Methods

The ^{WCVP (2022)} was downloaded from the website where it is published (wcvp.science.kew.org) and imported into a relational database (^{Codd 1990}). Several quick integrity tests were performed, such as establishing the primary key in the "kew_id" field, verifying that the content of each field belonged to its domain (e.g., that the “taxonomic_status” field contained only one of the values “Accepted”, “Synonym”, “Unplaced”). Then, some normalization steps were made, such as the separation of the year into a single field separated from the publication’s information.

After importing the source file with the validations and transformations described above, queries were made to determine totals, such as the number of names for each taxonomic category and the number of species with each taxonomic status. After obtaining the totals, which also served as quality control for subsequent queries, further analyses were designed and executed to quickly evaluate the quality of the information, such as identifying the existence of repeated records or pairs of inconsistent records (for example, homonyms or pairs of names that had different authors but were both recorded as “Accepted”).

Once carrying out the quick evaluation process of the quality of the information, queries were designed and applied to synthesize the information, such as the count of names for each publication year and the number of species published per decade. Finally, to estimate the completeness of the information, the names on the WCVP list were compared against the list of names of species of vascular plants in Mexico (^{Villaseñor 2016}, ^{Villaseñor & Meave 2022}) to determine how many names from the latter were missing from the former.

Estimating the number of species of vascular plants worldwide. The inclusion of the synonyms for each accepted species in the WCVP makes it possible to estimate the number of species yet to be discovered or published using the Chao₂ estimator (^{Colwell & Coddington 1994}). Chao₂ is a very simple non-parametric richness estimation index that proposes that

where S_est is the estimated number of species, S_obs is the number of currently known species, Q₁ is the number of species occurring in a single sample (singletons), and Q₂ is the number of species occurring in exactly two samples (doubletons). In this case, singletons (Q₁) are represented by the accepted species with a single synonym, while doubletons (Q₂) are accepted species with two synonyms. The estimate of the number of potential species calculated by this index could theoretically help to determine the extent of taxonomic work that remains to be done. The application of the Chao₂ estimator using the number of synonyms supports its heuristics because nonparametric richness estimates (like Chao₂) use only part of the available information on the distribution of species, for example, singletons and doubletons. In this case, the number of synonyms for each accepted species is also partial information and is not directly related to the number of accepted species. We consider the species estimate based on synonyms a heuristic method that allows exploring the assessment of the number of species.

Relative species richness. To compare the Mexican species richness by family against the worldwide richness, a relative species richness index was defined and calculated as follows:

Absolute species richness (ASR) = Number of species of the family in Mexico (according to ^{Villaseñor 2016} and ^{Villaseñor & Meave 2022}).

Relative species richness (RSR) = Number of species of the family in Mexico (ASR) / Number of species of the family worldwide.

Each family was assigned a category in a four-quadrant matrix depending on its RSR and absolute species richness (ASR) values in Mexico: I) High RSR and High ASR, II) High RSR and Low ASR, III) Low RSR and Low ASR, or IV) Low RSR and High ASR. The RSR and ASR values were categorized as high or low depending on whether they were above or below the average of each index, calculated as follows. The average RSR was calculated as the total number of species of all families in Mexico divided by the total number of species worldwide; the average ASR was calculated as the total number of species in Mexico divided by the total number of families in Mexico.

Finally, to order the list of Mexican families in a single dimension that combines their ASR and RSR, an index of relative importance (RI) was defined as the product of the relative species richness and the absolute richness:

where RI is the index of relative importance, ASR is the absolute species richness, and RSR is the relative species richness.

Results

Plant diversity in a global context. The WCVP contains a total of 999,713 species names and fills an important gap in the knowledge of our planet’s biodiversity, placing the knowledge and richness of the different regions of the world into context. This list reports 350,980 accepted species worldwide (Table 2); meanwhile, approximately 25,000 species have been reported for Mexico, so it can be estimated that 7 % of worldwide vascular plant diversity at the species level and 19.5 % of diversity at the genus level are found in Mexico. The Earth’s land surface is approximately 130,000,000 km², while Mexico covers about 2,000,000 km². Thus, about 7 % of the world’s total known vascular plant species are found in Mexico, which accounts for only about 1.5 % of the total land area, placing the floristic richness of the country more diverse than expected.

A historical accounting of taxonomic work around the world. The WCVP allows an analysis of the evolution of taxonomic work worldwide over time. Figure 1A shows an intense period of taxonomic work between 1820 and 1850, exceeding 2,000 genus names published per decade. Similarly, at the species level (Figure 1B), the decades from 1890 to 1930 had the highest numbers of new species reported per decade, with more than 50,000 new species published per decade. These numbers reflect the immense taxonomic work carried out around the world.

Figure 1 Number of new genus (A) and species (B) names published per decade reported in the WCVP. Both accepted names and synonyms are included. 

The publication of new species over time (Figure 2) also evidences the influence of modern systematics synthesis, which includes more evolutionary approaches in the taxonomy of vascular plants compared to earlier works. In the 1930s, taxonomists intensified the work of reclassifying previously described species. At the beginning of the 1970s, the number of basionyms increased, probably due to new systematics and molecular tools available to taxonomists.

Figure 2 The number of accepted species names published per decade and included in the WCVP. 

Almost half of the accepted species in the last four decades correspond to systematic re-arrangements rather than species that are completely new to science. As shown in Figure 2, the 2010 decade contains the highest number of accepted names (39,744), but 17,057 of those species were accepted based on previously published names, mainly constituting new combinations. In summary, the taxonomic activity aimed at the discovery of new species in the last decade was slightly higher than that of the decades from 1900 to 1930, but most of them constituted taxonomic adjustments derived from systematic studies.

Taxonomic information is continuously evolving. Because the WCVP is in a digital medium, the list of taxa can be analyzed automatically. For example, in this exercise, we surveyed duplicate records by searching for homonyms with the same author and same publication, but different id. The search only retrieved 58 pairs of duplicated records (116 records), which should be reviewed and updated or removed. This quick quality test shows that the inconsistencies in the list are minimal and exemplifies one of many possible tests that users of the information can design. As the information provided is improved and used, it will benefit other research areas that depend on better floristic taxonomic knowledge.

An additional quality test was the verification of publication years. We detected only seven records with 5-digit values (for example, 19713 instead of 1913); these errors are easy to correct based on the name of the journal, the volume, and the publication number. Another quick validation detected six pairs of records with the same species name but different authors, both recorded as accepted; for example, Helichrysum oligocephalum DC. and Helichrysum oligocephalum S.Moore.

There was no completeness test on the list of synonyms; more than half a million synonyms are reported for about 177,000 species. The species with the most synonyms is the potato, Solanum tuberosum, with 441 names, published from 1758 to 2006. The years 1930 and 1980 had the largest number of synonyms added, with 126 and 100 names respectively, mainly for infraspecific categories.

One interesting situation is the case of varieties for which there is no published species name. For example, Wigandia kunthii var. eukunthii Brand, Wigandia kunthii var. intermedia Brand, Wigandia kunthii var. macrophylla (Schltdl. & Cham.) Choisy, and Wigandia kunthii var. viscosa Brand, are all reported in the WCVP as synonyms of Wigandia urens (Ruiz & Pav.) Kunth, but there is no record of the species Wigandia kunthii. In total there are 2,820 subspecific category names that fall under this scenario.

Among the 350,980 accepted species names, 41 % are marked as "Reviewed" and the remaining 59 % as "In review". This illustrates the immense amount of work that is still required to obtain an even more precise list. Similarly, the number of homonyms (species with the same name but different authors) is 73,050, distributed among a total of 35,600 different accepted species names (on average two homonyms per species name). The name with the largest number of homonyms is Scorzonera graminifolia, for which nine homonyms are reported (Table 3). Another curious finding with respect to homonyms is that of the 223 pairs of homonymous species that belong to different families, only 32 of these pairs had one or both names reported as “Accepted”. In the remaining pairs, both names had the status of “Synonym”. Finally, since the list is constantly growing and being updated, it still contains a few spelling errors; for example, the species Solanum rudepannum is written with an intermediate hyphen, as Solanum rude-pannum.

The WCVP includes two families for which no accepted species are reported, and all their names have been considered as synonyms: Adiantaceae (two synonymous species of the family Pteridaceae) and Gigaspermaceae (with the genus Pomatotheca given the status of “Synonym” and labeled as “in review”).

The publication of names is mostly a solitary activity. The analysis documents that 86 % of the taxonomic names in the WCVP have been published by a single author, 12.6 % by two authors, and only 1.2 % involved three or more authors (Table 4). These data point out that taxonomic work is a predominantly individual activity, a situation that seems to be due to a lack of training for new taxonomists. As this exercise reveals, taxonomic knowledge is mainly learned through personal activity and to a lesser extent through participation in working groups.

The WCVP in the context of the flora of Mexico. Among the 25,105 accepted names of species recorded in Mexican territory, the WCVP recognizes 21,632 as accepted, 2,671 as synonyms, and 67 as “Unplaced”; 735 are not included in the WCVP. These missing names have probably not been included in the WCVP because they were recently published and have not yet been accessed in the database by the specialists in charge of reviewing the groups to which they belong (107 families). In just the past three years (2019-2021), about 300 new species have been described for Mexico, several of which have not yet been recorded in the WCVP. Without a doubt, these 735 names will be incorporated into the WCVP once they are carefully reviewed.

The consistency (86.2 %) among accepted names (21,632 out of 25,105) shows that unifying criteria in systematics is still a difficult task, which requires time and long academic discussions through publications. However, it is also encouraging to recognize that two different academic groups with different study areas and geographic scales have compiled lists of species names that share such a high percentage. As stated above, the comparison of the two lists shows that Mexican vascular plant diversity represents 7 % of the entire planet’s flora at the species level and 19.5 % of total genera.

Appendix I includes the number of species recognized and estimated for each family. The number of estimated species results in 436,013 species. As a result, we postulate 80 % of completeness of the floristic knowledge of the world flora, remaining to be discovered about 20 % additional species to those already included in the WCVP. Thus, it is predicted that some 85,033 species of vascular plants remain to be described and published. If the current rate of species description worldwide is about 3,000 species per year, then it will still take about 28 years of taxonomic activity to inventory all the vascular world's flora. This assumes that we will be able to overcome the impediments due to a lack of trained taxonomists engaged in the circumscription and description of species, as well as the current disregard for alpha taxonomy.

Relative species richness. The average RSR value for the Mexican families was 7.2 % (25,105 / 350,980), while the average value of ASR, that is, the average number of species per family, was 87.2. The number of families per quadrant in the RSR vs. ASR matrix (Figure 3) was: quadrant I: 37, II: 110, III: 115, and IV: 25. The 'Relative Importance' index (Appendix I), had a range of 0.003 (Lecythidaceae)-304.1 (Asteraceae), and ranges by quadrant were I: 7.3-304.1; II: 0.08-16.6; III: 0.003-4.15; IV: 2.11-52.05.

Figure 3 RSR ( ASR matrix containing the families of vascular plants of Mexico. The eight families with the highest ‘Relative importance’ (RI) index in each quadrant are listed. 

Of the ten families with the highest species richness in Mexico (high ASR), eight also had high RSR (i.e., placed in quadrant I): Asteraceae (IR = 304.1), Cactaceae (283.1), Fabaceae (189.4), Poaceae (132.9), Crassulaceae (95.1), Euphorbiaceae (81.7), and Asparagaceae (71.7). The two families with high ASR but low RSR (quadrant IV) were Orchidaceae (52.0) and Rubiaceae (38.1).

The ten families placed in quadrant II (low ASR; high RSR) and with high RI index values are Pinaceae (16.6), Zamiaceae (14.4), Lentibulariaceae (13.5), Fouquieriaceae (11.0), Phrymaceae (10.2), Dioscoreaceae (9.1), Aristolochiaceae (8.7), Selaginellaceae (8.7), Loasaceae (8.2), and Bignoniaceae (7.7). On the other hand, the ten families placed in quadrant III (low ASR; low RSR) with high RI index are Salicaceae (4.1), Geraniaceae (4.0), Moraceae (3.6), Loranthaceae (3.2), Caprifoliaceae (3.0), Violaceae (2.8), Papaveraceae (2.4), Oxalidaceae (2.4), Annonaceae (2.0), and Capparaceae (2.0).

Discussion

Plant diversity in a global context. The relevance of the WCVP lies mainly in the synthesis of a query platform for all the species known to date worldwide. It efficiently complements other consultation platforms that people interested in this type of information visit regularly, such as Plants of the World Online (powo.science.kew.org) or Tropicos (www.tropicos.org/home). In addition, there are lists of species published, but none with a worldwide context, being limited to regions of the world. Examples on the national scale are the flora of Brazil (^{BFG 2015}, ²⁰²²) or Mexico (^{Villaseñor 2016}) or at the continental scale, Europe (^{Tutin et al. 1964-1980}, https://eunis.eea.europa.eu/references/1780/species) or America (^{Ulloa-Ulloa et al. 2017}, http://legacy.tropicos.org/Project/VPA). The figure of 436,013 species estimated with the WCVP is quite close to the 400,000 estimated by the World Flora Online project (^{Borsch et al. 2020}) and to the 450,000 predicted by ^{Pimm & Joppa (2015)}.

The WCVP: an epistemological archive. In addition to constituting a worldwide list of taxonomic names of vascular plants, which facilitates further taxonomic work, the WCVP implicitly contains part of the history of the discovery of this plant diversity. It can be considered an epistemological archive of biodiversity; in that it contains the record of the various scientific approaches to taxa by different authors over time. It also contains the thread of its discussion and construction, since the relationships between synonymous and accepted names reflect such scientific work. It can be said that there has been an "epistemic compromise" (sensu ^{Granjou & Arpin 2015}) within the community of taxonomists whose different points of view are integrated into their practices and research networks and whose work has been reflected in the WCVP. For example, the decade of 1890 is the one recording the largest number of published names, while the decade of 2010 records the maximum number of accepted species. As shown in this text, many other statements can be deduced by analyzing the WCVP.

The checklist accounts for taxonomic work over 28 decades. From Linnaeus to the present day, more than 1.2 million taxonomic names of vascular plants have been published, of which almost a million correspond to species names, and of these, 35 % are considered accepted. Thus, taxonomic work has not traveled a direct and easy path towards biodiversity knowledge; the activities of describing and publishing have led to a solid and consensual knowledge, where the proposed species go through long and sinuous scrutiny, as indicated by 61 % of synonyms and other data available in the WCVP. It is precisely this epistemological archive that has been used here to estimate the number of species yet to be discovered for each family and described in Appendix I. The estimates are calculated based on the number of synonyms, that is, on the history of the construction of that knowledge. As a result of the long road that taxonomists have traveled with dedication and scrutiny, we may be approaching Linnaeus's dream -an inventory of species from all over the world- at least for vascular plants.

The continuous evolution of taxonomic information. It is not surprising that taxonomic information evolves and increases daily. However, it is important to recognize that this evolution is the result of two main impulses; the first is better taxonomic and evolutionary knowledge of the groups, and the second is more precise and accessible ways of reporting that knowledge. The first impulse allows an even more systematic and orderly way of compiling this great list of taxa; the second improves every day with better information and communication technologies.

The high quality of WCVP information. The quick quality tests applied here to the WCVP indicate that it contains high-quality information with minimal inconsistencies. The inclusion of a unique identifier for each name, whether accepted or synonymous (the kew_id field), facilitates communication between taxonomists and working groups, as well as the annotation of errata. The proportion of species names of the flora of Mexico not included in the checklist (735/25,105 = 2.9 %) serve as a quick estimate of its completeness (about 97 %).

The WCVP in the context of the flora of Mexico. The most updated list of vascular plant species for Mexico dates back more than five years (^{Villaseñor 2016}). The WCVP allows comparisons from different points of view that will help to improve both worldwide and Mexican lists. Although one can be considered a subset of the other, as reviewed in this study, this is not the case, since both contain some differences in the inclusion criteria and the definition of taxonomic status (e.g., accepted vs. synonym). In this non-exhaustive analysis, we calculated that Mexican vascular plant species richness represents about 7 % of the species richness of the entire planet. This percentage may be adjusted as both lists are refined, but above all, adjusted so that they are comparable. Another important parameter is the rate of description of new species for Mexico, which has recently been estimated at close to 100 per year (^{Alvarado-Cárdenas et al. 2021}).

The WCVP allows the floristic richness of a country to be placed in a global context. It facilitates contextualization of the species richness of each family in Mexico concerning world richness. The relative species richness (RSR), calculated by dividing the number of species in the country by the number of species in the world, can be a tool for planning floristic work considering geographical contexts. The relative importance index (RI), defined, as the product of the relative species richness and the absolute species richness, is a tool to order the families of a country in a way that prioritizes them considering the global context. One potential application of the RI would be its use as a criterion to prioritize future exploration and study work, especially for the flora of Mexico.

There are at least five efforts to create a database of the world's plants (^{Qian et al. 2022}); each one of them deserves an analysis of the content of their information like the one presented here.

As indicated on the WCVP website, its information constitutes a backbone, created from the unification of two major sources of information -the International Plant Names Index (IPNI) and the World Checklist of Selected Plant Families (WCSP). This backbone can be consulted and downloaded from the web page wcvp.science.kew.org, and constitutes, without a doubt, a milestone in the systematics of vascular plants and the knowledge of our planet’s plant diversity. Its consultation and analysis of the information contained will make it easier to carry out a better inventory of the great floristic diversity of Mexico, which is far from being completed.