Part 7

Now that the important ecological factors of the major environments have been outlined, we may examine the local distribution of amphibians in each of these. Beginning with the rainforest, we find only one fossorial species, Gymnopis mexicanus. A large number of species are found on the forest floor; characteristic inhabitants of the leaf litter are: Bufo valliceps, Eleutherodactylus rhodopis, Microbatrachylus pygmaeus, and Syrrhophus leprus. Other terrestrial amphibians usually are not scattered throughout the rainforest, as are those named immediately above, but instead inhabit areas of forest adjacent to ponds or streams; these species include: Bufo marinus, Eleutherodactylus natator, Eleutherodactylus rugulosus, Leptodactylus labialis, Leptodactylus melanonotus, Rana palmipes and Rana pipiens. The most striking ecological assemblage of amphibians in the rainforest is the arboreal group of species, including:

Bolitoglossa occidentalis Bolitoglossa platydactyla Eleutherodactylus alfredi Hyla baudini Hyla ebraccata Hyla loquax Hyla microcephala martini Hyla picta Phrynohyas modesta Phrynohyas spilomma Phyllomedusa callidryas taylori

In the savannas Rhinophrynus dorsalis, Engystomops pustulosus, and Gastrophryne usta are fossorial species. Bufo marinus, Leptodactylus melanonotus, Leptodactylus labialis, Rana palmipes, and Rana pipiens are found in the vicinity of permanent water in the savannas. Although the savanna habitat does not provide the ecological conditions for the existence of an arboreal fauna, many arboreal species from the surrounding rainforest utilize the extensive marshes and ponds in the savannas for breeding purposes. Thus, Hyla baudini, Hyla microcephala martini, Hyla picta, and Phrynohyas spilomma have been found breeding in savannas. In parts of savannas where clumps of trees surround depressions containing water throughout the year, individuals of the species named above, together with Hyla loquax and Phyllomedusa callidryas taylori, may not only breed, but remain throughout the year.

In the semi-arid scrub forest the same fossorial species as exist in the savannas are found. Likewise, Bufo marinus, Leptodactylus labialis, Leptodactylus melanonotus, and Rana pipiens are found near permanent water. Terrestrial species in this semi-arid environment include Bufo canaliferus, Bufo coccifer, Bufo marmoreus, Syrrhophus pipilans, and Diaglena reticulata. Of these, Syrrhophus pipilans sometimes inhabits low trees and bushes; the others may be fossorial. The arboreal species in the scrub forest include Hyla baudini, Hyla robertmertensi, Hyla staufferi, and Phyllomedusa dacnicolor.

Eleutherodactylus rugulosus and Hylella sumichrasti live along streams in the scrub forest. Hylella sumichrasti lays its eggs in these streams.

In comparing the ecological differences in the amphibian assemblages in the three major habitats, the most obvious difference is the great percentage of arboreal species in the rainforest as compared with savanna and scrub forest. Only four arboreal species are found in the scrub forest, none in the savannas, but eleven in the rainforest. Likewise, there is an absence of ground-dwelling forms in the arid habitats; in the latter the only terrestrial species are those that are found near water. A possible exception is Syrrhophus pipilans.

From the above analysis of ecological distribution we may see that the rainforest provides a variety of habitats for amphibians and that these habitats are suitable for amphibian life throughout the year. On the other hand, the savannas and scrub forests are characterized by extreme conditions of desiccation, a factor of considerable importance in limiting the ecological distribution of amphibians. However, there still is a diversity of amphibians in these semi-arid environments. Obviously, these species are adapted in various ways for survival during the dry season, at which time environmental conditions are such that the animals cannot carry on their normal activities.

Although there is not an abundance of data concerning the seasonal activity of the fauna, what is available shows some interesting correlations with the environments. During the dry season in the scrub forest there is essentially no amphibian activity; an occasional Rana pipiens may be seen along a river, or a Bufo marinus may be seen at night. In the rainforest the terrestrial-breeding amphibians are active during the dry season. Eleutherodactylus rugulosus is found at night or by day along streams. Eleutherodactylus rhodopis, Microbatrachylus pygmaeus, and Bufo valliceps are active during the day; these plus Bolitoglossa occidentalis, Bolitoglossa platydactyla, Eleutherodactylus alfredi, Eleutherodactylus natator, and an occasional Hyla are active at night.

With the onset of the heavy summer rains and the subsequent formation of breeding ponds, amphibian activity reaches a peak. This is especially noticeable in the semi-arid environments, where during the dry season there is little activity.

Among the anurans in the isthmus the four species of Eleutherodactylus, the two species of Syrrhophus, and the one species of Microbatrachylus are either known, or presumed, to lay eggs on the ground; these develop directly into small frogs. All of the other anurans deposit their eggs in water or attach them to objects over water (Phyllomedusa); these hatch into tadpoles, which later metamorphose into frogs. Hylella sumichrasti is known to breed only in streams. All of the other species breed in ponds, but at times some species deposit their eggs in streams; in this last group are Bufo valliceps, Bufo marmoreus, Phyllomedusa callidryas taylori, and Rana pipiens.

Although the ecological data are incomplete, they do show that ecological conditions differ greatly in the three major environments, different species of amphibians inhabit these environments, and that the fauna is ecologically diversified in each environment.

Distribution of the Fauna

Plotting the distributions of species of amphibians known to live in the lowlands of the Isthmus of Tehuantepec results in an array of geographic patterns. These may be analyzed with respect to those species that are restricted either to the Gulf lowlands or the Pacific lowlands, or those that occur on both the Gulf and Pacific lowlands. Furthermore, the distributions may be analyzed with respect to those species whose ranges extend from Mexico across the Isthmus of Tehuantepec into Central America, those that reach the isthmus from Central America but do not extend into Mexico proper, and those that reach the isthmus from Mexico but do not extend into Central America. It should be kept in mind that the following analysis is of the lowland inhabitants only. Species inhabiting the foothills and mountains will be discussed later.

1. SPECIES RESTRICTED TO THE GULF LOWLANDS. Of the 36 species of amphibians recorded from the Isthmus of Tehuantepec, nine (25 per cent) are in this group. Four of these (Eleutherodactylus alfredi, Syrrhophus leprus, Hyla loquax, and Hyla picta) live in the Gulf lowlands to the east and to the west of the isthmus. Three others (Hyla ebraccata, Hyla microcephala martini and Phyllomedusa callidryas taylori) are primarily Central American in their distribution and reach the northwestern limits of their ranges in the Gulf lowlands of the isthmus, whereas Bolitoglossa platydactyla and Eleutherodactylus natator reach the southern limits of their distributions in the isthmus.

2. SPECIES RESTRICTED TO THE PACIFIC LOWLANDS. This group includes six species, or 17 per cent of the amphibian fauna of the isthmus. Two of these (Bufo coccifer and Syrrhophus pipilans) range to the east and to the west of the isthmus on the Pacific lowlands. Two others (Bufo canaliferus and Hyla robertmertensi) range from the isthmus into Central America, and Diaglena reticulata and Phyllomedusa dacnicolor range on the Pacific lowlands of Mexico southeastward to the isthmus.

3. SPECIES THAT OCCUR ON THE PACIFIC AND GULF LOWLANDS. This group includes 19 species, or 53 per cent of the total amphibian fauna. Of these, nine species (25 per cent of the entire amphibian fauna) are widespread throughout the lowlands of Mexico and Central America; these are:

Gymnopis mexicanus Rhinophrynus dorsalis Bufo marinus Engystomops pustulosus Leptodactylus labialis Leptodactylus melanonotus Hyla baudini Hyla staufferi Rana pipiens

Four species occur on the Gulf lowlands to the east and to the west of the isthmus, but on the Pacific lowlands they occur only to the east; this group includes Bufo valliceps, Eleutherodactylus rhodopis, Phrynohyas modesta, and Phrynohyas spilomma. Three species live to the east and to the west of the isthmus on the Pacific lowlands, but only to the west on the Gulf lowlands; these include Eleutherodactylus rugulosus, Microbatrachylus pygmaeus, and Gastrophryne usta.

Six species that cross the isthmus live on the humid Gulf lowlands and on the humid lowlands of Chiapas and Guatemala, but not on the semi-arid Plains of Tehuantepec; these include Bolitoglossa occidentalis, Eleutherodactylus rhodopis, Microbatrachylus pygmaeus, Phrynohyas modesta, Phrynohyas spilomma, and Rana palmipes. Of these, Microbatrachylus pygmaeus also occurs in scattered humid environments to the west of the isthmus on the Pacific lowlands.

Two species are endemic to the isthmian region. Bolitoglossa veracrucis is known only from the humid northern slopes of the isthmus. Hylella sumichrasti occurs on the Pacific slopes of the isthmus and extends to the east into western Chiapas.

In analyzing the distribution of the amphibians with respect to those that are restricted to either the Pacific or Gulf lowlands or those that cross the continental divide in the isthmus, we find that 25 per cent of the species are restricted to the Gulf lowlands, 17 per cent are restricted to the Pacific lowlands, and 53 per cent cross the isthmus. In analyzing the distribution patterns with respect to those that extend across the isthmus of Tehuantepec from east to west, we find that 14 per cent of the species do not extend east of the isthmus into Central America and that 19 per cent do not range west of the isthmus into Mexico proper; 61 per cent of the species range to the east and to the west of the isthmus. Of the 36 species of amphibians inhabiting the isthmus only nine species (25 per cent) range across the isthmus, that is, occur on the Gulf and Pacific lowlands, and also range to the east and to the west of the isthmus. To these wide-ranging species the diversified environments of the isthmus do not present a barrier to distribution. The other 27 species (75 per cent) either do not cross the isthmus from east to west or from north to south; thus, probably in one way or another the isthmus presents a barrier to their distribution.

THE AMPHIBIAN FAUNA OF THE FOOTHILLS AND ADJACENT HIGHLANDS

To amphibians inhabiting the foothills and mountains of southern Mexico and northern Central America, the isthmus presents a great barrier to dispersal. For example, salamanders of the genus Thorius, the mexicanus and augusti groups of the genus Eleutherodactylus, the bistincta group of the genus Hyla, and the genus Tomodactylus occur on the Mexican Plateau and southward into the mountains of Oaxaca. Nevertheless, no members of these groups are present in the Guatemalan-Chiapan Highlands. The genera Chiropterotriton, Magnadigita, Pseudoeurycea, and Ptychohyla, as well as the eximia group of Hyla are represented by different species in the Guatemalan-Chiapan Highlands than in the mountains of Mexico on the other side of the isthmus. Several species of Plectrohyla occur in the Guatemalan-Chiapan Highlands, but none is known from the Mexican Highlands, although one species occurs in the Tuxtlas.

Living in the humid forests of the foothills are salamanders of the genus Lineatriton, frogs of the spatulatus group of Eleutherodactylus, Anotheca coronata, Hyla miotympanum, and Phyllomedusa moreleti. All of these occur in the foothills of the Sierra Madre Oriental in eastern Mexico and in Los Tuxtlas. Lineatriton, Hyla miotympanum, and the spatulatus group of Eleutherodactylus do not occur in the foothills of the Guatemalan-Chiapan Highlands; those amphibians reach the end of their ranges at the isthmus. Phyllomedusa moreleti and Anotheca coronata are found in the northern foothills of the Guatemalan-Chiapan Highlands, and Phyllomedusa moreleti is found in the foothills on the Pacific slopes of the Chiapan Highlands.

Although the above analysis is not so detailed as that of the lowland inhabitants, it does show that all of the genera and species of amphibians known to inhabit the foothills and highlands adjacent to the isthmus, only two species of amphibians cross the isthmus from one highland mass to the other. Thus, it is evident that the Isthmus of Tehuantepec presents a great barrier to dispersal of these groups of amphibians.

ESTABLISHMENT OF PRESENT PATTERNS OF DISTRIBUTION

From the foregoing analysis of geographical and ecological distribution in the Isthmus of Tehuantepec we may strive for an interpretation of the events that led to the establishment of patterns of distribution displayed not only by the amphibians, but other terrestrial vertebrates as well. The thesis that I am proposing below is based on the premise that in southern Mexico and northern Central America climatic fluctuation during the Pleistocene was of sufficient magnitude to cause vegetational shifts, both vertically and latitudinally, resulting in the establishment of alternating continuous and discontinuous lowland and highland environments, although this climatic fluctuation was not so great as to eliminate tropical lowland environments from the region. I feel that the present patterns of distribution of the amphibians in the Isthmus of Tehuantepec may be explained on this premise.

Many authors dealing with the herpetofauna of Middle America have followed Schuchert's (1935) suggestion of a seaway in the isthmus during the Cenozoic. Thus, Burt (1931), Duellman (1956, 1958a), Gloyd (1940), Oliver (1948), Smith and Laufe (1946), and Stuart (1941) employed the presence of a seaway to explain distribution and speciation in various genera. Durham, Arellano, and Peck (1952), Olson and McGrew (1941), and Stirton (1954) have provided geological evidence that there probably was no Cenozoic seaway in the Isthmus of Tehuantepec. Even if there were a seaway in the Pliocene or Miocene (the dating of this possible seaway is open to question), its presence is not necessary to explain the present patterns of distribution in the isthmus.