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 Petén, Guatemala (Stuart, 1935 and 1958), we find that there are more species recorded from the isthmus than from El Petén. Stuart found only 20 species of amphibians in both forest and savanna habitats in El Petén. 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 pronounced 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 quantities of organic matter (decaying leaves and rotting logs) maintain considerable quantities 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 grasses 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 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 México across the Isthmus of Tehuantepec into Central America, those that reach the isthmus from Central America but do not extend into México proper, and those that reach the isthmus from México 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 México 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 México 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 México 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.