A Distributional Study of the Amphibians of the Isthmus of Tehuantepec, Mexico Part 3

Oaxaquena, La.--Lat. 17 26'; long. 94 53'; elev. 80 m. A hacienda on the Rio Coatzacoalcos about 12 kilometers east of Jesus Carranza; rainforest [27].

Playas, Rio de las.--Lat. 18 08'; long. 94 07'; elev. 3 m.

The river (sometimes known as the Rio Tonola) forming the boundary between the states of Veracruz and Tabasco; rainforest [18].

San Lorenzo.--Lat. 17 44'; long. 94 42'; elev. 25 m. A village on the Rio Chiquito, about 30 kilometers southeast of Acayucan; rainforest [23].

Suchil.--Lat. 17 31'; long. 95 03'; elev. 40 m. A village on the Trans-isthmian Railroad, about 10 kilometers north of Jesus Carranza; rainforest [25].

Tecolapan.--Lat. 18 24'; long. 95 18'; elev. 275 m. A village on a small river of the same name in the western foothills of Los Tuxtlas; rainforest [5].

Tejada, Lerdo de.--Lat. 18 37'; long. 95 31'; elev. 60 m.

An agricultural village, 35 kilometers by road east-southeast of Alvarado; scrub forest, marshes, and sugar plantations [2]. Collections were made in a marsh, 5 kilometers west-northwest of the village.

Tlacotalpan.--Lat. 18 37'; long. 95 42'; elev. 3 m. A town at the confluence of the Rio San Juan and Rio Papaloapan; marshes and sugar plantations [3].

Tula.--Lat. 18 36'; long. 95 22'; elev. 150 m. A village near the western base of Los Tuxtlas; low evergreen forest and marshes [4]. Collections were made in a marsh 3 kilometers northwest of the village.

THE AMPHIBIAN FAUNA OF THE LOWLANDS

In presenting an account of the amphibian fauna of the lowlands of the Isthmus of Tehuantepec three items must be considered:

1. The composition of the fauna.

2. The ecology of the fauna.

3. The distribution of the fauna.

These items, together with similar data concerning the amphibians of the adjacent highlands, will form the basis for the subsequent discussion of the establishment of present patterns of distribution in the isthmian region.

_Composition of the Fauna_

The amphibian fauna of the lowlands of the Isthmus of Tehuantepec consists of 36 species definitely recorded from the area. These include one genus and species of caecilian, one genus, including three species of salamanders, and 14 genera and 32 species of anurans.

In comparison with the known amphibian fauna of the forested and savanna portions of El Peten, Guatemala (Stuart, 1935 and 1958), we find that there are more species recorded from the isthmus than from El Peten. Stuart found only 20 species of amphibians in both forest and savanna habitats in El Peten. Of the 36 species of amphibians known from the isthmus, 28 occur on the Gulf lowlands and live in forest or savanna habitats.

The geographic position of the isthmus with regard to major faunal areas in Middle America, and the diversity of the environment are important factors in understanding the presence of a large number of species of amphibians in the isthmus. The large number of species probably is a reflection of the diversity of the environment; this diversity is the result of fluctuation of climate, and thus environments, in the not too distant past. In no individual habitat, such as rainforest, savanna, or scrub forest, does the number of species approach the total for the region.

_Ecology of the Fauna_

In the preceding section on the description of the Isthmus of Tehuantepec I have outlined the major environments in the region. With respect to the distribution of amphibians we may recognize three major environments in the isthmus--rainforest, semi-arid scrub forest, and savanna. Each of these has varying combinations of physical and biotic factors that are important in the ecology of amphibians. Because of the importance of moisture, not only for the maintenance of life in these animals, but in most species their dependence on water for breeding purposes, this environmental factor is considered the most significant in the ecological distribution of amphibians. A second factor is the availability of necessary shelter, especially aestivation sites. These factors will be compared in the three major environments in the region.

Moisture is present in the environment in the form of free water or atmospheric moisture. With respect to the latter, it is well known that dense shaded forests have a considerably higher relative humidity than do open plains or areas with only scattered trees. Thus, the rainforests of the isthmus are characterized by a much higher relative humidity than are the savannas or semi-arid scrub forests. Although with regard to rainfall there is a p.r.o.nounced dry season in the regions supporting rainforest, there still remains considerable atmospheric moisture in this environment throughout the year. The dense foliage provides shade and protection from desiccating effects of wind and sunlight; furthermore the foliage contributes moisture by transpiration. The deep alluvial soils mixed with large quant.i.ties of organic matter (decaying leaves and rotting logs) maintain considerable quant.i.ties of moisture.

Conversely, the savannas and scrub forests have little atmospheric moisture during the dry season. In the former habitat there are few trees to provide shade or moisture through transpiration; in the latter most of the trees lose their leaves during the dry season. Thus, these environments are desiccated by the dry winds and direct sunlight.

Furthermore, the soils in these environments become dry and caked.

There is little or no terrestrial matter to hold moisture.

Free water in these environments is present in a variety of forms at different times of the year. During the dry season the more extensive marshes in the savannas persist; many ponds and most of the streams in the rainforest are permanent throughout the year. In the scrub forest all except the largest streams become dry during the dry season, and no ponds exist through the dry season. With the advent of the first heavy summer rains the stream beds fill with water, marshes expand, and many depressions become ponds (Pl. 5, fig. 2). At this time the amount of free water in the scrub forests and savannas greatly increases, much more so than that in the rainforests.

Environments are vertically stratified in the rainforests. There is the deep alluvial soil, the ground litter of leaves and decaying logs, the low bushes and small trees, and finally the tall trees of the forest.

Each of these provides certain types of shelter for amphibians. The moist soil and litter on the forest floor is an important microhabitat for fossorial and strictly terrestrial species. The dense foliage of the trees, tree holes, and bromeliads growing on the trees provide shelter for arboreal species. Arboreal and terrestrial bromeliads and the terrestrial elephant-ear plants (_Xanthosoma_) contain water in the axils of their leaves throughout the year and thus provide an important habitat for amphibians. The low, spiny, deciduous trees of the scrub forest and the gra.s.ses and scattered trees in the savannas provide little shelter. In the savannas there are depressions, some of which contain water throughout the year; these are often surrounded by trees providing refugia for amphibians during the dry season. In the scrub forest many species congregate along streams and in moist stream beds during the dry season.

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 mexica.n.u.s_. 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 l.a.b.i.alis_, _Leptodactylus melanonotus_, _Rana palmipes_ and _Rana pipiens_. The most striking ecological a.s.semblage 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 l.a.b.i.alis_, _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 l.a.b.i.alis_, _Leptodactylus melanonotus_, and _Rana pipiens_ are found near permanent water. Terrestrial species in this semi-arid environment include _Bufo ca.n.a.liferus_, _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 a.s.semblages 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 a.n.a.lysis 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 a.n.a.lyzed 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 a.n.a.lyzed 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 a.n.a.lysis 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 ca.n.a.liferus_ 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 mexica.n.u.s_ _Rhinophrynus dorsalis_ _Bufo marinus_ _Engystomops pustulosus_ _Leptodactylus l.a.b.i.alis_ _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_.