AMPHIBIANS AND REPTILES

Patrick T. Gregory and Linda A. Gregory

PERTINENT ASPECTS OF THE BIOLOGY OF AMPHIBIANS AND REPTILES

Amphibians and reptiles are an often overlooked element of the fauna in an area, although recent interest in the “declining amphibian problem” (Green, 1997a) has counteracted this somewhat. In addition to intrinsic interest in these organisms for their own sake, however, there are sound ecological reasons to be concerned about their preservation. Because amphibians and reptiles are ectotherms, they do not devote significant food intake to metabolic heat production and they are more efficient than endotherms (birds and mammals) in converting food to new biomass. Amphibians and reptiles thus reach tremendously high biomasses in some ecosystems; they therefore represent a potentially important food source for animals at higher trophic levels and may play a very important role in energy flow through the system (Pough, 1980).

Collectively, amphibians, both living and extinct, form a monophyletic group. That means that all members of the group are more closely related to each other than they are to animal species outside the group; another way of expressing it is that a monophyletic group includes an ancestor and all its descendants. Within the amphibian lineage, the living amphibians also generally are considered to be monophyletic and therefore to constitute a legitimate taxonomic group, although there continues to be debate on the issue (Duellman and Trueb, 1986; Zug, 1993). There are, conservatively, 4000-4500 species of amphibians recognized worldwide (Halliday and Adler, 1987; Stebbins and Cohen, 1995), largely anurans (frogs and toads, about 87 %), with smaller numbers of caudates (salamanders, 9 %) and apodans or caecilians (4 %). The latter group, consisting of limbless animals, both aquatic and terrestrial, is strictly tropical in distribution and need concern us no further here. Collectively, amphibians most often are characterized as having a complex life cycle (aquatic mating-aquatic eggs-aquatic larvae-metamorphosis to an aquatic/terrestrial juvenile-growth to sexual maturity-aquatic mating); in reality, however, there are numerous derived departures from this apparently ancestral pattern. For example, salamanders, including various species in the temperate zone, exhibit life cycles ranging from loss of the larval phase to attainment of reproductive maturity while maintaining an otherwise largely larval morphology (paedomorphosis). Paedomorphosis (at least in this particular sense) is not seen in anurans, but there are numerous examples of reduction or loss of the larval phase, often coupled with some form of parental care or other protection of eggs or larvae, especially in tropical species. Most species of anurans at higher latitudes of the north temperate zone exhibit the “typical” complex life cycle, which is seen in all species in the Montane Cordillera. Viviparous, or live-bearing , amphibians, also are known, especially among apodans, but none occur in our area. On the other hand, one of our species, the Tailed Frog, is unusual for an anuran in having internal fertilization, facilitated by the “tail” (really a modification of the cloaca), which acts as a copulatory organ (Fig. 1). Regardless of life cycle, most amphibians, because of their thin skin, desiccate easily; therefore, they are generally tied to moist habitats or restricted in their activity to cooler, moister parts of the day or year (Stebbins and Cohen, 1995). The overwhelming importance of water to amphibians is a highly significant influence on their patterns of distribution, at both local and regional scales (Fig. 2). Consequently, the draining of marshes and wetlands for development, including agriculture, is a significant threat to the amphibian populations of the Montane Cordillera.

Unlikeamphibians, reptiles, both living and extinct, constitute a paraphyletic group (a monophyletic group minus some members), in that their descendants, birds and mammals, are not usually included with them in the Class Reptilia (Zug, 1993). Furthermore, the extant forms of reptiles alone are paraphyletic. Based on phylogenetic (cladistic) criteria, therefore, “reptiles” do not comprise a valid taxonomic category and newer classifications do not recognize them as such. Nonetheless, we can still recognize them informally based on shared ancestral traits that both influence their ecology and provide the characteristics (e.g. “scaly” skins) that caused us to lump them together in the first place. Living “reptiles” consist of at least 6500 recognized species worldwide (Halliday and Adler, 1987), divided into turtles (4 %), crocodilians (0.3 %), tuataras (only two species, 0.03 %), amphisbaenians (2 %), lizards (57 %), and snakes (36 %). Crocodilians (alligators, crocodiles, etc.) and amphisbaenians (burrowing, mostly limbless) are restricted to the tropics and subtropics; tuataras (lizard-like) are found only on islands off mainland New Zealand. Thus, the reptilian fauna of the Montane Cordillera area is made up of turtles, lizards, and snakes, especially the latter. “Reptiles”, along with birds and mammals, are amniotes; that is, they produce an amniotic egg, which distinguishes them from the amphibians. The amniotic egg derives its name from the amnion, one of a series of membranes, which, together with a semi-rigid to rigid shell, allow the egg to be large relative to an amphibian egg, and more resistant to desiccation (Alexander, 1975). The nature of the egg is held to be one of the main factors that has allowed amniotes to become far less dependent on access to water than are amphibians in general. Reptiles also have a thicker, more cornified skin than amphibians; lipids in the skin help increase its resistance to water loss. Not surprisingly, reptiles and other amniotes are dominant vertebrates in hot dry deserts. However, in the Montane Cordillera and elsewhere, many species of reptiles are aquatic or partly aquatic, feed on prey associated with wetlands, or rely on fresh water to drink; thus preservation of wetlands, even in dry areas such as the Okanagan, is critical to conservation of reptiles. Furthermore, although the amniotic egg must be laid on land, the eggs of most reptiles still require access to soil moisture for development. Finally, because a shell is deposited around the fertilized egg in the oviduct, internal fertilization is a requirement (and therefore a characteristic of all amniotes). This development, in turn, facilitated the evolution of vivparity (or live-bearing of young), which has evolved independently many times in lizards and snakes and is of some significance in considering the question of how snakes deal with cool northern environments.