Endemic angiosperm lineages in Mexico: hotspots for conservation

 

Linajes de angiospermas endémicas en México: zonas de alto endemismo para la conservación

 

Victoria Sosa^1,4 and J. Arturo De-Nova^2,3

 

¹ Instituto de Ecología A.C., Biología Evolutiva, Apdo. postal 63, 91070 Xalapa, Veracruz, Mexico.

² Universidad Autónoma de San Luis Potosí, Instituto de Investigación en Zonas Desérticas, Altair 200, 78377 San Luis Potosí, Mexico.

³ Universidad Autónoma de San Luis Potosí, Facultad de Agronomía, km 14.5 carretera San Luis Potosí-Matehuala, 78321 San Luis Potosí, Mexico.

⁴ Autor para la correspondencia: victoria.sosa@inecol.edu.mx

 

Recibido en abril de 2012.
Aceptado en junio de 2012.

 

ABSTRACT

As a megadiverse country, Mexico harbors 4 to 8% of the flora of the world and of this, 51% is endemic. There is concern because several factors are impeding its conservation. In this paper, areas of endemism for the flowering plants of Mexico are identified to prioritize regions for conservation. To categorize zones for preservation, the approach followed takes biodiversity, weighted endemism and evolutionary history into account. Lineages of angiosperms, families, genera, and formal or informal groups within genera previously retrieved as monophyletic are selected to represent evolutionary history in equivalent spatial units. A database with 9416 entries based on specimens of species belonging to 259 monophyletic groups of angiosperms from Mexico was compiled, and their presence-absence recorded for every unit area. Species richness and weighted endemism index was calculated for each of these units. The results indicate that the majority of the regions with the highest indices of endemism have a dry climate with xeric vegetation, with the exception of two areas of tropical and temperate forests. They are: the northeastern rosette scrub in Nuevo León and Coahuila, gypsum grasslands in San Luis Potosí, the Sierra Gorda in Queréraro, Tolantongo in Hidalgo, the Tehuacán-Cuicatlán Valley in Puebla and Oaxaca, El Salto in Durango, Sierra de Quila in Jalisco, a western portion of the Balsas River Basin in Michoacán, Guerrero, Morelos and State of Mexico, the Tehuantepec area in Oaxaca, the Central Depression of Chiapas and El Triunfo in Chiapas. Some of the areas of endemism in the Chihuahuan Desert, Balsas River Basin, the Central Depression of Chiapas and the southern area of Oaxaca are not sufficiently protected. Approximately 340 species were microendemic, i.e. restricted to a single quadrat, and the Cactaceae account for the majority of the species on the Mexican Red List.

Key words: Cactaceae, Chihuahuan Desert, endemism, Mega-Mexico, xeric vegetation.

 

RESUMEN

México está considerado como uno de los países megadiversos y en su territorio se encuentran entre 4 y 8% del número de total de especies de plantas del mundo, de las cuales 51% son endémicas. Existe una gran preocupación sobre la conservación de la flora mexicana, ya que se han detectado varias actividades y factores que la amenazan. En este trabajo se identifican áreas de endemismo para las angiospermas de México con el objetivo de priorizar regiones para conservación. Para categorizar estas zonas se sigue el enfoque que toma en cuenta la biodiversidad, el índice de endemismo ponderado y la historia evolutiva. Se identificaron los linajes de angiospermas, ya sean familias, géneros, o grupos infragenéricos con o sin estatus taxonómico que previamente se habían determinado como monofiléticos para representar la historia evolutiva en unidades espaciales equivalentes. Se construyó una base de datos de 9416 registros de especies de 259 grupos monofiléticos de angiospermas restringidas a México y se registró su presencia en estas áreas. Para cada una se calculó la riqueza de taxones y el índice de endemismo ponderado. Los resultados muestran que la mayoría de las zonas de más alto endemismo están en climas secos, con vegetación xérica, con dos excepciones de vegetación tropical y templada. Los índices de endemismo ponderado más altos se localizaron en: el área norte de matorral rosetófilo en Nuevo León y Coahuila, matorrales gipsófilos en San Luis Potosí, la Sierra Gorda en Querérato, Tolantongo en Hidalgo, el Valle de Tehuacán-Cuicatlán en Puebla y Oaxaca, El Salto en Durango, la Sierra de Quila en Jalisco, la zona oeste de la Depresión del Balsas en Michoacán, Guerrero, Morelos y el Estado de México, la zona de Tehuantepec en Oaxaca y El Triunfo en Chiapas. Algunas áreas de endemismo en el Desierto Chihuahuense, en la Cuenca del Balsas y en la Depresión de Chiapas, así como del sur de Oaxaca no están suficientemente protegidas. Se registraron aproximadamente 340 especies con distribución restringida a un solo cuadrante y de éstas la mayoría de las que se incluyen en la lista de taxones amenazados de México pertenecen a las Cactaceae.

Palabras clave: Cactaceae, Desierto Chihuahuense, endemismo, Mega-México, vegetación xérica.

 

INTRODUCTION

Different criteria have been applied to prioritize areas for conservation and undoubtedly the most widely used is the concentration of endemic species undergoing increased habitat loss. The areas defined this way are known as "biodiversity hotspots" (Myers et al., 2000). Globally, thirty-five biodiversity hotspots, comprising 44% of the total number of species of vascular plants, have been identified and Mesoamerica is one of them (Myers et al., 2000; Mittermeier et al., 2011).

Other approaches for prioritizing areas for conservation have added the element of evolutionary history, because diversity and evolutionary history are unequally distributed in different areas of the world (Vane-Wright et al., 1991). Among these approaches, phylogenetic endemism uses phylogenetic diversity and weighted endemism as measures to identify areas for conservation based on equivalent spatial units (Rosauer et al., 2009). Additionally, attributes such as scarcity (Cadotte & Davies, 2010), local and global rarity of taxa (e.g. Crain et al., 2011), phylogenetic distinctiveness and isolation (e.g. Collen et al., 2011), phylogenetic diversity (Forest et al., 2007) and functional diversity (e.g. Devictor et al., 2010; Pio et al., 2011) have variously been incorporated to the evolutionary history approach.

In this paper, we identify areas of endemism for the flowering plants of Mexico to prioritize areas for conservation. The concept followed here for an area of endemism is that of a geographic region that includes the distributions of two or more monophyletic taxa with phylogenetic and distributional congruence (Harold & Mooi, 1994). Areas of endemism have several attributes: they have a single history, they are smaller than the entire study area, they do not overlap with other areas of endemism, they host at least two taxa with ranges restricted to the area and they are maximally congruent (Linder, 2001; Szumik et al., 2004; Ebach et al., 2008).

Thus, to categorize areas for preservation we follow an approach that combines biodiversity, weighted endemism and evolutionary history. Lineages of Mexican angiosperms, families, genera or formal and informal groups within genera that have been previously recognized as monophyletic were selected to represent evolutionary history in equivalent spatial units.

As a megadiverse country, Mexico houses 4 to 8% of the flora of the world and there is concern because several factors are impeding its conservation. The greatest threats to the flora of Mexico are intensification of habitat loss, the adverse effects of climate change and the overexploitation of the majority of habitats (Dávila et al., 2011).

Angiosperms were chosen because they are one of the most diverse groups of organisms in Mexico. Their diversity has been estimated at 24,500-29,000 species (Villaseñor, 2003; Espejo-Serna et al., 2004) and more than 50% are endemic to the country (Rzedowski, 1993). Furthermore, the genera distributed in Mexico have been documented (Villaseñor, 2004), and the floristic knowledge of the country has been recently summarized (Anonymous, 2009). The groups of angiosperms distributed in the area known as Mega-Mexico were used in this study. This biogeographic province was proposed by Rzedowski (1993) and includes, in addition to Mexico's current territory, the areas of the Sonoran Desert, the Chihuahuan Desert and the Tamaulipan scrub that lie in the United States of America, as well as those portions of Central America as far south as northern Nicaragua.

The families with the largest number of endemic Mexican genera are Cactaceae and Asteraceae (Turner, 1996-2010; Guzmán et al., 2003; Hernández & Gómez-Hinostrosa, 2011a,b). Setchellanthaceae, a monotypic family, only grows in Mexico (Iltis, 1999). In the monocots, a clade of geophyte genera in the Asparagaceae, the Milla clade, grows in Mega-Mexico (Gándara et al., 2009) and a group in the Crassulaceae, the Acre clade includes several genera exclusive to Mexico (Acevedo-Rosas et al., 2004; Carrillo-Reyes et al., 2010). Three related genera, Morkilia, Sericodes and Viscainoa in the Zygophyllaceae (Sheahan & Chase, 2006), and two genera in the Anacardiaceae, Bonetiella and Pseudosmodingium (Aguilar-Ortigoza et al., 2004) are found in this biogeographic province. In the Acanthaceae, nine genera and a clade within Ruellia are endemic to Mexico (Daniel, 1993; Tripp, 2010). In addition, among the more remarkable endemic groups of Mexico are clades of Bursera (Rzedowski et al., 2005; De-Nova et al., 2012), Agave and groups nested within this genus such as Manfreda, Polianthes and Prochnyanthes (García-Mendoza, 1995; Rocha et al., 2006, Good-Avila et al., 2006), the section Physodium in Melochia (Dorr & Barret, 1989), a clade in the Zea diploperennis group (Poaceae) (Buckler & Holtsford, 1996), and a clade of Yucca within the Sarcocarpa group (Pellmyr et al., 2007). Fouquieria and Leucophyllum are arid land groups in Mega-Mexico (Henrickson & Flyr, 1985; Schultheis & Baldwin, 1999). Enigmatic genera like Velascoa (Crossosomataceae) (Sosa & Chase, 2003), Chiangiodendron (Achariaceae) (Sosa et al., 2005), Enriquebeltrania (Euphorbiaceae) (De-Nova et al., 2006), Cerdia (Caryophyllaceae) (Sosa et al., 2006), Olmeca (Bambusoideae, Poaceae) (Dávila-Aranda et al., 2004; Ruiz-Sanchez et al., 2011), Peltophorum (Leguminosae) (Sousa, 2005), the parasitic Eremitilla (Orobanchaceae) (Yatskievych & Contreras-Jiménez, 2009), Echinopterys (Malpighiaceae) (Davis et al., 2001), Nowickea (Phytolaccaceae) (Martínez & McDonald, 1989), and Mexipedium (Orchidaceae) (Albert & Chase, 1992) are endemic to Mexico, to mention just a few examples. We recorded 259 monophyletic angiosperm groups endemic to Mega-Mexico.

The objectives of this paper are: 1) to identify the areas of endemism of the angiosperms of Mexico, using monophyletic groups to prioritize areas for conservation, and 2) to detect species from these natural groups with a restricted distribution to highlight the threatened taxa.

 

MATERIALS AND METHODS

Taxa

Mexican angiosperm lineages, families, genera, and infrageneric groups with or without formal taxonomic status were compiled based on the literature (Rzedowski, 1993; Villaseñor, 2004; Anonymous, 2009). Distribution records were obtained from herbarium specimens in ANSM, ENCB, HCIB, IBUG, IEB, MEXU, MO, NY, TEX, UAMIZ, US and XAL, and by consulting the Mexican Biodiversity Database (REMIB) (www.conabio.org).

Study area

The study area includes the entire country of Mexico. Even though the distribution of some groups extends into the south of the United States of America and northern Central America in Mega-Mexico, only the localities within Mexico were used. A system of land quadrats based on one degree squares was used to define arbitrary area units, resulting in a set of 237 area units with records of endemic taxa. The occurrence of every specimen of each monophyletic group in each quadrat was recorded. The data matrix had a total of 9416 georeferenced records. Quadrats with no records were eliminated. Species restricted to a single quadrat were identified as microendemics.

Areas of endemism

First, the number of species was added up for each quadrat to estimate its diversity (unweighted species richness, Pearson & Juliano, 1993; Kershaw et al., 1995). Then, the weighted endemism index, a method that weights species inversely to their distribution areas was also calculated (Linder, 2001).

Microendemic species

The species with a restricted distribution, i.e., those only found in a single quadrat, were recorded and of these the taxa on the Mexican Red List (Anonymous, 2010) were identified.

 

RESULTS

Areas of endemism

The data matrix included the presence/absence data for 878 species belonging to 259 monophyletic groups for 237 area units. The highest unweighted species richness values for each quadrat are shown in Table 1 and Fig. 1. The areas with the highest number of endemic species are in Tehuacán-Cuicatlán, in the eastern of the Balsas River Basin, in Tolantongo and Tepeapulco, Hidalgo and in the Sierra Gorda.

The weighted endemism values are listed in Table 1 and shown in Fig. 2. Eleven areas with the highest weighted endemism values (10.657-34.819) were identified: 1) A northeastern area of rossette scrub in Nuevo León and Coahuila (Ramos Arizpe, Aramberri, Galeana and Zaragoza); 2) an area of gypsum grasslands in San Luis Potosí, 3) the Sierra Gorda, Querétaro (extending to San Luis Potosí); 4) Tolantongo in Hidalgo, 4) the area of Tehuacán-Cuicatlán, Puebla and Oaxaca; 5) El Salto, Durango; 6) the Sierra de Quila in Jalisco; 7) the western area of the Balsas River Basin (Michoacán, Guerrero, Morelos, State of Mexico); 8) the Tehuantepec area, Oaxaca; 9) the Central Depression of Chiapas; 10) El Triunfo, Chiapas. Among the areas with high weighted endemism indices is the southern area of Baja California and the Sierra de Órganos, Zacatecas (Fig. 2).

Microendemics

Appendix lists the 340 species whose distribution is restricted to a single quadrat, with their threatened status indicated when applicable.

 

DISCUSSION

Rzedowski (1993) pointed out that the distribution of areas with endemic species for the flora of Mexico does not coincide with the distribution of biodiversity. He indicated that the endemic taxa are concentrated in areas of dry climate and this conclusion was reached based on species richness alone, without taking the historical element into account. The latter has been included in this paper by identifying areas of endemism shared by at least two monophyletic groups. Our results indicate that majority of the areas: the northeastern rosette scrub, the gypsum grasslands, the Sierra Gorda, the southern portion of the Chihuahuan Desert, the area of Tehuacán-Cuicatlán, the Sierra de Quila, the western area of the Balsas River Basin, the Tehuantepec area and the Central Depression of Chiapas, have a dry climate, corroborating Rzedowski's hypothesis. The endemic groups occur at low to middle elevations, in xeric vegetation. The only area with a tropical climate and a high weighted endemism index is El Triunfo in Chiapas, and El Salto in Durango has a temperate climate.

The Sierra Gorda was previously recognized as an area with significant endemism and it is included in the Mexican System of Natural Protected Areas, the SINAP (Arriaga-Cabrera et al., 2000). Additionally, the Tehuacán-Cuicatlán area is comprised of arid vegetation and is perhaps the most important biosphere reserve in Mexico (Arriaga-Cabrera et al., 2000). It is a floristic province, and an ecological island given the high number of endemics, estimated at 365 species (Méndez-Larios et al., 2004; Dávila et al., 2002).

The area of Metztitlán-Tolantongo was previously known for having high endemism and it was decreed as a biological reserve in 2000 (Hiriart-Valencia & González-Medrano, 1983; Arriaga-Cabrera et al., 2000). Furthermore, Sierra de Quila was earlier identified as a hotspot for conservation based on mammal distributional predictions as biodiversity surrogates (Sánchez-Cordero et al., 2005). In addition, the semiarid gypsum karstlands in north central Mexico characterized by a mosaic of shrubby communities and endemic gypsophile grasslands were formerly acknowledged as areas with elevated endemism and important to preserve (Henrickson & Johnston, 1986; Meyer et al., 1992; Huerta-Martínez & García-Moya, 2004).

The western area of the Balsas River Basin in Michoacán, Guerrero, Morelos and State of Mexico, is another region with high indices of endemism. Rodríguez-Jiménez et al. (2005) have identified 337 endemic species of vascular plants in this biogeographic province. Cañón del Zopilote and Infiernillo are two proposed areas for conservation in this province (Arriaga-Cabrera et al., 2000), yet they represent only a small area within the Balsas River Basin.

Several regions in Nuevo León harbor extremely large numbers of endemisms in the Cactaceae (Juárez et al., 2009), and a high concentration of narrowly distributed Asteraceae (González-Zamora et al., 2007; Alanís-Flores et al., 2011). Moreover, these regions coincided with one of the areas of high endemism identified in this study: the Northeastern rosette scrub.

Our results detected the Sierra La Laguna in Baja California Sur as a territory with high endemism. Plant diversity and endemism on the entire Baja California Peninsula have previously attracted attention (Riemann & Ezcurra, 2007). The notable endemism in the Sierra La Laguna, Baja California, had also been pointed out (León de la Luz & Breceda, 2006) and resulted in the Sierra La Laguna reserve being decreed one of Mexico's biosphere reserves (Arriaga-Cabrera et al., 2000).

El Triunfo is a biosphere reserve with elevated endemism and richness, for which approximately 1000 species of vascular plants have been reported. The area includes several habitats such as cloud, oak and tropical forest (Martínez-Meléndez et al., 2008; Pérez-Farrera et al., 2012). It was decreed as a reserve because it is considered to be a Pleistocene refugium for several tropical species (Arriaga-Cabrerra et al., 2000). As well, the pine and oak forests in El Salto have provided suitable habitats for several gymnosperm relict species (Valenzuela-Núñez & Granados-Sánchez, 2009).

The Central Depression of Chiapas has been earlier identified as an area with seasonally dry tropical forests and tropical oak forests where approximately 3.4% of the total number of vascular plant species in Mexico are distributed (Reyes-García & Sousa, 1997). Furthermore, among the habitats of Tehuantepec, the tropical dry forests harbor the largest diversity including several taxa of endemic angiosperm (Acosta et al., 2003; Pérez-García et al., 2010).

It is noteworthy that the majority of the areas with the highest endemism indices, such as El Salto (Durango), the Central Depression of Chiapas, Tehuantepec, (Oaxaca), and Tolantongo in the southern area of the Chihuahuan Desert are not protected under the SINAP scheme (Arriaga-Cabrera et al., 2000).

Microendemic species

It is crucial to take the rarity of species into account when setting conservation priorities (Mooers & Redding, 2009). It has been mentioned that in areas of endemism the species with restricted distributions are usually on the red lists (e.g., Argentina, Szumik et al., in press). The same happens in Mexico where we found that a large number of the species whose distribution is restricted to a single quadrat in our study area are included on the Mexican List of Threatened Species (Anonymous, 2010).

Most of the species on this list with a limited distribution are cacti. More than 900 species of Cactaceae are present throughout Mexico (Ortega-Baes & Godínez-Álvarez, 2006). This is one of the groups that are most used as ornamental plants and so have been continuously extracted from their habitats, with the result that they are now the most threatened group in Mexico (Gómez-Hinostrosa & Hernández, 2000; Hernández & Gómez-Hinostrosa, 2011a,b).

 

CONCLUSIONS

Future research should examine the probable causes of diversification for the angiosperm lineages in the areas of endemism in Mega-Mexico. For other areas with high degrees of endemism, such as the Andes, isolation caused diversification, similar in many respects to the floras of remote oceanic islands (Särkinem et al., in press) or along elevational gradients (Kessler, 2000). Climate was the factor that promoted speciation in Australia's areas of endemism (Ladiges et al., 2011), while tectonic stability in central and southern China influenced the permanence of areas of plant endemism (López-Pujol et al., 2011). In Sub-Saharan Africa, elevation range and low seasonality were core environmental predictors for centers of endemism (Jetz et al., 2004). Diversification in hotspots of biodiversity and endemism in Brazil were attributed to the effect of fire on vegetation (Simon et al., 2009), while serpentine soils and a benign climate favored endemism in California (Anacker & Harrison, in press).

Our results suggest that various causes promoted the diversification of several groups of plants in the areas of endemism, and a dry climate together with isolation are probably the most remarkable. Nine areas of endemism have a dry climate. Furthermore, the Central Depression of Chiapas and the Balsas River Basin are two areas that remained isolated, bordered by mountain ranges. In contrast, El Triunfo in Chiapas probably acted as a refugium for angiosperm lineages that remained there throughout the Pleistocene. Gypsum soils probably favored endemism in the northeastern rosette scrub and the grasslands of San Luis Potosí. However investigation is needed to corroborate these hypotheses.

It should be emphasized that hotspots do not necessarily coincide with species richness, the degree of threat or areas of endemism (Orme et al., 2005). The areas of endemism identified in our study do not coincide with the areas with elevated diversity of the flora of Mexico, as Rzedowski (1993) pointed out, and some of the areas of endemism in the Chihuahuan Desert, Balsas River Basin and the southern area of Oaxaca are not sufficiently protected.

 

ACKNOWLEDGEMENTS

We are grateful to Patricia Dávila and Jorge Meave del Castillo for their useful comments that improved the manuscript. We are grateful to Manuel Cuéllar and Ismael G. Valdivieso for their help designing the database, and to Rosario Landgrave for her invaluable help producing the maps, as well as to Manuel Cuéllar and Diego Angulo. Jerzy Rzedowski provided important information on the endemism of the Mexican flora; Carolina Calviño provided information on Apiaceae (Eryngium), Etelvina Gándara on Asparagaceae (Milla clade) and Scrophulariaceae (Leucophyllum); John Bain, Jose Panero, Edward Schilling and José Luis Villaseñor on Asteraceae; Salvador Arias and Héctor Hernández on Cactaceae; Mark Simmonds on Celastraceae; Pablo Carrillo Reyes and Raúl Acevedo-Rosas on Crassulaceae; Rafael Lira on Cucurbitaceae; Eduardo Estrada, Lourdes Rico and Jenny Sotuyo on Fabaceae; Susana Valencia on Fagaceae; Paul Peterson, María Elena Siqueiros and J. Travis Columbus on Poaceae; Rafael Fernández Nava on Rhamnaceae and Lynn Boss and Aarón Rodríguez on Solanaceae.

 

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