Base Cations And Northern Cricket Frogs: A Critical Relationship?
Alan R. Resetar, Field Museum of Natural History, Division of Amphibians and Reptiles, 1400 S. Lake Shore Dr., Chicago, IL 60605-2496; (312) 665-7768; FAX (312) 665-7932; aresetar@fieldmuseum.org
The occurrence during the 1970's of a marked decline in Great Lakes populations of the northern cricket frog (Acris crepitans) is generally accepted. This decline included populations in northeastern Illinois, northwestern Indiana and southern Michigan. A conclusive reason for this decline has not been found. Seven sites harboring A. crepitans within 175 kilometers of Gary, Indiana were visited between 1998 and 2004 to investigate the persistence of these populations despite the apparent disappearance of some intervening, documented populations. Comprised of natural and anthropogenic habitats in urban, industrial and agricultural locations, these sites shared an obvious characteristic - the presence of base cations in the form of either exposed limestone, marl soil, slag (a steel-manufacturing by-product), groundwater, possible wind blown particulates or a combination of these. It is hypothesized that such sites serve as "base cation refugia" that sustain A. crepitans populations by ameliorating the effects of past, current, chronic and episodic acid precipitation. Information reviewed in formulating this hypothesis includes: 1) the causes and effects of wetland acidification; 2) the role of base cations in the neutralization of acidity in wetlands; 3) the geographic pattern and chronology of acid precipitation in the Great Lakes region; 4) the apparent sensitivity to, or avoidance of, acidic water by A. crepitans; and 5) aspects of A. crepitans distribution in the context of wetland pH. Further field surveys and diatom-based pH reconstruction studies at both extant and historic A. crepitans sites could test the hypothesis in larger geographic and intersite contexts.