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Revista mexicana de biodiversidad

versão On-line ISSN 2007-8706versão impressa ISSN 1870-3453

Rev. Mex. Biodiv. vol.87 no.3 México Set. 2016

http://dx.doi.org/10.1016/j.rmb.2016.06.017 

Taxonomía y sistemática

Checklist of the native vascular plants of Mexico

Catálogo de las plantas vasculares nativas de México

José Luis Villaseñora  * 

a Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, Apartado postal 70-233, 04510 Ciudad de México, Mexico

Abstract:

An updated inventory of the native vascular plants of Mexico records 23,314 species, distributed in 2,854 genera, 297 families, and 73 orders. The flora includes 1,039 species of ferns and lycophytes, 149 gymnosperms, and 22,126 angiosperms. On average, the number of synonyms per species is 1.3 (mode = 1). The number of species places Mexico as the country with the fourth largest floristic richness in the world, although among the non-insular countries, by its number of endemic species (about 50%) is second only surpassed by South Africa. The species distribution among higher taxonomic categories, and the richness and endemism values in the 32 states of Mexico are discussed. This compilation allows us to assess the flora's contribution to the overall Mexican biodiversity.

Keywords: Biodiversity; Biomes; Endemism; Flora of Mexico; Floristic studies; Inventories

Resumen:

Un inventario actualizado de plantas vasculares nativas de México registra 23,314 especies, distribuidas en 2,854 géneros, 297 familias y 73 órdenes. La flora incluye 1,039 especies de helechos y licofitas, 149 gimnospermas y 22,126 angiospermas. En promedio se registran 1.3 sinónimos por cada nombre aceptado (moda = 1). Por su número de especies, México ocupa el cuarto lugar a nivel mundial; entre los países continentales ocupa el segundo por el número de especies endémicas (alrededor del 50%), sólo por debajo de Sudáfrica. Se discute la distribución taxonómica de las especies entre las distintas categorías taxonómicas superiores, así como los valores de riqueza y endemicidad entre los 32 estados del país. Esta recopilación permite evaluar la contribución de la flora a la biodiversidad de México.

Palabras clave: Biodiversidad; Biomas; Endemismo; Flora de México; Estudios florísticos; Inventarios

Introduction

The concept of biodiversity, applied to floristic richness, considers the number of taxa (categories of the taxonomic hierarchy) present in any geographical or administrative unit, such as county, state or country. With this number, it is possible to quantitatively evaluate diversity and compare it among areas. There are international agreements that prioritize the quantification of biodiversity of the signatory countries, especially those with poor or insufficient knowledge of biodiversity at the national and/or regional levels, as is the case of Mexico (Conabio, 2012).

Mexico has a long and growing tradition of studying its vascular flora, reflected in the significant increase in recent decades of specimens housed in national scientific collections and abroad, backed by an immense bibliography. However, the knowledge of national floristic richness is still unsatisfactory mainly due to the difficulty of synthesizing scattered information in such publications along with the lack of well-curated databases of specimens documenting this richness. It is also clear that most genera require additional taxonomic study (revisions or monographs), and large areas of land remain unexplored to date.

The first estimates of the vascular flora of Mexico, proposed more than 2 decades ago, quoted between 17,000 and 30,000 species (reviewed in Villaseñor, 2003). A decade ago, an extensive literature review led to an estimate of about 22,351 species of vascular plants (Villaseñor, 2003, 2004). Later, Llorente-Bousquets and Ocegueda (2008), collaborating with many specialists, published the first list of species of vascular plants of Mexico, which included 22,332 species, a figure remarkably similar to that reported by Villaseñor (2003). Their list was the first publication that documents in detail aspects of Mexican plant biodiversity (Conabio, 2008). Unfortunately, the exercise carried out by the Conabio (the Mexican Biodiversity Commission) has been little used, probably because the general public has limited access to such information, and the databases still have little impact on the presentation and management of biological information. Moreover, due to the dynamism of taxonomy, published scientific names are constantly changed due to updates and corrections, or added in publication of numerous new species. Therefore, the documented information on Mexico's floristic richness should be regularly updated through the publication of floristic lists or catalogs that synthesize information on the species reported.

A catalog (floristic list or checklist) represents a more or less critical summary of the information gathered or known about the plant species (or other taxonomic designation) of a region (Nimis, 1996), and it may vary in content or approach. Sometimes, as in this work, they only list the scientific names collected for the country; on other occasions they provide additional information, such as representative specimens, literature for particular taxonomic groups, synonymy, or specific comments aimed to clarify doubts or taxonomic conflicts (see, for example, Dávila et al., 2006; García-Mendoza & Meave, 2011; Guzmán, Arias, & Dávila, 2003; Ibarra-Manríquez, Villaseñor, & Durán, 1995; Villaseñor, Ortiz, Beutelspacher, & Gómez-López, 2013). Usually, a basic species list is what is first published for any region, so it always requires a critical evaluation. The reliability of the existing literature is an issue for any catalog and unreliable primary sources result in biases or difficulties in compiling lists; in addition, it is practically impossible to verify all species identifications, and the number of reviews or monographs consulted or available is relatively low. A further problem is the use of different taxonomic criteria; specialists do not always coincide in the circumscription of species, genera or even taxonomic categories of higher rank, and reconciling these different treatments is not trivial. Sometimes an inventory follows one of these criteria, while another may prefer an alternative approach. Expert opinions help clarify uncertainties, especially when the geographical distribution of species reveals errors of reference in a given regional inventory. Ultimately, it is up to catalog users to judge the reliability of names and additional information presented.

Many people are skeptical of the scientific value of catalogs, especially scholars of biodiversity that require information that catalogs do not provide directly. However, among the merits of catalogs is that they synthesize a wealth of information accumulated throughout the history of botany and exploration of any territory. Listing species names is key to accessing a world of additional information on species, including aspects of natural history and current and potential uses. For taxonomists, lists are certainly valuable in order to consider the number of species to study in a review or monograph, and facilitate the inclusion of many species that have been overlooked in previous treatments. For ecologists and phytogeographers, catalogs are the first step to forming an informed opinion on the relationships between certain floras, and attempting to explain the causes, origins and evolution of diversity.

The aim of this contribution is to provide an updated catalog of the native vascular plant species of Mexico. This catalog is expected to serve as a basis for a better understanding of the Mexican flora and to promote more comprehensive floristic and taxonomic studies of groups or regions that require more detailed inventory or systematic work. As Nimis (1996) points out, catalogs are catalysts for new research projects and questions, and their relevance is not limited to floristic or taxonomic studies. The intention of this list of species was 2-fold: first, to document the current state of floristic knowledge, and second, to provide a basic reference that specialists can use to compare their data and more efficiently perform future taxonomic reviews. By examining the information provided here, it will soon be possible to handle more precise information on the floristic richness of the country and progress toward the long-awaited goal of having a flora of Mexico.

Materials and methods

The catalog is the result of the review of over 2,500 references covering different aspects of the flora of Mexico. Among them are the numerous fascicles published by different regional taxonomic treatments for the country. They include, for example, Flora of El Bajío and adjacent regions (>190 fascicles and >30 additional fascicles), Flora of Veracruz (>150), Flora of the Tehuacán-Cuicatlán Valley (>130), Flora of Guerrero (>60), Flora of Jalisco (>20), Flora Mesoamericana (5 volumes) and Flora Novo-Galiciana (17 volumes). The protologues of many species that had never been mentioned in previous inventories or floristic treatments were also consulted. For example, in the last decade (2006-2015) 924 brand new species occurring in Mexico have been described, and 656 species have undergone name changes due to the proposal of new taxonomic combinations. Very few of these 1,580 names had been mentioned in publications of inventories or vegetation studies, and their inclusion in the catalog derives directly from the publications where they were originally described.

The compilation of this catalog has also benefited from the large body of published state and regional inventories published. In his 2 early-century reviews, Villaseñor (2003, 2004) reported the major contributions of these publications to the flora of Mexico, and pointed out the 13 states lacking statewide inventories at the time. Interestingly, updated floristic lists have been published for 5 of them since (Ciudad de México - formerly known as Distrito Federal [Rivera-Hernández & Flores-Hernández, 2013]; Jalisco [Ramírez-Delgadillo et al., 2010]; Nuevo León [Villarreal & Estrada, 2008]; Oaxaca [García-Mendoza & Meave, 2011]; and Puebla [Rodríguez, Villaseñor, Coombes, & Cerón, 2014]). Moreover, 284 local and regional inventories from all over the country have been reviewed (reference list available from the author upon request). It is unfortunate that many important works are available in libraries only as “gray literature” (e.g. technical reports, unpublished undergraduate and graduate theses; Corlett, 2011). Figure 1 illustrates the geographic distribution of these reviewed inventories; the area included in each inventory is represented by a circle centered at the intersection of the long and wide axes of the surveyed area. Thus, the size of the circle is proportional to the studied surface.

Figure 1 Mexican localities with floristic inventories (N=284). The size of the circle is proportional to the studied surface in each inventory. Total surface covered by the studies is 378,887km2 (19.2% of Mexico's territory). 

Undoubtedly, another invaluable source of data was the intense review of the library and specimens housed at the National Herbarium (MEXU) at the Instituto de Biología, UNAM and internet databases (IPNI, REMIB, The Plant List, Tropicos, Unibio, etc.). Finally, numerous experts also lent valuable support reviewing draft lists of their specialty and providing relevant information on species important literature that is difficult to access or not yet recorded in the consulted information sources. Many of them (Table 1) should be cited as responsible for these groups in the catalog, though any nomenclatural discrepancies and the accepted name included in the list are the sole responsibility of the author.

Table 1 Specialists reviewers of preliminary lists of genera or families, or providers of relevant information crucial for elaborating the checklist. 

Specialist Taxonomic group or region
Leonardo Alvarado-Cárdenas Apocynaceae
William Anderson Malpighiaceae
Salvador Arias Cactaceae
Jesús Balleza-Cadengo Flora of Zacatecas
Theodore M. Barkley Senecio, Asteraceae
Attila Borhidi Rubiaceae
María Goreti Campos-Ríos Boraginaceae
Germán Carnevali Flora of the Yucatán Peninsula
Fernando Chiang-Cabrera Lycium, nomenclatural aspects
Thomas F. Daniel Acanthaceae
Arturo de Nova-Vázquez Flora of San Luis Potosí
Ricardo de Santiago Melastomataceae
María del Rosario García-Peña Cunila, Lamiaceae
José García-Pérez Flora of San Luis Potosí
Carlos Gómez-Hinostrosa Cactaceae
Socorro González-Elizondo Flora of Durango
Martha Gual-Díaz Flora of the Humid Mountain Forest
Ulises Guzmán Cactaceae
Héctor M. Hernández-Macías Cactaceae
Guillermo Ibarra-Manríquez Ficus, Moraceae
Jaime Jiménez-Ramírez Flora of Guerrero
Verónica Juárez-Jaimes Asclepiadoideae
Geoffrey A. Levin Acalypha, Drypetes, Euphorbiaceae, Putranjivaceae
Lucía G. Lohmann Bignoniaceae
José Antonio López-Sandoval Flora of the state of Mexico
Emily J. Lott Flora of Jalisco, Flora of Oaxaca
Martha Martínez-Gordillo Euphorbiaceae, Lamiaceae, Peraceae, Phyllanthaceae
Mahinda Martínez Solanaceae
Esteban Martínez-Salas Arecaceae, Triuridaceae
Angélica Ramírez-Roa Gesneriaceae
Jerónimo Reyes-Santiago Crassulaceae, Flora of Oaxaca
Lourdes Rico-Arce Mimosoideae
Gerardo Salazar-Chávez Orchidaceae
Mario Sousa-Sánchez Fabaceae
Rafael Torres-Colín Caesalpinoideae
Ivonne Sánchez del Pino Amaranthaceae
Victor W. Steinmann Euphorbiaceae
Jesús Valdés-Reyna Poaceae
Susana Valencia-Avalos Quercus, Fagaceae
Alejandra Vasco-Gutiérrez Elaphoglossum, Dryopteridaceae
Rito Vega-Aviña Flora of Sinaloa
Thomas L. Wendt Polygalaceae
Sergio Zamudio-Ruiz Flora of the Bajío

† Deceased.

Results

Floristic richness nationwide

So far more than 54,000 names to compile the final list have been reviewed, which is included as Appendix 2. The number of accepted species is 23,314, distributed in 73 orders, 297 families, and 2,854 genera (Table 2). In addition, there are 1,414 subspecific categories (subspecies, varieties or forms) that were not considered in the analysis; since there is uncertainty in assigning these taxa appropriately to the states, biomes or regions where they were reported, I preferred to document diversity to the species level. Considering these subspecific taxa would have increased floristic richness to about 24,728 native taxa. The higher taxonomic categories (families and orders) agree with the most recent classification proposals. For ferns, lycophytes, and gymnosperms, the arrangement follows the proposals of Christenhusz, Chun, and Scheider (2011) and Christenhusz, Reveal, et al. (2011), and angiosperms are arranged following the proposal of the APGIII (APG III, 2009; Chase & Reveal, 2009; Haston, Richardson, Stevens, Chase, & Harris, 2009; Wearn, Chase, Mabberley, & Couch, 2013).

Table 2 Taxonomic distribution of the native vascular flora of Mexico. 

  Orders Families Genera Species
Ferns and Monilophytes 14 41 134 1,039
Gymnosperms 5 6 14 149
Angiosperms 54 250 2,706 22,126
  73 297 2,854 23,314

Appendix 1 contains the families of vascular plants in the accepted classification schemes. Families follow a numerical order, defining a linear sequence in accordance with recent classification proposals. For each family a summary of the number of genera and species accepted is presented. The genera and species are listed in Appendix 2, with species endemic to Mexico being denoted with an asterisk (*).

The list does not include naturalized exotic species, many of which escaped from cultivation. Villaseñor and Espinosa-García (2004) documented the presence in Mexico of 618 exotic species. Currently, this figure needs to be revised, because similar to the case of native species, in addition to recent taxonomic changes, some 300 additional species have been documented. Consequently, summing the richness of species, subspecific taxa, and naturalized exotics, the number of vascular plants of the flora of Mexico totals more than 25,700 taxa of infrageneric rank. Finally, the list includes 48 species and 2 genera of hybrid origin (nothospecies and nothogenera).

Determining accepted names in the catalog required the revision of many additional names considered under synonymy. Considering that 31,263 synonyms were found, the average number of synonyms per accepted species is 1.3 (mode = 1); in other words, on average each species has been described more than once. It is important to emphasize that the synonymy search was not exhaustive and is almost certainly an underestimate. The ratio is based solely on the names retrieved from the floristic-taxonomic literature for the flora of Mexico, without considering relevant information for species which also occur outside the national territory. The numbers of species accepted and considered as synonyms in different taxonomic groups is variable; however, it should be noted that few orders comprise a lower number of synonyms than accepted species. Table 3 summarizes the number of accepted species and synonyms recorded in each order to date.

Table 3 Number of Mexican species accepted and number of synonyms recorded, by order of the 3 main groups of vascular plants. 

  Species accepted Synonyms
Ferns and Lycophytes
Cyatheales 21 59
Equisetales 6 12
Gleicheniales 7 23
Hymenophyllales 51 90
Isoetales 7 1
Lycopodiales 21 68
Marattiales 6 8
Ophioglossales 16 18
Osmundales 2 5
Polypodiales 782 1,473
Psilotales 2 3
Salviniales 12 21
Schizaeales 27 45
Selaginellales 79 57
Gymnosperms
Araucariales 3 7
Cupressales 30 53
Cycadales 50 20
Ephedrales 7 3
Pinales 59 137
Angiosperms
Alismatales 191 295
Apiales 243 207
Aquifoliales 21 19
Arecales 105 200
Asparagales 1,892 2,040
Asterales 3,150 4,240
Austrobaileyales 2 2
Brassicales 295 502
Buxales 7 2
Canellales 2 4
Caryophyllales 1,360 2,555
Celastrales 96 98
Ceratophyllales 2 2
Commelinales 140 212
Cornales 93 83
Crossosomatales 11 8
Cucurbitales 263 253
Chloranthales 1 1
Dilleniales 7 19
Dioscoreales 83 61
Dipsacales 69 96
Ericales 512 780
Fabales 2,012 3,114
Fagales 200 255
Garryales 9 12
Gentianales 1,238 1,562
Geraniales 45 29
Gunnerales 3 1
Huerteales 4 1
Icacinales 10 4
Lamiales 2,322 2,744
Laurales 160 235
Liliales 83 96
Magnoliales 85 63
Malpighiales 1,292 1,763
Malvales 568 875
Myrtales 625 790
Nymphaeales 14 12
Oxalidales 61 95
Pandanales 9 11
Picramniales 12 9
Piperales 322 274
Poales 1,952 2,509
Proteales 8 20
Ranunculales 197 239
Rosales 476 636
Sabiales 12 5
Santalales 166 161
Sapindales 421 530
Saxifragales 427 233
Solanales 704 1,002
Vitales 38 50
Zingiberales 64 76
Zygophyllales 41 75

Table 4 shows the 25 families and genera with the most species. Comparing these lists with those provided by Villaseñor (2004), only Bignoniaceae, Cucurbitaceae and Scrophulariaceae are excluded, which were listed before among the families of angiosperms with the largest numbers of genera (Villaseñor, 2004). Notably, this list now also includes a family of ferns (Pteridaceae). Moreover, all genera with the largest numbers of species (Villaseñor, 2004) continue to be ranked highest; the only exception is the genus Opuntia, which was reduced from 90 to 72 species due to the segregation of the genus Cylindropuntia (18 species).

Table 4 The 25 most diverse families and genera of Mexican vascular plants. 

Family Species
Asteraceae 3,057
Fabaceae 1,903
Orchidaceae 1,213
Poaceae 1,047
Euphorbiaceae 714
Rubiaceae 707
Cactaceae 677
Lamiaceae 601
Malvaceae 527
Asparagaceae 445
Bromeliaceae 426
Apocynaceae 418
Cyperaceae 416
Solanaceae 407
Acanthaceae 385
Crassulaceae 372
Convolvulaceae 295
Piperaceae 245
Pteridaceae 214
Amaranthaceae 211
Brassicaceae 210
Plantaginaceae 209
Apiaceae 208
Melastomataceae 204
Rosaceae 195
Species included 15,306

Genus Species
Salvia 328
Euphorbia 245
Tillandsia 237
Quercus 174
Mammillaria 169
Ageratina 165
Verbesina 165
Agave 160
Ipomoea 159
Dalea 146
Solanum 143
Piper 136
Sedum 133
Echeveria 132
Croton 127
Muhlenbergia 118
Epidendrum 117
Stevia 117
Carex 114
Mimosa 112
Desmodium 110
Peperomia 109
Acalypha 108
Begonia 106
Bursera 94
Justicia 94

The 25 most diverse families include 15,306 species, representing 65.6% of Mexico's total floristic richness. The additional 34.4% is distributed unevenly among the remaining 272 families. It is worth noting that 44 families are represented in Mexico by a single species, 23 by 2 species, and 45 by 3 to 5 species; consequently, rare or poorly represented families in Mexico constitute an important fraction of the plant diversity of Mexico.

Statewide floristic richness

An important question is whether the floristic richness of Mexico is distributed evenly throughout the country or if there are areas hosting a disproportionately high richness, and if so, where are they located? Some studies have tried to answer this question, but without considering all of the flora of Mexico (for example, Luna-Vega, Espinosa, Rivas, & Contreras-Medina, 2013; Rzedowski, 1991). The identification of extremely rich areas is the recurring theme of studies focused on conserving biodiversity. This paper provides a synthesis of floristic richness at the state level in order to identify the different levels of diversity by political entity (Table 5).

Table 5 Species richness of vascular plants in the 32 Mexican political states. EndMex=species endemic to Mexico; EndEdo=endemic to Mexico and restricted to the state; ExclEdo=non-endemic but restricted to the state. 

State Families Genera Species EndMex EndEdo ExclEdo
Aguascalientes (AGS) 147 690 1,871 658 2 1
Baja California (BCN) 151 797 2,336 447 140 572
Baja California Sur (BCS) 159 752 1,988 682 242 1
Campeche (CAM) 172 917 2,369 177 4 2
Chiapas (CHIS) 270 1,912 8,790 1,635 403 877
Chihuahua (CHIH) 176 1,091 4,291 1,197 89 54
Coahuila (COAH) 161 982 3,780 1,202 141 61
Colima (COL) 219 1,267 4,333 1,734 32 0
Ciudad de México (CDMX) 150 698 1,978 681 0 0
Durango (DGO) 191 1,145 4,472 1,861 80 5
Guanajuato (GTO) 176 965 3,206 1,350 7 0
Guerrero (GRO) 238 1,507 6,551 2,715 237 13
Hidalgo (HGO) 227 1,332 4,734 1,755 16 0
Jalisco (JAL) 235 1,541 7,155 3,353 182 4
México (MEX) 212 1,311 5,177 2,155 37 2
Michoacán (MICH) 219 1,394 5,885 2,588 85 16
Morelos (MOR) 197 1,063 3,491 1,242 23 1
Nayarit (NAY) 209 1,160 3,964 1,571 27 2
Nuevo León (NLE) 170 1,028 3,740 1,350 52 9
Oaxaca (OAX) 266 1,946 10,229 4,071 760 90
Puebla (PUE) 247 1,483 5,232 1,935 67 1
Querétaro (QRO) 215 1,289 4,411 1,691 11 0
Quintana Roo (QROO) 165 905 2,276 168 6 13
San Luis Potosí (SLP) 226 1,441 5,413 1,994 42 2
Sinaloa (SIN) 200 1,118 3,736 1,458 69 2
Sonora (SON) 183 1,158 4,106 1,081 77 59
Tabasco (TAB) 198 1,036 2,826 212 16 23
Tamaulipas (TAMS) 218 1,309 4,278 1,267 65 43
Tlaxcala (TLAX) 119 503 1,297 417 0 0
Veracruz (VER) 271 1,956 8,497 2,498 238 59
Yucatán (YUC) 150 803 1,900 157 6 7
Zacatecas (ZAC) 179 1,045 3,705 1,599 12 0
MEXICO 297 2,854 23,314 11,600 3,167 1,924

Table 5 presents an update of the vascular plant richness by state provided by Villaseñor and Ortiz (2014), supplemented with data for ferns and gymnosperms. Oaxaca, Chiapas, Veracruz, Jalisco and Guerrero respectively, continue to occupy the top 5 ranks of species richness. However, the accumulated information for other states, thanks to the collaboration with experts working with data on their floristic richness, has substantially changed compared to values recorded earlier.

Figure 2 illustrates the distribution of species, genus and family richness for each state synthesized in Table 5. The values are divided by the logarithm (log10) of the state's surface area, a procedure that essentially standardizes mean species richness per 10 km2. A diversity gradient from south to north is depicted, with some states of central Mexico breaking from this gradual shift of richness. Similarly, the 2 major peninsulas of the country (Baja California and Yucatán) had the lowest richness values for all 3 taxonomic levels evaluated.

Figure 2 Richness of species, genera, and families in Mexican states. Figures are standardized by area by dividing the total values by the logarithm (log10) of the state's surface (S). 

Apparently, most states showed a roughly equal proportion of species per genus (species/genus ratio; mean = 3.5, minimum = 2.3, maximum = 5.2), except Oaxaca, for which the largest ratio was recorded. The variation is more noticeable when comparing the proportion of species per family (minimum = 10.8, maximum = 38.3; average = 8.2; Fig. 3).

Figure 3 Proportion of species described by genera and families in the 32 political states of Mexico. 

Endemism

The total percentage of endemic species of Mexico among all vascular plants had not been previously reported. Most estimates are primarily for flowering plants. For this set, Rzedowski (1991) estimated their level of endemism at 52%. Subsequently, Villaseñor and Ortiz (2014) estimated the value at 50.4%. The figures included in this paper suggested that the level of endemism for all vascular plants is 49.8% (Table 5). Given the consistency between Rzedowski's estimate and these updated figures, levels of endemism are likely around 50%; in other words, about half of the flora of Mexico is endemic to its territory.

Another interesting finding from these analyses is that 3,167 species endemic to Mexico (13.6% of the total floristic richness and 27.3% nationwide endemics) are found only in 1 state. If we add to this figure those species that are not endemic to Mexico, but only found in a single Mexican state (classified here as restricted species), mostly at the northern or southern edge of the species’ geographic range, the proportion of rare species (species found in a single state) reaches 21.8% of the national floristic richness. Table 5 indicates how many of these rare species are recorded in each one of the states of Mexico.

Floristic richness in the major biomes of Mexico

In agreement with the uneven distribution of floristic richness statewide, irregular distribution patterns are also evident at larger scales, although it is feasible to recognize areas with characteristic species assemblages. For example, Villaseñor and Ortiz (2014) described 5 major biomes into which the vegetation types in Mexico are grouped, ranging in value from 5,296 species in the tropical humid forest (THUF) to 8,824 species in the temperate forest (TEMF). Here, the species richness distribution is assessed by biome, with special consideration of the species characteristic to each one. Species are classified as being restricted to (or characteristic of) a biome when distributed in only 1 biome or in 2 contiguous biomes (e.g., humid mountain forest (HUMF) and TEMF, or HUMF and THUF).

Table 6 shows the numbers of characteristic species, recorded by biome in each Mexican state. Highlighted in bold typeface are the 5 states contributing with the largest richness values to each biome. There are states where 1 or more of the biomes are not represented; for example, in Aguascalientes there is neither humid mountain forest (HUMF) nor seasonally dry tropical forest (SDTF) and there in neither HUMF nor temperate forest (TEMF) in Quintana Roo. However, the richness of these biomes is recorded anyway in these states because many species are found in more than 1 biome and are therefore distributed even in states where their principle biome is lacking. For example, most species reported for the HUMF in Aguascalientes are shared between HUMF and TEMF, the latter of which occurs in the state. Other species, for example Acaciella angustissima (Mill.) Britton & Rose (Fabaceae), Sambucus nigra L. (Adoxaceae), Tecoma stans (L.) Juss. ex Kunth (Bignoniaceae), or Zanthoxylum fagara (L.) Sarg. (Rutaceae), are widely distributed and have broad ecological tolerance, and are therefore found in almost every states and all 5 biomes in Mexico. On the other hand, of the 6,763 species reported by Villaseñor and Ortiz (2014) for the HUMF, 3,941 are characteristic of HUMF, found only in HUMF or in HUMF and only 1 more (adjacent) biome. Chiapas excells over all remaining states by having the largest number of HUMF characteristic species (2,399), and it is followed by Oaxaca (2,302) and Veracruz (1,875). In contrast, of the 6,852 species reported for the xeric scrubland (XESC), 4,614 are characteristic of the biome, highlighting Coahuila with the largest number of XESC characteristic species (1,764), followed by Chihuahua (1,461) and Sonora (1,411). Table 6 shows the values and percentages of characteristic species by biome in each of the states. In turn, Figure 4 shows the general distribution of characteristic species for each biome in Mexico as a whole. As in Figure 2, richness values are standardized per 10 km2 by dividing the total values by the logarithm (log10) of the state's surface.

Table 6 Native vascular plant species characteristic of each of the 5 main biomes of Mexico (occurring in only 1 or 2 biomes). In bold the highest number of characteristic species. HUMF=humid mountain forest; TEMF=temperate forest, THUF=humid tropical forest; SDTF=seasonally dry tropical forest; XESC=xeric scrubland. 

State HUMF TEMF THUF SDTF XESC
Aguascalientes (AGS) 83 471 12 122 430
Baja California (BCN) 16 300 3 263 1,282
Baja California Sur (BCS) 24 188 11 470 959
Campeche (CAM) 201 81 757 387 78
Chiapas (CHIS) 2,399 1,233 2,542 945 226
Chihuahua (CHIH) 153 1,450 22 426 1,461
Coahuila (COAH) 146 1,103 26 182 1,764
Colima (COL) 547 746 336 852 181
Ciudad de México (CDMX) 243 578 11 67 255
Durango (DGO) 302 1,515 57 414 1,078
Guanajuato (GTO) 247 839 34 234 713
Guerrero (GRO) 1,070 1,321 587 1,251 348
Hidalgo (HGO) 758 1,126 194 232 869
Jalisco (JAL) 875 1,877 473 1,237 695
México (MEX) 634 1,373 154 624 482
Michoacán (MICH) 725 1,364 334 1,053 432
Morelos (MOR) 386 697 118 469 238
Nayarit (NAY) 387 802 281 683 221
Nuevo León (NLE) 220 1,101 39 175 1,395
Oaxaca (OAX) 2,302 1,959 1,912 1,396 875
Puebla (PUE) 792 971 439 510 801
Querétaro (QRO) 519 968 170 285 871
Quintana Roo (QROO) 198 68 742 383 71
San Luis Potosí (SLP) 587 1,241 290 393 1,382
Sinaloa (SIN) 264 754 163 808 392
Sonora (SON) 108 1,076 45 700 1,411
Tabasco (TAB) 453 126 1,107 243 48
Tamaulipas (TAMS) 368 886 205 320 1,083
Tlaxcala (TLAX) 138 371 6 23 146
Veracruz (VER) 1,875 1,406 1,948 705 807
Yucatán (YUC) 110 55 528 382 92
Zacatecas (ZAC) 215 1,085 49 355 947
MEXICO 3,941 5,823 3,343 3,225 4,614

Figure 4 Map of the 5 main biomes of Mexico and richness values in each of them. Values were standardized by area by dividing the total values by the logarithm (log10) of the state's surface. Biomes following the definition of Villaseñor and Ortiz (2014)

Discussion

Mexico's total floristic richness (23,314 species) places it fourth in the world, behind Brazil (32,000 species, BFG, 2015), China (29,000 species; Qian & Ricklefs, 1999) and Colombia (24,000 species; Rangel, 2015). Villaseñor (2003) positioned Mexico in fifth place, below South Africa, although the most recent account for that country reported the existence of 20,000 native species (Germishuizen, Meyer, Steenkamp, & Keith, 2006). Also noteworthy is the high level of endemism of the flora of Mexico; although recent values reduce the percentage of endemism to 49.8%, it still occupies a privileged place among the world's countries, reporting a native flora that is very restricted to its political territories. Excluding archipelagos and other island territories that are characterized by large proportions of endemic species, Mexico is apparently surpassed only by South Africa among mainland countries in its proportion of endemic species (57.1%).

Reviewing the literature, especially protologues, has been instrumental for the inclusion of many species that have been previously forgotten or overlooked in our floristic knowledge, and continuing searches among journal publications allows the inclusion of the many recently described species occurring in Mexico whose inclusion in international databases (IPNI, The Plant List, Tropicos, etc.) is not immediate, and in fact sometimes it never happens. Consequently, search protocols should be established to increase the efficacy in incorporating these taxa into national databases, as this would allow the continuous updating of catalogs like the one presented in Appendix 2. Moreover, dealing with the diverging species concepts used by different specialists has not been trivial; the decision to accept or reject a name often reflects experience in a particular group or personal preferences.

It is interesting to contrast the species richness between northern and southern halves of the country (Fig. 2); this contrast more or less identifies the latitude of contact between the Nearctic and Neotropical realms. In turn, the decrease in richness in extreme northwest and southeast of Mexico, on the Baja California and Yucatán peninsulas, respectively, is likely the result of the peninsular effect, which causes peninsular territories to be less diverse than the continental nearby mainland due to their geographic isolation and the lack of important biomes (Gaston & Williams, 1996). Also interesting is the existence of some low-diversity states embedded within a matrix of high-diversity sites; this is the case of Tlaxcala, which had richness values well below its neighboring states at all 3 taxonomic levels. It would be revealing to further explore the cause of these low richness values; discerning whether this is due to lower collecting effort, land use change, or other factors would be informative and could modify the results reported here.

The data in Table 6 point out the large number of species that are found in only 1 or 2 biomes; thus, relatively few species actually show a wide ecological tolerance. Surely, this widely distributed component includes weedy species and those that tolerate disturbance, commonly occurring in successional plant communities known as secondary vegetation. For example, some 2,640 species are currently documented as having weedy and/or ruderal behavior (Villaseñor, unpublished data), of which 1,544 (58.5%) are reported from more than 2 biomes. Similarly, of more than 1,000 species known to grow in secondary vegetation, over 700 (70%) are also reported as occurring in 3 or more biomes.

Like other mega-diverse countries, the results reported here suggest that a large percentage of Mexico's flora tends to have high values of rarity, as indicated either by the breadth of its distribution (species recorded in few states; Table 5) or its habitat specificity (species recorded in few biomes; Table 6). Therefore, it should not come as a surprise to conclude that many species remain to be encountered before we can fully assess vascular plant diversity in Mexico (Villaseñor, 2015). With each new intensive exploration of a given territory (and sometimes by pure serendipity) anywhere in the country, new species are discovered and recorded, as a consequence of their narrow geographic and/or habitat distributions.

Concluding, any catalog, in the format of a flora, a taxonomic review, or a monograph, is constantly changing. The number of species reported here does not reflect the final number of different plants existing in Mexico. In fact, it may never be possible to explore any country in its entirety. This is particularly true for nations with orographies as complex as that of Mexico, so that each newly explored region will unavoidably reveal surprises. Even those regions that are better-researched are important sources of new floristic information (Ertter, 2000). Moreover, the dynamism of taxonomy and systematics, revived in recent decades by the implementation of new techniques and methods of analysis (cladistics, evolutionary biogeography, molecular biology, phylogeography, etc.) leads to constant changes in circumscriptions and taxonomic rearrangements, which requires periodic updates to the floristic information. The compilation of information in databases and the new tools provided by the informatics of biodiversity (Bisby, 2000; Graham, Ferrier, Huettman, Moritz, & Peterson, 2004; Soberón & Peterson, 2004), have made the curation and maintenance of taxonomic information more efficient, allowing for easier updates to catalogs like the one included here, and ultimately leading to the accumulation of more and better knowledge of our ever changing floristic richness.

Importantly, the sources of information for scholars of biodiversity and its conservation comes from 4 main sources: (1) floras, reviews and monographs; (2) catalogs and inventories; (3) databases, and (4) unpublished experience of experts. These sources are not, however, independent of one another; floras, taxonomic revisions and monographs rely on catalogs, which rely on experts to carry them out, and databases rely on all 3 of these sources for their reliable development, curation, and management. For this reason, countries whose biodiversity is high but poorly known, such as Mexico, should continue to support inventory studies, so that these other sources, including personal intellectual enrichment of specialists, continue to improve and facilitate the exploration and proper evaluation of the country's natural capital. Scientific collections and databases with a reliable curatorial level are the basis for taxonomic and systematic work; they constitute what May (1990) calls the bricks and mortar for building the biodiversity knowledge on which ecologists, biogeographers and analysts of biodiversity and conservation depend.

Completing the flora of Mexico is a huge undertaking that requires the accumulation of a vast amount of information, from data derived from field explorations, inventories, preparation of regional floras and taxonomic reviews, to the creation of databases of well-curated data. Floristic checklists, as one of these components, report the numbers of species recorded in the area of study and reflect regional species richness on which many studies assessing patterns of diversity, endemism (biogeographic), conservation, and macroecological patterns, among others, depend. They all rely on the quality of data, which must be reported with the utmost seriousness and a deep understanding of the floristic and taxonomic aspects essential to the enterprise. The identification and correct application of the names of plants is central to many areas of research in natural sciences and since the coining of the concept of biodiversity, in government agencies as well. Producing lists of threatened species, such as the Mexican NOM-054 (Semarnat, 2010), the IUCN Red List of Threatened Species (Walter & Gillett, 1998), and the international agreements regulating international trade of biological specimens (CITES), requires updated information on plants and their names.

The exercise carried out in this work, which aims to standardize the names used in the flora of Mexico, will hopefully serve as a useful framework for refining floristic information. Ultimately, this will provide benefits not only to the taxonomist community, but also to many other colleagues who use, require and routinely consult the kind of information presented here. Finally, paraphrasing the great botanist Arthur Cronquist, this catalog holds the work of our predecessors and will be the support of future floristic and taxonomic work.

Acknowledgments

Víctor Sánchez-Cordero, Director of the Institute of Biology, Universidad Nacional Autónoma de México (UNAM), and Gerardo Salazar, Head of its Department of Botany, supported this project with interest, energy and resources to bring it to its conclusion. The experienced and outstanding technical support from Enrique Ortiz, who prepared the maps and collaborated in the design, management and curation of the information in databases, facilitated the analysis and presentation of the data discussed in this work. Also, the technical assistance of Joselin Cadena, Guadalupe Segura, and Maribel Paniagua, allowed an efficient and accurate search of missing and necessary information to conduct better analysis and interpretation of results. Numerous colleagues, specialists of different plant groups (Table 1), revised preliminary or final versions of taxonomic groups included in this catalog; it was often difficult to reconcile their criteria with those used here to achieve homogeneity in integrating of the list. Thus, I assume full responsibility for what is quoted in the catalog, but thank them for their selfless support. Fernando Chiang critically reviewed the manuscript and the list of species, greatly improving the first and minimizing errors in the second. Emily J. Lott, Teresa Terrazas, Jorge Meave, and Claudio Delgadillo critically read the manuscript and their comments improved it considerably. Lynna M. Kiere reviewed and edited the English version of the paper. Some information was obtained through various projects that gave financial and human support; among the supporting institutions are Conabio (the Mexican Biodiversity Commission), Conacyt (the Mexican Council For Science And Technology), Senasica-Sagarpa (the National Service for Food Health and Quality of the Ministry for Agriculture, Animal Husbandry and Fisheries), Unibio (the Unit for Biodiversity Informatics), and the Institute of Biology, UNAM. Finally, thanks to all botanists (several thousand) not cited in the paper, collectors and taxonomists, ecologists and biogeographers, who with their contributions and publications, generated the combined wealth of information reflected in this study.

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** Peer Review under the responsibility of Universidad Nacional Autónoma de México.

Appendix 1.

Systematic arrangement of the native vascular plants of Mexico. The number of the families corresponds to the linear arrangement proposed by APG III (2009), Chase and Reveal (2009), Christenhusz, Chun, et al. (2011), Christenhusz, Reveal, et al. (2011), Haston et al. (2009) and Wearn et al. (2013). In parentheses, the first number indicates the number of genera and the second the number of species recorded for the family in Mexico.

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Appendix 2.

Native species of vascular plants of Mexico. An asterisk (*) indicates those species endemic to the country. State acronyms as in Tables 5 and 6. ND = data not available.

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Received: March 07, 2016; Accepted: May 06, 2016

*E-mail address: vrios@ib.unam.mx

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