Amphibians and Reptiles of the Pacific Lowlands Biogeographic Province of Mexico: Diversity, Similarities, and Conservation

Abstract

The Pacific Lowlands (PL) of Mexico hosts a rich diversity of amphibians and reptiles, comprising 326 native species (76 amphibians and 250 reptiles) across 45 families. This region is significant from a biodiversity standpoint, accounting for 23.3% of Mexico’s total amphibian and reptile species, with 30 species being endemic to the PL. Notably, the PL ranks sixth in amphibian and reptile diversity among Mexico’s fourteen biogeographic provinces. The species composition indicates a complex biogeographic relationship for the province, which shares the highest percentages of species with the neighboring Transition Zone provinces, while also acting as a vital corridor for species adapted to varied environmental conditions. Despite its diversity, 24 species in the PL are of conservation concern, highlighting threats from habitat loss, urbanization, and the impacts of introduced species in the province. Our findings underscore the ecological significance of the PL and the urgent need for conservation efforts to protect its unique biodiversity.

1. Introduction

Mexico stands out as one of the megadiverse countries in the world, recognized for its exceptional diversity of flora and fauna, a result of its location between the Nearctic and Neotropical biogeographic regions [1,2]. This diversity is enriched by the complex topography and the presence of two oceans, factors that have facilitated the evolution of numerous species adapted to diverse environments over short distances (México Megadiverso. https://www.biodiversidad.gob.mx/pais/quees, accessed on 1 June 2024) [3]. Mexico is organized into 14 biogeographic provinces, among which the Pacific Lowlands (PL) stands out by containing the largest area of seasonally dry tropical forest in the country [4,5]. The PL runs from the center of Sonora to north-to-southeastern Chiapas on the border with Guatemala to the south. This province is one of the richest in biodiversity, although it also faces growing conservation threats due to accelerated deforestation driven by tourism development, livestock, and agriculture [6].

The seasonally dry tropical forests of the PL support approximately 35% of the mammal species, 33% of the bird species, 34% of the reptile species, and 25% of the known amphibian species from Mexico [6]. The seasonally dry tropical forests of western Mexico, including the Balsas Basin, an independent biogeographic province, and the Chiapas Depression, part of the Veracruzan biogeographic province, host an estimated 301 species of amphibians and reptiles [7]. This represents a significant percentage (21.5% = 301/1399) of the total herpetofauna reported for the country [8].

Despite the significant biodiversity and presence of unique ecosystems like the seasonally dry tropical forest, the Pacific Lowlands of Mexico has not been comprehensively studied with regard to its amphibian and reptile species. Previous studies have addressed parts of this biogeographic province. For example, [6] highlighted conservation issues within the seasonally dry tropical forest of western Mexico, emphasizing its biodiversity. However, their scope was broader and did not exclusively focus on amphibians and reptiles. Ref. [9] assessed the conservation status of the herpetofauna of the Pacific Lowlands and the neighboring Balsas Basin and Chiapas Depression, the latter forming part of the Veracruzan province. Similarly, Ref. [7] analyzed patterns of diversity of amphibians and reptiles across the Pacific Lowlands, including the seasonally dry tropical forest regions of the Balsas Basin and Chiapas Depression, noting an inverse relationship between latitude and diversity. Our study, however, stands apart by reporting the diversity of amphibians and reptiles across the entire Pacific Lowlands biogeographic province, but not including neighboring provinces, and comparing this diversity with adjacent provinces. We highlight the conservation status, distribution, and endemism of amphibian and reptile species within this province, stressing the urgent need for conservation efforts amidst growing anthropogenic pressures. Such an approach helps establish a basic understanding of the region’s herpetofaunal diversity, as well as identify biogeographic provinces or regions in particular need of conservation. Determining the similarity of the herpetofaunas of the PL and its neighboring provinces can help determine at what scale conservation planning should be carried out (e.g., across provinces or within provinces). Focusing on the biologically relevant biogeographic provinces (e.g., [5,10]) is likely more appropriate than focusing on politically determined states.

2. Methods

2.1. Physiography

The PL stretches in a long, narrow strip along the Pacific Coast of Mexico, spanning parts of several Mexican states, including Sonora, Chihuahua, Sinaloa, Durango, Nayarit, Zacatecas, Jalisco, Colima, Michoacán, Guerrero, Oaxaca, and Chiapas, as well as portions of Guatemala, El Salvador, Honduras, Nicaragua, and Costa Rica. This province also includes two sets of islands in the Pacific Ocean: the Revillagigedo Archipelago (Isla San Benedicto, Isla Socorro, Isla Clarión, and Isla Roca Partida) and the Marias Islands off the coast of Nayarit (modified from [5]) (Figure 1).

Covering an area of 187,113 km², the PL extends between 14.5339° and 30.2719° latitude and −92.0702° to −114.7602° longitude. The province has a perimeter of 26,517 km and borders eight other Mexican biogeographic provinces (Table 1): the Chihuahuan Desert, the Sonoran, the Sierra Madre Occidental, the Transvolcanic Belt, the Sierra Madre del Sur, the Balsas Basin, the Veracruzan, and the Chiapas Highlands. Along its western edge, the PL is bordered by the Pacific Ocean. Along its eastern border, the PL is primarily bounded by mountain ranges: the Sierra Madre Occidental in the north, the Transvolcanic Belt in the center, and the Sierra Madre del Sur and the Chiapas Highlands to the south, except for where it borders the Balsas Basin and the Isthmus of Tehuantepec [11]. According to the shapefile map provided by [10], the PL has an elevational range of up to 2200 m, although it generally does not exceed 400 m above sea level (modified from [5]). The PL is notable for having the greatest latitudinal range in Mexico, at 15.73°, as well as the largest perimeter and the highest number of neighboring provinces (Figure 1).

The PL contains a diverse range of vegetation types, including the subtropical dry forests found in Sonora and the northern part of Sinaloa, seasonal dry tropical forests, mangroves along the coast, shrublands, and cultivated fields (Figure 2; [5,9]). However, the predominant vegetation type is the seasonally dry tropical forest, which runs continuously from central Sonora to southeastern Chiapas, along the border with Guatemala [12,13]. The prevailing climate in the region is warm subhumid, with average monthly temperatures ranging from 16 °C to 30 °C and average monthly precipitation varying from 300 to 2000 mm, with a dry season typically lasting between six and ten months per year (Figure 3; [7,14]).

2.2. Methodology

We updated the species lists for amphibians and reptiles for all of the Mexican states included in the Pacific Lowlands (PL) biogeographic province (Sonora, Chihuahua, Sinaloa, Durango, Nayarit, Zacatecas, Jalisco, Colima, Michoacán, Guerrero, Oaxaca, and Chiapas) provided by [17] and updated with [18,19,20,21,22,23,24,25]. We used the definition of the PL biogeographic province of [5,10,26,27] to use the state checklists to create a species list for the PL. We used [8] for species lists of the neighboring provinces to the PL. We follow Amphibian Species of the World (https://amphibiansoftheworld.amnh.org/index.php accessed on 17 June 2024) [28] and AmphibiaWeb (http://amphibiaweb.org accessed on 17 June 2024) [29] for amphibian names and the Reptile Database (http://www.reptile-database.org accessed on 17 June 2024) [30] for reptile names. In addition, we recorded the conservation status and population trends of each species based on the International Union for Conservation of Nature’s (IUCN) Red List 2024-1 (https://www.iucnredlist.org/ accessed on 21 June 2024) [31], Secretaría del Medio Ambiente y Recursos Naturales (SEMARNAT) [32], and environmental vulnerability scores (EVSs; [33,34]).

We used hierarchical clustering analyses based on Jaccard’s Similarity Coefficients for Binary Data as the distance metric and the single-linkage method (nearest neighbor) to generate cluster trees of the PL and the neighboring biogeographic provinces (Sonoran, Chihuahua Desert, Sierra Madre Occidental, Transvolcanic Belt, Sierra Madre del Sur, Balsas Basin, Veracruzan, and Chiapas Highlands) for amphibians and reptiles separately. We also used the species lists to calculate pair-wise Jaccard distances for the PL and its eight neighboring biogeographic provinces for amphibians and reptiles separately. In addition, we obtained geospatial estimates using the map of biotic provinces of Mexico by [10] on a Lambert Conformal Conic projection in the Datum Word Geodetic System 1984 (WGS84) in ArcGIS 10.8.1 (Environmental Systems Research Institute, Inc., Redlands, CA, USA) for the length of shared borders between the biogeographic provinces using the Polygon Neighbor Tool and the straight-line distance between the centroids of the biogeographic provinces using the Feature to Point Tool and Point Distance (Table 1). We used non-parametric Spearman’s ρ tests to examine correlations among Jaccard distance estimates for amphibians, reptiles, the length of shared borders, and the distance between the centroids of the biogeographic provinces for all pairs of the PL and its neighboring provinces. Cluster analyses were performed using Systat 13.2 (Systat Software Inc., San Jose, CA, USA), and all other statistical analyses were performed using JMP 16.2 (SAS Institute, Cary, NC, USA).

3. Results and Discussion

3.1. Species Richness

The Pacific Lowlands (PL) is home to 326 native species of amphibians and reptiles, comprising 76 amphibians and 250 reptiles, representing 45 families. Of these, 13 are amphibians (11 anurans, 1 salamander, and 1 caecilian) and 32 are reptiles (including 2 crocodiles, 16 lizards, 8 snakes, and 6 turtles), with a total of 126 genera (26 amphibians and 100 reptiles) (Supplementary Table S1; Table 2 and Table 3). According to [8], Mexico harbors 1399 native amphibian and reptile species (435 amphibians and 964 reptiles), distributed among 55 families (16 amphibians and 39 reptiles) and 210 genera (55 amphibians and 155 reptiles), findings similar to those reported by [35]. Consequently, the PL contains 80.0% (44/55) of the families, 81.3% (126/155) of the genera, and 23.3% (326/1399) of the species of amphibians and reptiles found in Mexico. For amphibians, the PL has representatives of 81.3% (13/16) of the families, 47.3% (26/55) of the genera, and 17.5% (76/435) of the species in Mexico; for reptiles, it includes 82.5% (32/39) of the families, 64.5% (100/155) of the genera, and 25.9% (250/964) of the species.

Compared to the other thirteen biogeographic provinces of Mexico, the PL ranks sixth in amphibian and reptile diversity, following the Sierra Madre del Sur (517 species), Transvolcanic Belt (427), Sierra Madre Oriental (382), Chiapas Highlands (354), and Veracruzan (340) [8]. Specifically, for amphibians, the PL ranks sixth, following the Sierra Madre del Sur (186), Transvolcanic Belt (154), Sierra Madre Oriental (123), Chiapas Highlands (112), and Veracruzan (93) [8]. The PL ranks fourth in reptile diversity, following the Sierra Madre del Sur (331), Transvolcanic Belt (271), and Sierra Madre Oriental (259) [8].

Six amphibian and twenty-four reptile species are endemic to the PL. Additionally, six introduced species are present in the PL: common bullfrog (Rana catesbeiana), stump-toed gecko (Gehyra mutilata), common house gecko (Hemidactylus frenatus), Mediterranean house gecko (Hemidactylus turcicus), mourning gecko (Lepidodactylus lugubris), and Brahminy blindsnake (Indotyphlops braminus). How these non-native species arrived in Mexico, and in particular, the PL, is somewhat unclear (e.g., H. frenatus, [36]). However, some species probably arrived via aquaculture (e.g., escape and international trade, R. catesbeiana, [37,38]). Other species likely arrived inadvertently as hitchhikers on ships carrying cargo from around the world (e.g., I. braminus, [39]) or by other means of anthropogenic transport (e.g., H. turcicus, [40]). At least some of these non-native species have the potential to negatively impact native amphibians and reptiles. For example, non-native R. catesbeianus may be a reservoir for amphibian pathogens such as Batrachochytrium dendrobatidis and ranavirus (e.g., [37,38,41,42]) or parasites [43]. Other non-native species may outcompete native species (e.g., H. turcicus, [44]).

3.2. Distribution

Thirty-nine of the seventy-six amphibian species are endemic to Mexico, with six (7.9%) endemic to the PL. Apart from the six species endemic to the PL, all amphibians in the PL inhabit biogeographic provinces in both the Neotropical Region and the Transition Zone, with only some extending into provinces of the Nearctic Region. The 37 amphibian species in the PL that are not endemic to Mexico can be categorized into three groups: (1) 10 species are shared with the United States, (2) 23 species have their northernmost distribution in Mexico and extend south into Central or South America, and (3) 4 species are widely distributed throughout the Americas (Smilisca baudinii, Hypopachus variolosus, Rhinella horribilis, and Leptodactylus fragilis).

Of the 250 reptile species found in the PL, 129 are endemic to Mexico, with 24 of these species being endemic to the PL, 5 of which are restricted to islands: Socorro Island tree lizard (Urosaurus auriculatus), Clarion lizard (Urosaurus clarionensis), María Cleofas leaf-toed gecko (Phyllodactylus cleofasensis), Clarion Island whip snake (Masticophis anthonyi), and Islas Revillagigedo nightsnake (Hypsiglena unaocularus). Twenty-nine of the reptile species from the PL that are endemic to Mexico have broad distributions across the country, occupying provinces in the Nearctic and Neotropical Regions and the Transition Zone. Seventy reptile species of the PL that are endemic to Mexico inhabit provinces in the Neotropical Region and the Transition Zone. Two reptile species of the PL that are endemic to Mexico, the four-toed worm lizard (Bipes canaliculatus) and the Balsas spiny-tailed iguana (Ctenosaura clarkii), have a distribution that is limited to the Balsas Basin and the PL. The 121 species of reptiles inhabiting the PL that are not endemic to Mexico can be categorized into four main groups: (1) 37 species shared with the United States, (2) 69 species whose distribution extends from Mexico south into Central or South America, (3) 10 species that are distributed from the United States to Central or South America, and (4) 6 species of marine reptiles that have circumglobal, oceanic distributions (Hydrophis platurus, Caretta caretta, Chelonia mydas, Eretmochelys imbricata, Lepidochelys olivacea), and Dermochelys coriacea).

Although the PL is part of the Neotropical Region, its transitional nature is reflected in the mix of its species, many of which are distributed across multiple provinces. At 26,517 km, the PL has the longest perimeter of the Mexican biogeographic provinces and spans a latitudinal gradient of 15.73°, the highest in the country. These conditions facilitate contact with eight other biogeographic provinces, creating dispersal routes that allow for the exchange of species adapted to diverse environmental conditions, including tropical, temperate, and arid climates.

Despite its melting pot nature, the PL is still home to 30 species of amphibians and reptiles that are endemic to the PL, representing 2.1% of the species in Mexico (30/1399) and 3.4% of the endemic species of Mexico (30/892) [8]. Thus, while the PL serves as a significant corridor for diverse species, it also hosts a small yet unique subset of species that are adapted to its specific conditions. Indeed, the PL is one of the most important regions of endemism in Mexico [45].

3.3. Comparison with Neighboring Provinces

The Pacific Lowlands (PL) shares the highest percentage of its amphibian and reptile species with the four biogeographic provinces of the Transition Zone with which it has contact: Sierra Madre del Sur, Transvolcanic Belt, Chiapas Highlands, and Sierra Madre Occidental (Table 2). It shares fewer species with the two biogeographic provinces of the Neotropical Region with which it has contact: Balsas Basin and Veracruzan (Table 2). It shares the lowest percentages of species with the two provinces of the Nearctic Region with which it has contact: the Chihuahuan Desert and the Sonoran (Table 2). This pattern is similar when amphibians and reptiles are considered separately (Table 2). The PL contributes a relatively low percentage to the regional pool of amphibian and reptile species (i.e., the PL and its eight neighboring provinces) (Table 2). This percentage is slightly higher for reptiles than for amphibians (Table 2). In general, the contribution of PL to the regional species pool is low except for Caecilians and turtles (Table 2).

The Jaccard distances between pairs of provinces for amphibians and reptiles were highly positively correlated (Figure 4; n = 36, Spearman’s ρ = 0.7924, p < 0.0001), suggesting amphibian and reptiles may have similar patterns of distribution. The Jaccard distances between pairs of provinces for amphibians were positively correlated with the length of the shared border (Figure 5A; n = 36, Spearman’s ρ = 0.6189, p < 0.0001) and negatively correlated with the distance between their geographic centroids (Figure 5C; n = 36, Spearman’s ρ = −0.6659, p < 0.0001). Similarly, the Jaccard distances between pairs of provinces for reptiles were positively correlated with the length of the shared border (Figure 5B; n = 36, Spearman’s ρ = 0.6831, p < 0.0001) and negatively correlated with the distance between their geographic centroids (Figure 5D; n = 36, Spearman’s ρ = −0.8251, p < 0.0001). These results suggest that the similarity among biogeographic provinces is likely due, at least in part, to proximity. Proximity of biogeographic provinces would allow greater exchange of species and increased shared environmental conditions. Similar patterns have been observed among all Mexican states [17] and biogeographic provinces [8].

The cluster analysis for amphibians resulted in two main groups of provinces (Figure 6A). One group includes the PL, Sierra Madre Occidental, Balsas Basin, the Chihuahuan Desert, and the Sonoran. The second group of provinces comprises provinces in southern and eastern Mexico: Veracruzan, Chiapas Highlands, Transvolcanic Belt, and Sierra Madre del Sur. The results of the cluster analysis of the reptiles differed from those obtained for the amphibians (Figure 6B). For reptiles, the Chiapas Highlands and Veracruzan form a pair of provinces. This pair joins a group of provinces that include a trio of provinces (Transvolcanic Belt, Sierra Madre del Sur, and the Balsas Basin) that are paired with the PL. This large group is joined by the pair of the Chihuahuan Desert and the Sierra Madre Occidental. The Sonoran then joins all the other provinces. Morrone [5] reported that the PL’s main affinities are with the Balsas Basin and Veracruzan, although it is also related to the Sierra Madre del Sur and the Chiapas Highlands.

3.4. Conservation Status

A total of 24 (7.4%) of the 326 species of amphibians and reptiles native to the PL are included in a category of conservation concern on the IUCN Red List (i.e., Vulnerable, Endangered, or Critically Endangered). Thirty-one species are placed in some category of protection by the Secretaría del Medio Ambiente y Recursos Naturales (SEMARNAT [32]), excluding those categorized as not listed (NL) and subject to special protection (Pr), with the latter category equivalent to the Least Concern (LC) category of the IUCN. Moreover, 104 species are considered high risk according to the environmental vulnerability score (EVS) (Figure 7, Table 3).

Of the 76 amphibian species in the Pacific Lowlands, 4 (5.3%) are listed on the IUCN Red List, 2 as Vulnerable: whistling frog (Eleutherodactylus teretistes) and northern roundgland toad (Incilius cycladen). Two are listed as Endangered: jeweled toad (Incilius gemmifer) and yellowbelly mushroomtongue salamander (Bolitoglossa flaviventris). All except B. flaviventris are endemic to Mexico [28,31]. However, B. flaviventris is limited to a small area in extreme southeastern Mexico and western Guatemala. The primary threats to these four species include habitat loss due to urbanization and expanding agricultural cultivation (Table 4). Incilius cycladen may also face a threat from the amphibian chytrid fungus, Batrachochytrium dendrobatidis [31]. Additionally, eight amphibian species have not yet been evaluated (Eleutherodactylus erythrochomus, E. jamesdixoni, E. nebulosus, Hyalinobatrachium viridissimum, Rana adleri, R. cora, R. floresi, and R. hillisi), with six of them described recently between 2020 and 2023 (E. erythrochomus [46], E. jamesdixoni [18], R. adleri [22], R. cora [23], R. floresi [23], R. hillisi [22]).

For reptiles, 20 (8.0%) out of the 250 species inhabiting the Pacific Lowlands are included in some category of conservation concern on the IUCN Red List, with 30 species listed by SEMARNAT [32] and 90 considered to be at high risk according to the EVS (Figure 7). Thirteen of the twenty species included in the IUCN Red List are Vulnerable (VU); four are Endangered (EN) (Urosaurus auriculatus, Isthmian dwarf boa (Ungaliophis continentalis), blackbelly garter snake (Thamnophis melanogaster), and Chelonia mydas); and three are Critically Endangered (CR): Oaxaca spiny-tailed iguana (Ctenosaura oaxacana), Masticophis anthonyi, and Eretmochelys imbricata [31] (Table 4). Twenty-two species are classified as Data Deficient (DD), and another thirty-six have not been evaluated [31]. Among the 20 species listed by the IUCN, 11 are categorized as high-risk by the EVS, and 10 are listed under a threatened category by SEMARNAT [32] (Table 3).

Although the percentage of amphibian and reptile species in a category of conservation concern is relatively low in the PL (7.4%), several studies have highlighted significant threats in the PL that affect not only amphibians and reptiles, but its wildlife in general. Indeed, there are high levels of anthropogenic threats in the PL influencing amphibian species that are Mexican endemic amphibians [47]. Refs. [6,48] reported high rates of deforestation driven by tourism development, livestock farming, and agriculture. Ref. [49] pointed out that the terrestrial vertebrate species of western-central Mexico are facing conservation problems due mainly to habitat loss and fragmentation. These authors, along with [31], agree that the primary challenge facing wildlife in the PL is the rapid loss of habitat in one of Mexico’s most fragile regions, which hosts the largest proportion of seasonally dry forest in the Americas [6,7,9]. The seasonal tropical dry forest is experiencing the second highest rate of deforestation (2%) in Mexico [7,9,50]. In addition, the expansion of coffee and citrus crops, overgrazing, deforestation, wildlife exploitation, extensive livestock farming, mining, and increasing water demands are all threatening the region’s seasonally dry forests [51]. The distribution of endemic amphibian and reptile species in Mexico, including those from the PL, has decreased by 80% and 70%, respectively, due to human-induced habitat loss [52,53]. For the sea turtles of the PL, the presence of pathogens is a potential threat to these populations [54,55,56,57], as might chemical contaminants [58]. In addition, the shallow, nearshore ocean habitats need protection since young sea turtles may use these habitats [59]. The islands of the Revillagigedo Archipelago have probably been impacted by the introduction of various non-native mammals, such as cats, pigs, and sheep; and the removal of these non-native species is likely to improve the status of native amphibians and reptiles [60].

Based on the information on the threats to the herpetofauna of the PL, and especially for those species of particular conservation concern (see Table 4), it is clear that the primary focus of any conservation efforts should be maintaining and hopefully restoring native habitats throughout the PL. In addition, preventing further introductions and spread of non-native species, particularly trout and domesticated mammals (e.g., sheep, pigs, cats), needs to be emphasized. Where possible, the control or removal of established populations of non-native species is likely to be of benefit to the native herpetofauna. For specific species, such as Ctenosaura clarki, Ctenosaura oaxacana, Xenosaurus grandis, and Trachemys yaquia, expansion, strengthening, and enforcement of the existing laws governing the harvesting or taking of reptiles and amphibians for food or the pet trade are needed, as is continued expansion of efforts (e.g., turtle-excluder devices) to reduce fisheries’ bycatch of sea turtles.

4. Conclusions

The Pacific Lowlands (PL) of Mexico is a biodiversity hotspot for amphibians and reptiles, containing a rich assemblage of 326 native species, including 76 amphibians and 250 reptiles. This region is significant, accounting for 23.3% of the country’s amphibian and reptile species. The presence of 30 endemic species emphasizes the PL’s unique ecological significance in providing critical habitats that support specialized adaptations. The distribution patterns of these species reveal a dynamic interplay between local and regional biodiversity. The PL is part of a transitional zone fostering connectivity among diverse biogeographic provinces, thus enhancing species dispersal and genetic exchange. While the PL supports a wealth of species, its contribution to the regional species pool is relatively modest, suggesting that its ecological role is more about facilitating connections between regions than serving as a primary reservoir of biodiversity. The conservation status of species in the PL raises concerns. With approximately 7.4% of species listed as Vulnerable, Endangered, or Critically Endangered, habitat loss due to urbanization, agriculture, and invasive species acting as predators or competitors for native species pose significant threats. This highlights the urgent need for targeted conservation efforts to protect both endemic and non-endemic species. In particular, as evidenced by the summary in Table 4, efforts continue to be needed to reduce the expansion of anthropogenic destruction of habitats and the spread and establishment of non-native species, both through increased control of aquaculturally important species, such as R. catesbeiana and non-native trout, and through tighter control and monitoring of international transport. Overall, the PL represents not only a vital area for amphibian and reptile diversity but also an ecological bridge facilitating interactions across various regions. The continued assessment of species distributions and conservation statuses is essential for preserving the ecological integrity of this unique province and the biodiversity it harbors. Protecting this area will require collaborative efforts among conservationists, policymakers, and local communities to mitigate threats and ensure the long-term sustainability of its rich biodiversity.

Author Contributions

Both authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by J.A.L.-E. and G.R.S. The first draft of the manuscript was written by J.A.L.-E. and G.R.S., and both authors commented on previous versions of the manuscript. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

All of the data that support the findings of this study are available in the main text.

Acknowledgments

We are grateful to Alejandra Núñez Merchand from the National Commission for the Understanding and Use of Biodiversity (CONABIO) for kindly creating and providing the biogeographic provinces map used in this publication and for generating the centroids, distances between centroids, territorial area, perimeter, contact area, and extreme coordinates for each biogeographic province of Mexico; to Christy McCain for providing logistical support in her laboratory at the Museum of Natural History at the University of Colorado, Boulder; and to Jesús Sigala-Rodríguez for allowing us access to an unpublished list of amphibians and reptiles of Zacatecas. We also thank three anonymous reviewers for their helpful comments on the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Footnotes

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Figures and Tables

Figure 1. Topographic map of the Pacific Lowlands biogeographic province of Mexico [11].

Figure 2. Vegetation map of the Pacific Lowlands biogeographic province of Mexico [15].

Figure 3. Climate map of the Pacific Lowlands biogeographic province of Mexico [16].

Figure 4. The correlation between the Jaccard distance of amphibians and reptiles among the PL and its neighboring biogeographic provinces of Mexico with the trend line and 95% confidence intervals.

Figure 5. The relationships between Jaccard distances for length of shared border and distance between centroids of the PL and its neighboring biogeographic provinces and the Jaccard distances of amphibians (A,C, respectively) and reptiles (B,D, respectively) with the trend line and 95% confidence intervals.

Figure 6. Cluster trees for amphibians (A) of the PL and its neighboring biogeographic provinces and (B) cluster trees for reptiles of PL and its neighboring biogeographic provinces. Clusters are identified with Roman numerals.

Figure 7. Percentage of amphibian and reptile species with conservation concern status (International Union for Conservation of Nature’s (IUCN) [31]), protected by the Mexican government (Secretaría de Medio Ambiente y Recursos Naturales (SEMARNAT) [32]), or deemed to have a high environmental vulnerability score (EVS [33,34]) for the Pacific Lowlands biogeographic province of Mexico.

Table 1

Surface area in km² of the Pacific Lowlands (PL) and each of its eight neighboring biogeographic provinces; contact area (shared border) in km between the PL and each of its eight neighboring biogeographic provinces; latitudinal centroid in degrees (°) of the PL and each of its eight neighboring biogeographic provinces; distances between the centroid in km between the PL and each of its eight neighboring biogeographic provinces; elevational range in m of the PL and each of its eight neighboring biogeographic provinces; shared species between the PL and each of its eight neighboring biogeographic provinces; and number of species of the PL and each of the neighboring provinces. Abbreviations: surface (surface area in km²); contact (length of shared border in km); longitude (longitude of the centroid); latitude (latitude of the centroid); distance (distance between centroids in km); shared species (number of shared species); species richness (number of amphibian and reptile species).

Neighboring Provinces	Surface (km2)	Contact (km)	Longitude (°)	Latitude (°)	Distance (km)	Elevation Range (m)	Shared Species	Species Richness
Pacific Lowlands	187,112.90	-	−105.558	20.35502	-	2200	-	326
Sonoran	119,762.79	1709	−112.265	30.07016	1265	1800	66	124
Chihuahuan Desert	578,001.54	1760	−103.73	26.19336	669.7	2600	73	261
Sierra Madre Occidental	171,195.06	11,506	−106.334	25.78151	603.1	3000	119	216
Transvolcanic Belt	82,839.90	1272	−100.551	19.73688	526.3	5200	161	427
Balsas Basin	76,135.84	152	−99.7366	18.37809	647.7	2600	118	206
Sierra Madre del Sur	93,606.91	4149	−100.206	17.70266	633.8	3400	186	517
Chiapas Highlands	27,879.28	490	−92.2252	16.7383	1460	3600	129	348
Veracruzan	191,451.14	83	−96.1699	19.05299	991.4	2200	106	341

Table 2

Summary of the number of species shared between the Pacific Lowlands and neighboring biogeographic provinces (not including introduced species). The percents of the Pacific Lowlands shared by neighboring provinces are given in parentheses. Total refers to the number of species found in the Pacific Lowlands and eight neighboring provinces (i.e., regional species pool), and the number in parentheses in this column is the percent of the regional species pool found in the Pacific Lowlands—indicates either the Pacific Lowlands or its neighboring province has no species in the taxonomic group, or none of that specific taxon is shared between the provinces; thus, no value for shared species is provided. Abbreviations of the biogeographic provinces are PL (Pacific Lowlands); SMS (Sierra Madre del Sur); TVB (Transvolcanic Belt); Chiapas (Chiapas Highlands); SMO (Sierra Madre Occidental); BB (Balsas Basin); Ver (Veracruzan); CD (Chihuahuan Desert); and SON (Sonoran).

	PL	SMS	TVB	Chiapas	SMO	BB	Ver	CD	SON	TOTAL
Amphibia	76	47 (61.8)	42 (55.3)	33 (43.4)	34 (44.7)	31 (40.8)	24 (31.6)	18 (23.7)	15 (19.7)	400 (19)
Anura	71	45 (63.4)	40 (56.3)	29 (40.8)	34 (47.9)	31 (43.7)	22 (31)	18 (25.4)	15 (21.1)	255 (27.8)
Bufonidae	14	8 (57.1)	6 (42.9)	6 (42.9)	8 (57.1)	4 (28.6)	4 (28.6)	6 (42.9)	6 (42.9)	33 (42.4)
Centrolenidae	1	1 (100)	1 (100)	1 (100)	-	-	1 (100)	-	-	1 (100)
Craugastoridae	8	7 (87.5)	6 (75)	4 (50)	4 (50)	6 (75)	3 (37.5)	2 (25)	-	43 (18.6)
Eleutherodactylidae	14	7 (50)	5 (35.7)	3 (21.4)	4 (28.6)	2 (14.3)	-	-	-	40 (35)
Hylidae	12	11 (91.7)	9 (75)	5 (41.7)	6 (50)	8 (66.7)	3 (25)	4 (33.3)	3 (25)	94 (12.8)
Leptodactylidae	3	2 (66.7)	2 (66.7)	3 (100)	1 (33.3)	2 (66.7)	3 (100)	2 (66.7)	1 (33.3)	3 (100)
Microhylidae	3	2 (66.7)	2 (66.7)	2 (66.7)	3 (100)	2 (66.7)	2 (66.7)	1 (33.3)	1 (33.3)	6 (50)
Phyllomedusidae	2	2 (100)	2 (100)	1 (50)	1 (50)	1 (50)	1 (50)	-	-	3 (66.7)
Ranidae	12	4 (33.3)	6 (50)	3 (25)	6 (50)	5 (41.7)	3 (25)	2 (16.7)	3 (25)	28 (42.9)
Rhinophrynidae	1	1 (100)	-	1 (100)	-	1 (100)	1 (100)	-	-	1 (10)
Scaphiopodidae	1	-	1 (100)	-	1 (100)	-	1 (100)	1 (100)	1 (100)	3 (33.3)
Caudata	3	1 (33.3)	-	2 (66.7)	-	-	1 (33.3)	-	-	142 (2.1)
Ambystomatidae	-	-	-	-	-	-	-	-	-	17 (0)
Plethodontidae	3	1 (33.3)	-	2 (66.7)	-	-	1 (33.3)	-	-	123 (2.4)
Salamandridae	-	-	-	-	-	-	-	-	-	1 (0)
Sirenidae	-	-	-	-	-	-	-	-	-	1 (0)
Gymnophiona	2	1 (50)	2 (100)	2 (100)	-	-	1 (50)	-	-	3 (66.7)
Dermophiidae	2	1 (50)	2 (100)	2 (100)	-	-	1 (50)	-	-	3 (66.7)
Reptilia	250	139 (55.6)	120 (48)	96 (38.4)	86 (34.4)	87 (34.8)	83 (33.2)	55 (22)	52 (20.8)	814 (30.7)
Crocodylia	2	-	-	1 (50)	-	1 (50)	1 (50)	-	1 (50)	3 (66.7)
Alligatoridae	1	-	-	-	-	-	-	-	-	1 (100)
Crocodylidae	1	-	-	1 (100)	-	1 (100)	1 (100)	-	1 (100)	2 (50)
Squamata	226	133 (58.8)	114 (50.4)	91 (40.3)	82 (36.3)	82 (36.3)	74 (32.7)	53 (23.5)	43 (19)	762 (29.7)
Lacertilia	98	52 (53.1)	40 (40.8)	35 (35.7)	33 (33.7)	31 (31.6)	18 (18.4)	17 (17.3)	14 (14.3)	385 (25.5)
Anguidae	2	2 (100)	2 (100)	1 (50)	2 (100)	1 (50)	1 (50)	2 (100)	-	42 (4.8)
Anolidae	12	5 (41.7)	1 (8.3)	5 (41.7)	1 (8.3)	2 (16.7)	1 (8.3)	1 (8.3)	-	54 (22.2)
Bipedidae	2	-	-	-	-	1 (50)	-	-	-	2 (100)
Corytophanidae	2	2 (100)	2 (100)	2 (100)	-	1 (50)	2 (100)	-	-	6 (33.3)
Crotaphytidae	1	-	-	-	1 (100)	-	-	-	1 (100)	7 (14.3)
Dibamidae	-	-	-	-	-	-	-	-	-	1 (0)
Diploglossidae	-	-	-	-	-	-	-	-	-	4 (0)
Eublepharidae	3	2 (66.7)	1 (33.3)	1 (33.3)	1 (33.3)	2 (66.7)	1 (33.3)	-	-	7 (42.9)
Gymnophthalmidae	1	-	-	1 (100)	-	-	-	-	-	1 (100)
Helodermatidae	4	1 (25)	1 (25)	2 (50)	3 (75)	1 (25)	-	-	1 (25)	4 (25)
Iguanidae	7	4 (57.1)	2 (28.6)	4 (57.1)	2 (28.6)	3 (42.9)	2 (28.6)	1 (14.3)	2 (28.6)	13 (53.8)
Phrynosomatidae	27	13 (48.1)	12 (44.4)	6 (22.2)	14 (51.9)	9 (33.3)	-	10 (37)	6 (22.2)	121 (22.3)
Phyllodactylidae	9	4 (44.4)	3 (33.3)	2 (22.2)	2 (22.2)	1 (11.1)	1 (11.1)	-	1 (11.1)	18 (50)
Scincidae	10	6 (60)	8 (80)	4 (40)	4 (40)	3 (30)	4 (40)	2 (20)	-	30 (33.3)
Sphaerodactylidae	3	2 (66.7)	1 (33.3)	3 (100)	-	-	3 (100)	-	-	3 (100)
Teiidae	10	7 (70)	6 (60)	3 (30)	3 (30)	6 (60)	2 (20)	1 (10)	3 (30)	39 (25.6)
Xantusidae	4	3 (75)	-	1 (25)	-	1 (25)	-	-	-	24 (16.7)
Xenosauridae	1	1 (100)	1 (100)	-	-	-	1 (100)	-	-	9 (1.1)
Serpentes	128	81 (63.3)	74 (57.8)	56 (43.8)	49 (38.3)	51 (39.8)	56 (43.8)	36 (28.1)	29 (22.7)	377 (34)
Boidae	3	2 (66.7)	2 (66.7)	2 (66.7)	1 (33.3)	2 (66.7)	1 (33.3)	-	1 (33.3)	5 (60)
Colubridae	53	31 (58.5)	30 (56.6)	22 (41.5)	26 (49.1)	24 (45.3)	26 (49.1)	19 (35.8)	17 (32.1)	125 (42.4)
Dipsadidae	40	29 (72.5)	24 (60)	20 (50)	9 (22.5)	13 (32.5)	19 (47.5)	9 (22.5)	2 (5)	129 (31)
Elapidae	10	5 (50)	5 (50)	4 (40)	3 (30)	3 (30)	1 (10)	2 (20)	3 (30)	16 (62.5)
Leptotyphlopidae	4	4 (100)	2 (50)	1 (25)	1 (25)	3 (75)	1 (25)	1 (25)	1 (25)	12 (33.3)
Loxocemidae	1	1 (100)	1 (100)	1 (100)	-	1 (100)	1 (100)	-	-	1 (100)
Natricidae	4	2 (50)	4 (100)	2 (50)	3 (75)	1 (25)	2 (50)	3 (75)	1 (25)	28 (14.3)
Typhlopidae	-	-	-	-	-	-	-	-	-	1 (0)
Viperidae	13	7 (53.8)	6 (46.2)	4 (30.8)	6 (46.2)	4 (30.8)	5 (38.5)	2 (15.4)	4 (30.8)	60 (21.7)
Testudines	22	6 (27.3)	6 (27.3)	4 (18.2)	4 (18.2)	4 (18.2)	8 (36.4)	2 (9)	8 (36.4)	49 (44.9)
Cheloniidae	4	-	-	-	-	-	3 (75)	-	4 (100)	5 (80)
Chelydridae	-	-	-	-	-	-	-	-	-	1 (0)
Dermatemydidae	-	-	-	-	-	-	-	-	-	1 (0)
Dermochelyidae	1	-	-	-	-	-	1 (100)	-	1 (100)	1 (100)
Emydidae	5	-	1 (20)	1 (20)	1 (20)	-	1 (20)	-	1 (20)	15 (33.3)
Geoemydidae	2	2 (100)	2 (100)	2 (100)	1 (50)	1 (50)	1 (50)	-	-	3 (66.7)
Kinosternidae	9	4 (44.4)	3 (33.3)	1 (11.1)	2 (22.2)	3 (33.3)	2 (22.2)	2 (22.2)	2 (22.2)	18 (50)
Testudinidae	1	-	-	-	-	-	-	-	-	4 (25)
Trionychidae	-	-	-	-	-	-	-	-	-	1 (0)
Total	326	186 (57.1)	162 (49.7)	129 (39.6)	120 (36.8)	118 (36.2)	107 (32.8)	73 (22.4)	67 (20.6)	1214 (26.9)

Table 3

Summary of native species present in the Pacific Lowlands biogeographic province of Mexico by family, order or suborder, and class. Status summary indicates the number of species found in each IUCN conservation status in the order DD, LC, VU, NT, EN, CR [31]. See Table 2 for abbreviations; in some cases, species have not been assigned a status by the IUCN, and therefore these may not add up to the total number of species in a taxon). Mean EVS is the mean environmental vulnerability score; scores ≥ 14 are considered high vulnerability [33,34] and conservation status in Mexico according to Secretaría de Medio Ambiente y Recursos Naturales (SEMARNAT) [32] in the order NL, Pr, A, P.

Scientific Name	Genera	Species	IUCN	Mean EVS	SEMARNAT
Class Amphibia			DD, LC, NT, VU, EN, CR		NL, Pr, A, P
Order Anura	24	71	0, 60, 0, 2, 1, 0	10	55, 15, 1, 0
Bufonidae	3	14	0, 12, 0, 1, 1, 0	9.6	12, 2, 0, 0
Centrolenidae	1	1	0, 0, 0, 0, 0, 0	10	1, 0, 0, 0
Craugastoridae	1	8	0, 8, 0, 0, 0, 0	11.8	8, 0, 0, 0
Eleutherodactylidae	1	14	0, 10, 0, 1, 0, 0	15	10, 4, 0, 0
Hylidae	10	12	0, 12, 0, 0, 0, 0	8.8	9, 2, 1, 0
Leptodactylidae	2	3	0, 3, 0, 0, 0, 0	4.7	3, 0, 0, 0
Microhylidae	2	3	0, 3, 0, 0, 0, 0	6.3	2, 1, 0, 0
Phyllomedusidae	1	2	0, 2, 0, 0, 0, 0	12	2, 0, 0, 0
Ranidae	1	12	0, 8, 0, 0, 0, 0	8.9	3, 5, 0, 0
Rhinophrynidae	1	1	0, 1, 0, 0, 0, 0	8	0, 1, 0, 0
Scaphiopodidae	1	1	0, 1, 0, 0, 0, 0	3	1, 0, 0, 0
Order Caudata	1	3	0, 2, 0, 0, 1, 0	13.3	1, 2, 0, 0
Plethodontidae	1	3	0, 2, 0, 0, 1, 0	13.3	1, 2, 0, 0
Order Gymnophiona	1	2	0, 2, 0, 0, 0, 0	11.5	0, 2, 0, 0
Dermophiidae	1	2	0, 2, 0, 0, 0, 0	11.5	0, 2, 0, 0
Subtotal	26	76	0, 64, 0, 2, 2, 0	10.2	56, 19, 1, 0
Class Reptilia
Order Crocodylia	2	2	0, 1, 0, 1, 0, 0	15	0, 2, 0, 0
Alligatoridae	1	1	0, 1, 0, 0, 0, 0	16	0, 1, 0, 0
Crocodylidae	1	1	0, 0, 0, 1, 0, 0	14	0, 1, 0, 0
Order Squamata	87	226	19, 163, 3, 6, 3, 2	11.8	133, 70, 23, 0
Suborder Lacertilia	28	98	8, 69, 1, 4, 1, 1	12.6	60, 28, 10, 0
Anguidae	2	2	0, 2, 0, 0, 0, 0	8	0, 2, 0, 0
Anolidae	1	12	3, 8, 0, 0, 0, 0	14.1	7, 4, 1, 0
Bipedidae	1	2	0, 2, 0, 0, 0, 0	15.5	0, 2, 0, 0
Corytophanidae	2	2	0, 2, 0, 0, 0, 0	8	1, 1, 0, 0
Crotaphytidae	1	1	0, 1, 0, 0, 0, 0	12	1, 0, 0, 0
Eublepharidae	1	3	0, 2, 0, 0, 0, 0	13	2, 0, 1, 0
Gymnophthalmidae	1	1	0, 1, 0, 0, 0, 0	9	0, 1, 0, 0
Helodermatidae	1	4	0, 2, 1, 1, 0, 0	13	2, 2, 0, 0
Iguanidae	3	7	0, 5, 0, 1, 0, 1	14.1	2, 1, 4, 0
Phrynosomatidae	5	27	0, 24, 0, 1, 1, 0	12.7	23, 3, 1, 0
Phyllodactylidae	1	9	0, 5, 0, 0, 0, 0	14	5, 2, 2, 0
Scincidae	3	10	3, 4, 0, 0, 0, 0	12	7, 3, 0, 0
Sphaerodactylidae	2	3	0, 2, 0, 0, 0, 0	11	1, 2, 0, 0
Teiidae	2	10	0, 8, 0, 0, 0, 0	11.4	7, 3, 0, 0
Xantusidae	1	4	2, 1, 0, 0, 0, 0	12.3	2, 1, 1, 0
Xenosauridae	1	1	0, 0, 0, 1, 0, 0	9	0, 1, 0, 0
Suborder Serpentes	59	128	11, 94, 2, 2, 2, 1	11.3	73, 42, 13, 0
Boidae	2	3	0, 1, 0, 0, 1, 0	10	2, 1, 0, 0
Colubridae	25	53	4, 42, 1, 0, 0, 1	10.7	36, 12, 5, 0
Dipsadidae	20	40	4, 32, 0, 0, 0, 0	11	25, 15, 0, 0
Elapidae	4	10	1, 8, 0, 1, 0, 0	13.9	2, 7, 1, 0
Leptotyphlopidae	2	4	0, 1, 0, 0, 0, 0	7.7	4, 0, 0, 0
Loxocemidae	1	1	0, 1, 0, 0, 0, 0	10	0, 1, 0, 0
Natricidae	1	4	0, 3, 0, 0, 1, 0	10.3	1, 0, 3, 0
Viperidae	4	13	2, 6, 1, 1, 0, 0	14.2	3, 6, 4, 0
Order Testudines	11	22	3, 3, 2, 6, 1, 1	14.3	7, 8, 1, 6
Chelonidae	4	4	0, 0, 0, 2, 1, 1	-	0, 0, 0, 4
Dermochelyidae	1	1	0, 0, 0, 1, 0, 0	-	0, 0, 0, 1
Emydidae	2	5	1, 0, 0, 2, 0, 0	17.8	4, 1, 0, 0
Geoemydidae	1	2	0, 0, 1, 0, 0, 0	11	0, 1, 1, 0
Kinosternidae	2	9	2, 3, 1, 0, 0, 0	12.7	2, 6, 0, 1
Testudinidae	1	1	0, 0, 0, 1, 0, 0	-	1, 0, 0, 0
Subtotal	100	250	22, 167, 5, 13, 4, 3	12	140, 80, 24, 6
Total	126	326	22, 229, 5, 15, 6, 3	11.6	196, 99, 25, 6

Table 4

Summary of the species of amphibian and reptile species from the Pacific Lowlands that are listed in a category of conservation concern (VU: Vulnerable; EN: Endangered; CR: Critically Endangered). Population Trend: Stable (=); Decreasing (↓); Increasing (↑); Unknown (?) by [31]. The population trend and a summary of the major conservation threats for each species as determined by [31] are also provided.

Class Amphibia	IUCN	Main Threats
Order Anura
Bufonidae
Incilius cycladen	VU (↓)	Habitat alteration due to agricultural clearance and agrochemical pollution
Incilius gemmifer	EN (=)	Habitat loss due to small-scale agricultural expansion, wood extraction, urban expansion
Eleutherodactylidae
Eleutherodactylus teretistes	VU (?)	Habitat loss due to agricultural expansion
Order Caudata
Plethodontidae
Bolitoglossa flaviventris	EN (↓)	Habitat loss due to agricultural and urban expansion, settlements for refugees and livestock
Class Reptilia
Order Crocodilia
Crocodylidae
Crocodylus acutus	VU (↑)	This species is recovering thanks to the conservation actions taken in the countries in which it ranges
Order Squamata
Suborder Lacertilia
Helodermatidae
Heloderma alvarezi	VU (↓)	Persecution by human, habitat loss (seasonally dry forest), climate change
Iguanidae
Ctenosaura clarki	VU (?)	Habitat loss and pet trade
Ctenosaura oaxacana	CR (↓)	Habitat loss and consumption of its meat by humans
Phrynosomatidae
Urosaurus auriculatus	EN (↓)	Habitat loss due to introduced species: sheep overgrazing and cats predating on this lizard
Urosaurus clarionensis	VU (?)	Habitat loss due to introduced species: rabbits and sheep overgrazing and cats possibly predating on this lizard
Xenosauridae
Xenosaurus grandis	VU (↓)	Pet trade and habitat loss
Suborder Serpentes
Boidae
Ungaliophis continentalis	EN (?)	Habitat loss and pet trade
Colubridae
Masticophis anthonyi	CR (?)	Habitat loss due to introduced species: rabbits, sheep, and pigs
Elapidae
Micrurus ephippifer	VU (↓)	Habitat loss due to agricultural expansion
Natricidae
Thamnophis melanogaster	EN (↓)	Water pollution due to agriculture and industry
Viperidae
Crotalus stejnegeri	VU (↓)	Habitat loss due to extensive commercial logging and agricultural expansion
Order Testudines
Chelonidae
Caretta caretta	VU (↓)	Fibropapilloma (linked with pollution and habitat degradation) and pollution
Chelonia mydas	EN (↓)	Fibropapilloma (linked with degradation of coral reefs)
Eretmochelys imbricata	CR (↓)	Fibropapilloma
Lepidochelys olivacea	VU (↓)	Fibropapilloma and fishery bycatch
Dermochelyidae
Dermochelys coriacea	VU (↓)	Fibropapillomaand fishery bycatch
Emydidae
Trachemys ornata	VU (↓)	No information available, but habitat loss and chemical pollution are likely
Trachemys yaquia	VU (↓)	Habitat loss; it is likely being harvested for food
Family Testudinidae
Gopherus evgoodei	VU (↓)	Habitat loss due to the conversion of native thornscrub to Buffelgrass

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/d16120735/s1, Table S1: Amphibians and reptiles of Pacific Lowlands (PL) biogeographic province of Mexico.

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