Acid Tolerance in Connecticut Wood Frogs (Rana sylvatica)

Abstract

-Acid tolerance limits were measured in wood frog (Rana sylvatica) embryos and larvae from a population located in New London, Connecticut. Among embryos, relative hatching success was high and not significantly different in solutions with pH 7.2-7.6, pH 6.0, pH 5.0 and pH 4.0. However, at pH 3.75 relative hatching success dropped to 50% and less than 1% of embryos survived at pH 3.5 or below. At pH 4.0 the number of developmental abnormalities among surviving embryos increased significantly, and at pH 3.75 35% of surviving embryos appeared abnormal. Larvae were more acid tolerant than embryos, and this tolerance appeared to increase as the larvae developed. Measurements of acidity in the natural breeding habitat indicated that considerable temporal and spatial variation may occur within a single pond, but the levels of acidity measured did not exceed the species' tolerance limits. The effect of hydrogen ion concentration, or pH, on freshwater ecosystems has recently been the focus of intensive study (Drablos and Tollan, 1980; Hutchinson and Havas, 1980). In spite of a growing interest in environmental acidity and its influence on plants and animals, one large group of aquatic vertebrates, the amphibians, has received relatively little attention. Many temperate zone frogs and salamanders exhibit complex life history patterns in which courtship and spawning typically occur in small ephemeral bodies of water, embryos and larvae develop in the aquatic environment, and metamorphosis results in a transition to terrestrial habitats. (There are, however, many variations of this pattern; see Salthe and Mecham, 1964 and Dent, 1968). Aquatic life history stages may be particularly susceptible to acidity, but the importance of this environmental factor in limiting the natural distributions of amphibians is not yet clearly understood. Gosner and Black (1957) first provided evidence that frog species differ in their ability to survive acidic conditions. They measured the acid tolerance limits of 11 species from New Jersey, and found that those frogs breeding in the naturally acidic waters of the Pine Barrens were more acid tolerant than those breeding only in more neutral waters. Two other investigations have also examined embryo survival of several amphibian species in acid bog water, and both detected differences among species in bog water tolerance (Karns, 1983; Saber and Dunson, 1978). Pough and Wilson (1977) tested acid tolerance in two salamander species, and found that embryos of the spotted salamander (Ambystoma maculatum) displayed increased mortality in even slightly acidic pond water. Jefferson's salamander (Ambystoma jeffersonianum) appeared to be more acid tolerant. Furthermore, Pough (1976) correlated pond acidity with embryo survivial in field populations of Ambystoma maculatum and suggested that this animal is in danger of localized extinction as a result of increasing acidity by acid precipitation. However, Cook (1978) found that populations of spotted salamanders from the Connecticut Valley of Massachusetts were considerably more acid tolerant. Thus geographical variation in acid tolerances is implicated for this species. In other studies, Schlichter (1980) investigated acid tolerance in Rana pipiens, Porter and Hakanson (1976) evaluated the impact of acid mine drainage on the distribution of Bufo boThis content downloaded from 157.55.39.128 on Mon, 18 Jul 2016 05:41:09 UTC All use subject to http://about.jstor.org/terms