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: