Identifying priority areas for conservation IN MEXICAN tropical deciduous forest based on tree species Eva M. Cué-Bär, José Luis Villaseñor, Juan J. Morrone and Guillermo Ibarra-Manríquez SUMMARY The main objective was to identify and rank areas for the conservation of exclusive, or nearly exclusive, tree species of the tropical deciduous forest in Mexico, a land cover type greatly endangered worldwide. A list of 425 tree species (67.5% endemic to Mexico), including 56 families and 185 genera registered at the state level, was compiled from an exhaustive revision of specialized floristic literature. The conservation status of these species was assessed by registering their presence in the tropical deciduous forest areas, located within the limits of Mexican natural protected areas. Parsimony analysis of endemicity led to identify 16 areas of endemism, supported by 54 synapomorphies and 73 autapomorphies (72.4% endemic to Mexico). Protected
ndemism is related to geographical or ecological events resulting in the isolation of populations from one or several species (vicariance). Because of this, islands, mountains, and deserts are particularly rich in endemic taxa (Major, 1988; Brown and Lomolino, 1998). Explanations for the high number of endemic plant taxa in Mexico include that some areas behave like ecological islands, as well as events and environmental conditions in the geological past that lead to suggest that several regions acted as refuges in pre-Pleistocene or Pleistocene times (Wendt, 1993). Rzedowski (1991a) points
areas include a low to medium proportion both of widespread tree species (129 species, 30.3%) and species characterizing areas of endemism (56 species out of 127, 44.1%). In the latter group, only 7 species are included within a risk category. Analyses of complementarity (total richness of species and of species within areas of endemism) and of phylogenetic diversity (families and genera) are coincident in assigning high conservation priority for areas in the states of Chiapas, Guerrero, Jalisco, Michoacán, Oaxaca, and Yucatán. Based on the results, specific recommendations are provided for the development of strategies of tree conservation in the Mexican tropical deciduous forest.
to the tropical deciduous forest (TDF) as a clear example of an ecological island, because of its flora exclusivity originating through geographical isolation. In Mexico, this type of vegetation, encompassing tropical subdeciduous and thorn forests, includes nearly 6000 species of phanerogamous plants, equivalent to 20% of the total flora of the country, and nearly 40% of endemic species (Rzedowski, 1991a, b). Moreover, a significant richness and level of endemism has been attributed to vertebrate species in the Mexican TDF (Ceballos and García, 1995). It has been hypothesized that the TDF has been the place of diversi-
fication of numerous plant lineages. At the family level, the Asteraceae, Bignoniaceae, Burseraceae, Cactaceae, Convolvulaceae, Eu phorbiaceae, Fabaceae, Malvaceae, and Ru biaceae are relevant cases (Rzedowski, 1978, 1991a, b; Gentry, 1995). More precise data are available for the Euphorbiaceae, with 384 species registered in Mexico, of which 23% is present in the TDF and 36% of these species being exclusive to this land cover type (Martínez-Gordillo et al., 2002). At the genus level Bursera (Burseraceae) is a remarkable case, given that 92% of its 82 Mexican species are basically restricted to these forests (Rosalinda Medina personal communication), with 6
KEY WORDS / Biogeography / Complementarity Analysis / Neotropics/ Parsimony Analysis of Endemicity / Phylogenetic Diversity / Received: 02/15/2006. Modified: 09/01/2006. Accepted: 09/04/2006.
Eva M. Cué-Bär. M.S. in Biological Science, University of Leipzig, Germany. Ph.D. student, Universidad Nacional Autónoma de México (UNAM). Address: Antigua Carretera a Pátzcuaro Nº 8701. Col. San José de la Huerta. 58190 Morelia, Michoacán, México. e-mail:
[email protected] José Luis Villaseñor. Ph.D. in Plant Systematics, The Claremont Graduate School, USA. Researcher, UNAM, Mexico. e-mail:
[email protected] Juan J. Morrone. Ph.D. in Natural Sciences, Universidad Nacional de La Plata, Argentina. Profesor, UNAM, Mexico. e-mail:
[email protected] Guillermo Ibarra-Manríquez. Ph.D. in Sciences, UNAM, Mexico. Researcher, UNAM, Mexico. e-mail:
[email protected]
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areas of high concentration of species having limited distribution (Rzedowski et al., 2005). Additional evidence for the floristic relevance of the TDF is also provided by the results of a census of the woody plant flora in 0.1ha areas in Chamela (Jalisco) and Caleta (Michoacán), located along the Pacific Ocean coast (Lott et al., 1987; Trejo and Dirzo, 2002); such regions are among the 10 most species-rich worldwide (Gentry, 1995; Phillips and Miller, 2002; Trejo and Dirzo, 2002). Areas of endemism have been given several meanings in the biogeographic literature (e.g. Müller, 1973; Platnick, 1991; Morrone, 1994; Posadas and Miranda-Esquivel, 1999). In general, they may be defined as areas presenting congruent and non-random distribution patterns for two or more taxa, which may be or may not be ecological or phylogenetically related. The complexity of identifying areas of endemism increases in proportion to the number of taxa being analyzed. Morrone (1994) suggested the use of the parsimony analysis of endemicity method (PAE) to locate endemism areas based on their shared taxa (usually species), and finding the most parsimonious solution under the assumption that the resulting groups of species have a common history (Rosen, 1988; Morrone, 1994; Morrone and Crisci, 1995). The PAE approach has been applied to several taxa and regions of the world (e.g. Linder, 2001; GarcíaBarros et al., 2002; Rovito et al., 2004; Silva et al., 2004). In Mexico, several plant and animal groups have been analyzed at different scales (Morrone et al., 1999, 2002; Luna-Vega et al., 1999; Morrone and Escalante, 2002; Escalante et al., 2003; Espadas et al., 2003; Rojas et al., 2003). It has been suggested that parsimony analyses of endemicity may help choose areas for conservation (Posadas and Miranda-Esquivel, 1999; Cavieres et al., 2002; Méndez-Larios et al., 2005), by establishing which areas are unique regarding their composition, and giving priority to those areas displaying a high degree of endemicity. In conservation biology, criteria and methods to select and to assign priority for conservation purposes to a particular region have been increasing in number and complexity. Among the most utilized options for selecting and ranking areas for conservation are iterative methods based on the principle of complementarity that allows the conservation of the largest number of species within a given region and in the minimum possible number of areas (Margules et al., 1988; Margules and Pressey 2000). This approach has been used for diverse groups
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of Mexican plant taxa (Villaseñor et al., 1998, 2003; Lira et al., 2002; Dávila et al., 2004). The high floristic diversity and level of endemicity of the TDF, in addition to its ample distribution in Mexico, point out the need to localize and prioritize its areas of endemism. The importance of such aim is enhanced considering that the TDF is one of the most threatened tropical ecosystems, due mainly to extensive grazing and agriculture (Janzen, 1988; Murphy and Lugo, 1995; Trejo and Dirzo, 2000; Miles et al., 2006). Trees were selected in this study because they meet the requirements for being considered as an indicator group of biodiversity (Stork, 1994), and their relevant role in structure and several ecological functions in the Mexican TDF (Miranda and Hernández-X., 1963; Rzedowski, 1978; Lott et al., 1987; Trejo and Dirzo, 2000). Therefore, the objectives of this study are: 1) to localize the areas of endemism of tree species in the Mexican TDF and the species characterizing them, 2) to establish the priority for conservation of the states that include these endemic areas, based on their species richness and phylogenetic diversity, and 3) to asses the protection level enjoyed by the TDF trees and their risk category.
Materials and Methods This forest type is distributed in Mexico along a nearly continuous strip along the Pacific slopes from Sonora (~28ºN) to Chiapas (ca. 14º30’N), whereas in the Gulf of Mexico it has a discontinuous distribution from Tamaulipas to the Yucatán Peninsula. In addition, it is present in several central states and the southern tip of the Baja California Peninsula (Figure 1), from sea level to 2220masl (Rzedowski, 1978; Trejo and Dirzo, 2000). The total area covered by the TDF has been estimated to be ~817% of the total country area (Rzedowski, 1978, 1991a). The TDF is present in 10 of the 11 morphotectonic provinces established for Mexico by Ferrusquía-Villafranca (1993), being absent only in the Chihuahua-Coahuila Plateaus and Ranges province. It grows mainly in rocky, shallow soils or in soils with textures ranging from clay to sand, acid to slightly alkaline, rich in organic matter, and of clear to dark color (Rzedowski, 1978). Climates favoring TDF are dry to semi-dry, warm (Aw or Bs) and with mean annual temperature above 19oC, mean annual precipitation of 400-1350mm, and 6-9 months with precipitation
