Areas of endemism identified with the EA by grid-cells

The analysis based on grid-cells identified 8 areas of endemism and 7 consensus areas located along the TVB and towards the central and east of the BB (Table 3, Fig. 2). The endemicity index (E) values vary from 1.8 to 4.63 and 25 species (16.5% from total) showed high e values, characterizing such areas of endemism (Table 3). Figure 2 shows the placement of the consensus areas, which are described as follows:

Western TVB. In the western portion of the TVB coincided 2 consensus areas (2 and 3), including 9 grid-cells supported by 7 TVB strict endemic species (Ageratina jocotepecana B.L. Turner, Ageratum platypodum B.L. Rob., Cosmos ramirezianus Art. Castro, Harker & Aarón Rodr., C. deficiens (Sherff) Melchert, C. longipetiolatus Melchert, Grindelia sublanuginosa Steyerm., and Stevia mascotensis Soejima & Yahara).

Central-Eastern TVB. In this area coincided 3 consensus areas (4, 5 and 7), which included 17 grid-cells located at central and eastern TVB. This area is supported by 13 species, 7 of them TVB strict endemics, 1 BB strict endemic, and 5 shared by both provinces (Table 3).

Central BB. This area comprises 6 consensus areas including 5 grid-cells situated at the central part of BB and is supported by 2 strict endemic species to this province (Chrysanthellum involutum Paul G. Wilson and Eryngiophyllum pinnatisectum Paul G. Wilson).

Eastern BB. This area includes one consensus area comprising 5 grid-cells located in the eastern part of BB and is supported by 3 strict endemic species of this province: Dendroviguiera guerrerana (Panero & E.E. Schill.) E.E. Schill. & Panero, Lasianthaea squarrosa (Greenm.) K.M. Becker, and Stevia mitopoda B.L. Rob.

Figure 2 Areas of endemism identified in the biogeographic provinces BB and TVB. A and B show the results obtained with the endemicity analysis (EA) and C and D those obtained with the parsimony analysis of endemicity (PAE). In some cases, there is spatial coincidence between the areas of endemism, for which the overlapping areas are indicated separated by a “comma”. 

Areas of endemism identified with the EA by hydrological sub-basins

The analysis based on hydrological sub-basins revealed the occurrence of 5 areas of endemism and 5 consensus areas (Table 3, Fig. 2B). The Endemicity index values vary from 2.23 to 3.63 and 18 species characterizing such areas of endemism showed high e values (Fig. 2B). Figure 2 displays the placement of the consensus areas, which are described as follows:

Eastern-Central area. This area includes 16 grid-cells located in the eastern-central part of the study area and comprises 3 consensus areas (1, 3 and 4). Such grids comprise part of 11 hydrological sub-basins (Río Atoyac-A, Río Balsas-Mezcala, Río Grande de Amacuzac, Río Moctezuma, as well as small parts of the Río Balsas-Zirándalo, Río Cutzamala, Río Jamapa, Río Lerma-Toluca, Río Papagayo, Río Tecolutla and Río Tlapaneco [Fig. 2B]). Eleven species defining this consensus area of which 4 are strict endemics of the TVB, 3 are strict endemics of BBand 4 are shared by both provinces (Table 3).

Western-Central area. This area includes 9 cell grids located at the western-central portion of the study area and comprises 2 consensus areas (2 and 5). These grid cells occupy parts of 10 hydrological sub-basins (Lago de Pátzcuaro-Cuitzeo and Lago de Yuriria, Río Coahuayana, Río Lerma-Chapala, Río Tepalcatepec, Río Tepalcatepec-Infiernillo, as well as small parts of the Río Cutzamala, Río Lerma-Salamanca, Río Lerma-Toluca, Río Nexpe, and Río Tacámbaro). The area is supported by 7 species, 4 of them TVB strict endemics (Astranthium laetificum De Jong, Brickellia leonis Rzed. & Calderón, Eremosis solorzanoana (Rzed. & Calderón) Pruski, and Viguiera moreliana B.L. Turner), 2 BB strict endemics (Pectis leavenworthii Standl. and Roldana glinophylla H. Rob. & Brettell) and 1 distributed both in BB and TVB (Acourtia michoacana (B.L. Rob.) Reveal & R.M. King).

Areas of endemism identified with PAE by grid-cells

A number of 50 equally parsimonious trees based on 151 species and 87 grid-cells were obtained, all them with a TBR score of 194 (Fig. 3A). The strict consensus of these trees recovered a single area of endemism with 4 exclusive species (Fig. 2C). This area records 4 TBV strict endemic species (Cosmos deficiens, C. jaliscensis, C. longipetiolatus, and Stevia mascotensis). In addition, the analysis showed 1 candidate grid-cell to be considered a “micro-area of endemism” because it contains at least 2 TBV strict endemic species (Cosmos ramirezianus and Grindelia sublanuginosa) (Table 4, Fig. 2C). This micro-area of endemism is located at the western part of the province, close to the previous area of endemism (Fig. 2C).

Figure 3 Cladograms obtained with the parsimony analysis of endemicity (PAE) rooted on Operative Geographic Units (OGU´s). A) Strict consensus from 50 equally parsimonious trees using 87 grid-cells. A close-up of the branches of interest is shown on the right. B) The tree obtained using the 30 hydrological sub-basins. A close-up of the branches of interest is shown on the right. In bold the species restricted to the grid-cells are indicated; the white circles represent homoplasy. Above the branches the numerical identifier of the species as cited in Table 1 are shown. 

Areas of endemism identified with PAE by hydrological sub-basins

The PAE analysis using the 27 sub-basins resulted in 100 parsimonious trees, recovering 3 areas of endemism supported by 7 endemic species (Table 4, Fig. 3B). One area of endemism is located in the western section of the studied region, corresponding to the Río Ameca-Atenguillo and Río Ameca-Ixtapa sub-basins (Fig. 2D). Both basins share 2 TBV strict endemic species (Cosmos jaliscensis and C. longipetiolatus). A second area of endemism comprises 2 sub-basins placed at the western-central portion, corresponding to the Río Tepalcatepec and Río Tepalcatepec-Infiernillo sub-basins (Fig. 2D); this area is supported by 3 species, 2 of them BB strict endemics (Pectis leavenworthii and Simsia tenuis) and 1 shared by both biogeographic provinces (Piqueria glandulosa) (Table 4). Finally, at the middle of the analyzed sub-basins, is located the third area of endemism, forming part of the Río Cutzamala and Río Grande de Amacuzac sub-basins, supported by 2 endemic species distributed both in BB and TVB provinces (Table 4; Fig. 2D).

Relations between areas of endemism by AE and PAE

Even using the same data of the Asteraceae species endemic to the biogeographic provinces under study, differences were obtained both in the EA and in the PAE, in the number of areas of endemism identified as well as in the number of species that support them (^{Morrone, 2014b}). However, both methods identified areas of endemism in the western portion of the TVB, when carrying out the EA by cells (Fig. 2A), and when analyzing the data with the PAE by cells and hydrological sub-basins (Fig. 2C, D). It should be noted that ^{Estrada-Márquez et al. (2020)} mentioned that the volcanic activity of the TVB has occurred asynchronously along its surface, beginning in its western portion and continuing more recently to the east, highlighting it as an older area in the TVB, a region in which both methods identified the areas of endemism previously indicated.

Particularly, endemicity analysis with both grid-cells and hydrological sub-basins, revealed important sites as areas of endemism. Although they are not characterized by a large number of species (Table 3), they show coincidences in specific areas at the center and west of the study area, specifically in the portion corresponding to the TVB (Fig. 2A, B). PAE identified a single area of endemism based on grid-cells division; however, the analysis carried out allowed to identify one so-called micro-area of endemism, which contains 2 endemic species not shared with contiguous grid-cells. This micro-area is located at the western portion of the study area, near to the only identified area of endemism through the PAE (Fig. 2C).

Areas of endemism in the Transmexican Volcanic Belt province

The placement of most of the areas of endemism towards the eastern portion of the TVB, coincides with other results, for example those obtained by ^{Gámez et al. (2012)} studying the mastofauna of the region. Similar results were reported by ^{Villaseñor et al. (2006)} with mosses and Senecios (Asteraceae). The high richness localities identified by these authors show important geographic congruence with the areas of endemism identified in this study. The areas of endemism here found are included within the East biogeographic district proposed by ^{Escalante et al. (2007)} based on mammal data.

At the western and central portions of the TVB, the areas of endemism identified form part of those found by ^{Estrada-Márquez et al. (2020)} with a set of Magnoliophyta species. These authors identified such areas of endemism towards the center of the country, particularly in the BB and TVB provinces, and highlighted the relevance of Asteraceae species in identifying them. Likewise, ^{Rodríguez et al. (2018)} pointed out the existence of areas of endemism and centers of high species richness for several groups of angiosperms in the western portion of TVB, coinciding several of them with the areas of endemism found in this study. In the same way, the regions of high vascular plant species richness found by ^{Rzedowski (2020)} in Jalisco and Michoacán, coincide with the areas of endemism here identified. In summary, the Transmexican Volcanic Belt has been recognized as an area rich in endemic mammals and other groups (^{Ceballos & Navarro, 1991}), as well as a center of diversification, endemism and biogeographic transition (^{Gámez et al., 2012}). Similarly, ^{Villaseñor and Ortiz (2007)}, studying the family Asteraceae, coincide that the TVB is a center of diversity and endemism for the family.

Areas of endemism in the Balsas Basin province

Our results support previous findings, pointing out the central-eastern portion of the BB as an important area of endemism. In a similar way, the studies of ^{Castro-Torreblanca et al. (2014)} with birds reported the eastern region as of the 3 highest species concentration centers. ^{Rodríguez-Jiménez et al. (2005)} pointed out that states that concentrate more than 50% of BB´s endemic vascular plant species are Guerrero, Estado de México and Michoacán, which coincide in part with the area of endemism identified in this region. ^{Montaño-Arias et al. (2018)} discussed the high endemism values found in the tropical dry forests of the western portion of the BB. The BB areas of endemism found in this study agree with such statement, placing the western part of the BB as the richest in endemic species. Finally, the areas of endemism found towards the central portion of BB coincide with the results of ^{Sosa and De-Nova (2012)}, who identified one area of endemism towards the center of the BB province based on a set of Mexican angiosperm species.

Natural Protected Areas and areas of endemism

Within the study area, 45 Natural Protected Areas (NPAs) include all or part of their territory, 35 in the TVB, 5 to the BB and 5 sharing surfaces between both provinces (Table 5, Fig. 4). Most of the NPAs are located towards the center and east of the TVB, while in BB a single area is located towards its west part and the other concentrated towards its east portion (Fig. 4). The areas of endemism identified with both EA and PAE indicate that 38 NPAs include portions of the endemism areas (Table 5).

Based on the EA by grid-cells, 33 NPAs matched with them, while for the analysis using hydrological subbasins, 29 NPAs matched with their surface (Table 5). In contrast, the area of endemism identified with PAE using grid cells, as well as the micro-area of endemism matched with 2 NPAs; the same analysis based on hydrological subbasins matched with 19 NPAs (Table 5). A figure of 14 NPAs coincided in 3 out of 4 analyses carried out to identify the areas of endemism (Table 5); among them stand out the Corredor Biológico Chichinautzin, El Tepozteco, Insurgente José María Morelos, Sierra de Huautla and Z.P.F.T.C.C. de los ríos Valle de Bravo, Malacatepec, Tilostoc y Temascaltepec (Table 5).

Figure 4 Natural protected areas (NPAs) circumscribed to the biogeographic provinces BB and TVB. 

Implications for the conservation of Asteraceae

Endemism has been of particular relevance for the design and delimitation of protected areas. Due to their geographic restriction, endemic species are sensitive to the modification of their environment, particularly if it is due to anthropogenic agents (^{Alcántara & Paniagua, 2007}). The Asteraceae family does not only stand out for its endemism, but because it is well known taxonomically, it is easy to sample, it has representatives of all types of growth forms, and it is distributed over a wide range of habitats. It has also turned out to be a good surrogate for country’s biodiversity (^{Villaseñor et al., 1998}, ²⁰⁰⁷). Thus, conservation of its species as well as the ecosystems in which they are found would also contribute to conserve much of the Mexican plant diversity, especially that threatened by several anthropogenic and natural factors.

Despite the fact that 84% of the NPAs already established inside the study region matched with several of the areas of endemism identified in this study, there are still important regions to be considered due to their endemism and actually not protected by any NPA. Such is the case of the central and southeast portion of the BB, as well as some regions of its western portion; the same situation can be observed within the TVB in its western portion, where there are not enough NPAs (Fig. 4). For this reason, we propose that those areas of endemism whose surface does not have any official protection, have some strategies for their conservation especially due to their coincidence with regions identified as important for different biotic groups (plants, birds and mammals), highlighting them as potentially irreplaceable areas, one of the utmost criteria for the definition of priority areas for conservation (^{Méndez-Larios et al., 2005}).