy 1984; Ramamoorthy and Elliott 1993), Castilleja (c. 50 species), Eupatorium sensu lato (c. 220 species), Senecio sensu lato (c. 180 species), Stevia (c. 70 species) and Muhlenbergia (c. 100 species). The cloud forests, which cover less than 1% of the territory, have been an important theatre of speciation for epiphytes - the most diverse and prolific have been Epidendrum, Peperomia and Tillandsia.
Mexico is also an important centre of evolution for weeds. Unlike countries such as Canada, U.S.A., Uruguay and Argentina, in which almost all of the weedy flora is composed of introduced species, it is remarkable that in most of Mexico native weeds strongly prevail. A great number of them have preserved their endemic character. Several representatives of genera such as Argemone, Sicyos, Euphorbia, Physalis, Solanum, Bidens and Melampodium are weedy and in an active state of evolution.
Mexico has been a notable laboratory for the interaction of plants and human cultures for thousands of years, dating back to the development of pre-Columbian civilizations in Mesoamerica (Bye 1993; Hernández-Xolocotzi 1993). In 1990 there were an estimated 50 indigenous cultures, with a population near 10 million. These ethnic groups occupy almost all of the main ecological zones or zonal habitats recognized in the territory, and have a profound knowledge of local and regional plants (Table 26). Mexico is considered one of the world's main centres of genetic diversification of cultivated plants, and a centre of origin for agriculture - 116 plant species have been domesticated (Hernández-Xolocotzi 1993). The large number of Mexican and foreign researchers interested in ethnobotany in Mexico has stimulated Mexican scientists to initiate their own research (Toledo 1995b). By May 1991 there were 116 studies published on the knowledge and uses of plants among 26 different indigenous groups in the country (Toledo and Cortés, unpublished), plus numerous studies on useful wild and cultivated plants.
The long interaction between indigenous cultures and the flora in Mexico is likely to yield a considerable number of plants with some degree of modern utility. Recent studies include a catalogue of more than 700 species of edible wild plants (J. Arellano, pers. comm.); a database with more than 3000 medicinal species (Argueta 1992); and a list of 1500 useful plants of the humid tropical flora (Toledo et al. 1992). So far over 5000 useful vascular plants have been documented for Mexico.
Mexico's natural plant communities and their botanical richness have undergone great modification, as in other countries in Latin America. The main cause of species loss is deforestation. Mexico has lost a large portion of its original forest cover, particularly during the 1960s, and especially in the tropical lowlands. As in the rest of the Latin American countries, expansion of cattle-ranching has been the leading factor by far in the loss of tropical and temperate forests (Toledo 1992). Other factors causing deforestation are forest fires, agriculture, over-exploitation of the forests and urban expansion.
Although there are no definitive statistics on deforestation rates in Mexico, a recent review on this problem offers the best source of information (Masera, Ordóñez and Dirzo 1992, 1995). Slightly more than 8000 km² of closed forests were lost each year during the mid 1980s, leading to an overall deforestation rate of 1.56% per year (Table 27 and Table 28). This ranks Mexico as the country with the third highest rate of deforestation (after Brazil and Indonesia) - a dramatically accelerating phenomenon in tropical nations during the 1980s (World Resources Institute 1992). Gone are 2450 km² of temperate forests and 5590 km² of tropical forests, affecting extensive areas of four of the main zonal habitats or ecological zones and their floras.
According to the World Conservation Monitoring Centre's plants database, over 1200 species in Mexico are threatened (Table 29). The families with the most threatened species recorded are Cactaceae, Orchidaceae and Palmae. The status of these species results from diverse actions, mainly destruction of habitat through deforestation, illegal commerce of plants, and industrial and urban pollution (especially of aquatic plants).
Gómez-Pompa et al. (1994) report that more than 385 areas are protected in Mexico, and can be grouped into seven broad management categories: (1) protection areas; (2) forest protection zones (for: sierras and their forests; watersheds; streams, rivers and lakes; reservoirs; national irrigation systems; farms and ranches; cities; and reforestation); (3) reserves, variously as: Forest Reserves, Ecological Reserves, UNESCO-MAB or solely national-level Biosphere and Special Biosphere Reserves; (4) parks, with their many variations; (5) Natural Monuments; (6) Refuges; and (7) Biological Stations. They provide a list of the c. 340 protected natural areas that have been created by federal decree, as well as the biologically most important areas created by State decree or private initiative. Forty-nine areas are identified as having priority for conservation; most of them are administered under the responsibility of the Secretaría de Medio Ambiente, Recursos Naturales y Pesca (SEMARNAP) [formerly the Secretaría de Desarrollo Social (SEDESOL), Subsecretaría de Ecología].
Although Mexico has protected some natural areas since the turn of the century (Vargas 1984), establishment of a national protection system for nature is recent. In 1982 the government created the Secretaría de Desarrollo Urbano y Ecología (SEDUE), followed by founding the Sistema Nacional de Areas Protegidas (SINAP) which is now under SEMARNAP. SINAP is the organization specifically in charge of administration of the country's natural reserves. SINAP recognizes nine different categories of protected areas and by 1989 administered 66 natural areas totalling 57,301 km², representing 2.9% of the national territory. The major protected areas under SINAP in 1989 were 9 UNESCO-MAB Biosphere Reserves (44,739 km²), 43 National Parks (6890 km²) and 10 Flora and Fauna Protected Areas (4864 km²) (Table 30).
An analysis of the distribution of land area officially protected in relation to the main natural zonal habitats or ecological zones was made by overlaying the distribution of the protected areas on the Toledo et al. 1989 map of ecological zones (Table 31).
The zonal habitat most under protection is the arid and semi-arid zone, with more than 30,000 km² distributed in 15 reserves. This sizeable coverage is the result of recent establishment of two large Biosphere Reserves: in Baja California (Ojo de Liebre BR) and in Sonora (El Pinacate y Gran Desierto de Altar BR). Nevertheless, there is still not enough protection provided for the xerophytic flora in other parts of Mexico, such as in Tamaulipas, Querétaro, the Mezquital Valley, Veracruz, and especially the Tehuacán-Cuicatlán Valley (Puebla and Oaxaca) (CPD Site MA4) - which is endowed with the most exclusive flora in Mexico (Dávila et al. 1991). It is important to protect enough areas of the arid and semi-arid ecological zone, which has a series of well-defined phytogeographic patterns and richness in endemism and unusual biological forms (Rzedowski 1973).
The humid tropical zone is the second most protected zone in Mexico, with 12,500 km² in six areas. Two Biosphere Reserves include almost the totality of the protected forest: Sian Ka'an (5280 km²) and Calakmul (7230 km²), both on the Yucatán Peninsula. Considering the profound ecological transformation that this zone has endured in Mexico, attention is drawn to the need to protect two other areas of importance: Los Tuxtlas region in Veracruz (Dirzo 1991) and the Uxpanapa-Chimalapa region on the border of Veracruz, Oaxaca and Chiapas (CPD Site MA2). If these two regions are protected and considered together with the Montes Azules BR which includes medium to high forests (see Lacandon Data Sheet, CPD Site MA1), this zone will be well represented in Mexico's system of protected natural areas.
The subhumid temperate zone, dominated by pine and/or oak forests, is third with 3250 km² and 22 areas protected. In relative terms, these areas protect a minimum portion of this zonal habitat, which is considered from a floristic point of view (by numbers of species and endemics) to be the most important in Mexico (Rzedowski 1991). The protected areas cover only a little more than 1% of this zone, perhaps less, because with the exception of La Michilía BR in Durango (see Upper Mezquital River Region Data Sheet, CPD Site MA8), the remainder is in many small National Parks, which tend to be poorly managed (Vargas 1984).
Although the areas that fall within the humid temperate zone (with mesophyllous mountain forests and firs) are only represented by El Triunfo BR (c. 300 km²) in Chiapas, it is likely that this zonal habitat type will be better represented - if El Cielo BR (1445 km²) in Tamaulipas (in CPD Site MA9) and the Sierra de Manantlán (1396 km²) in Jalisco (CPD Site MA6) are included under SINAP. But even if 1000 km² (i.e. 10% of the total) of this zonal habitat can be designated, the only way to provide adequate protection for the flora - some 3000 species - would be to integrate a network of reserves through the country, since the species are distributed in an ecologically insular pattern (which explains the high degree of endemism). At present there are more than a dozen studies on the flora of different sites in Mexico's humid temperate zonal habitat (see Puig, Bracho and Sosa 1983; Luna-Vega, Almeida and Llorente 1989), from which the criteria for its conservation can be generated.
Almost unprotected is Mexico's subhumid tropical zone (primarily lowland deciduous forest), home to an estimated 6000 plant species, with 40% endemism - 2400 species. Presently, SINAP protects 1100 km², which corresponds roughly to two flora and fauna protection areas, both on the Yucatán Peninsula: Río Celestún (590 km²) and Río Lagartos (480 km²). The lowland forests on the Pacific slope, which have been much altered by agriculture and cattle-raising, lack much protection. The same can be said about the Balsas Depression, with an exceptional endemic flora, and the dry zone in the centre of Veracruz.
The last region distinguished is composed of multizonal areas constituting 8460 km², where there is no clear predominance of an ecological zone, but a mosaic of habitats. Basically this regional complexity is represented by three Biosphere Reserves: Sierra de Manantlán BR and El Cielo BR (where mesophyllous mountain forest and pine and oak forests dominate) and Montes Azules BR (which mainly covers a tropical humid area, with ecological islands of temperate forest and lower deciduous forest).
Centres of Plant Diversity and Endemism
Eleven exclusively Mexican centres of plant diversity and one binational centre have been selected. Two identified plant centres encompass primarily north-western Mexico as well as the borderlands of south-western U.S.A. (within Rzedowski's Mega-Mexico 1): the Sonoran Desert, including Baja California (Mexico) - where the sectoral CPD centre has been selected (MA12), and the Apachian/Madrean region of south-western North America. The location of each of the 12 areas is shown on the regional map of Middle America (Map 11). These selected centres of plant diversity that involve Mexico are:
MA1. Lacandon Rain Forest Region
MA2. Uxpanapa-Chimalapa Region
MA3. Sierra de Juárez, Oaxaca
MA4. Tehuacán-Cuicatlán Region
MA5. Canyon of the Zopilote Region
MA6. Sierra de Manantlán Region and Biosphere Reserve
MA7. Pacific Lowlands, Jalisco: Chamela Biological Station and Cumbres de Cuixmala Reserve
MA8. Upper Mezquital River Region, Sierra Madre Occidental
MA9. Gómez Farías Region and El Cielo Biosphere Reserve
MA10. Cuatro Ciénagas Region
MA11. Apachian/Madrean Region of south-western North America
MA12. Central Region of Baja California Peninsula
These sites cover five of the principal terrestrial ecological zones of the country. More than one-third of Mexico's flora is represented in them, including an important percentage of endemic species. Additionally, the primarily U.S. California Floristic Province (CPD Site NA16), including the vegetation type of vernal pools (CPD Site NA16g), extends into north-western Baja California. Portions of Mexico's cold (alpine) zone have not been covered (cf. McDonald 1993) - several National Parks contain samples, nor have grasslands (cf. Valdés-Reyna and Cabral-Cordero 1993) or the wetlands zone (cf. Lot-H., Novelo and Ramírez-García 1993).
There are 65 main wetland sites in Mexico designated to be of special importance and meriting international attention (Scott and Carbonell 1986). This list is headed by the Usumacinta-Grijalva Delta, a vast marsh-mangrove wetland in southern Mexico with 10,000 km² of distributory riverine channels, freshwater lagoons, swamps and seasonally inundated marshes. In addition, the coastal zone of Mexico, extending along 10,000 km, contains over 125 lagoons (Lankford 1977; Contreras 1985). Tropical coastal lagoons are now considered to be among the most biologically important ecosystems on Earth - within them a wide variety of nutrients are concentrated that form essential support for a great diversity of plants and animals (e.g. fishes, crustaceans, molluscs). In addition, most of the tropical wetlands of Mexico are extremely important for the breeding, passage and/or wintering of a rich diversity of waterfowl.
Some of the 12 sites listed above now function as reserves, and others are in the process of becoming protected areas under SINAP. When the areas are provided adequate protection, they will constitute much of the basic network of protected areas where all the phytogeographic regions of Mexico are represented. However, it is essential to identify additional sites in parts of the country that have special natural conditions and concentrations of endemic plants. For example, Gómez-Pompa et al. (1994) list 62 natural areas recommended for protection by Mexican experts. The 12 areas that have been recognized as centres of plant diversity and the other areas specifically mentioned do not represent all of the important plant centres in Mexico. Comprehensive coverage will require considerably more work.
This overview was written by Dr. Victor M. Toledo (Universidad Nacional Autónoma de México (UNAM), Centro de Ecologia, Apdo.Postal 70-275, 04510 Mexico, D.F., Mexico); Dr. Jerzy Rzedowski (Instituto de Ecologia, Centro Regional del Bajío, Apdo. Postal 386,61600 Pátzcuaro, Michoacán, Mexico); and Jane Villa-Lobos (Smithsonian Institution, Department of Botany, NHB-166, Washington, D.C. 20560, U.S.A.)
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