Part 6

Jesus Carranza (formerly Santa Lucrecia).--Lat. 17 deg. 27'; long. 95 deg. 02'; elev. 80 m. A town and railroad junction in the middle of the isthmus; rainforest . Most of Dalquest's specimens came from varying distances from Jesus Carranza along the Rio Coatzacoalcos and its tributaries.

Minatitlan.--Lat. 17 deg. 58'; long. 94 deg. 32'; elev. 15 m. An oil refinery center on the Rio Coatzacoalcos; savanna .

Naranjo.--Lat. 17 deg. 35'; long. 95 deg. 07'; elev. 100 m. A village on the Trans-isthmian Highway, 45 kilometers south of Acayucan; rainforest and palm forest .

Novillero.--Lat. 18 deg. 16'; long. 95 deg. 59'; elev. 10 m. A village on the Rio Papaloapan; scrub forest and grassland .

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

Playas, Rio de las.--Lat. 18 deg. 08'; long. 94 deg. 07'; elev. 3 m. The river (sometimes known as the Rio Tonola) forming the boundary between the states of Veracruz and Tabasco; rainforest .

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

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

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

Tejada, Lerdo de.--Lat. 18 deg. 37'; long. 95 deg. 31'; elev. 60 m. An agricultural village, 35 kilometers by road east-southeast of Alvarado; scrub forest, marshes, and sugar plantations . Collections were made in a marsh, 5 kilometers west-northwest of the village.

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

Tula.--Lat. 18 deg. 36'; long. 95 deg. 22'; elev. 150 m. A village near the western base of Los Tuxtlas; low evergreen forest and marshes . 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 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.