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.