ABSTRACT
As amphibians have declined more than other vertebrates and are important environmental bio-indicators for aquatic ecosystems, we must understand how attributes of such ecosystems affect individual frogs, populations, and multi-species communities. For aquatic-breeding frogs, pond physical/chemical properties influence all life-stages and therefore populations, species, and communities. However, studies have focussed on tadpoles, considered few water variables, and not considered relationships between water attributes and frog populations or communities. Inter-specific differences in how water attributes affect tadpole populations, and hence possibly adult choice of breeding site, should reflect habitats where different species occur, but has been little investigated. We studied the only frog species on Broughton Island, off the east coast of Australia: the threatened Green and Golden Bell Frog Litoria aurea (GGBF); and the widespread and abundant Striped Marsh Frog Limnodynastes peronii (SMF). Suitability of pond breeding habitat, measured by encounter rates during 57 visits over 17 years with different life stages, depended on depth, salinity, pH and temperature, with responses by the two species in essentially opposite directions. For both species, encounter rates increased with increasing depth and with decreasing salinity. Encounter rates peaked at neutral to alkaline pH for GGBF (i.e., 6.8 to 8.0 or higher) and acidic pH for SMF (i.e., 5.5 to 6.0). As water temperature increased, encounter rates increased for GGBF and decreased for SMF.
Frog community structure depended on average pond temperature and how it changed over time. If recorded temperature, averaged over the year, was less than 22°C or declined to below this level, the GGBF population, as indexed by numbers of adults observed and occurrences of earlier life stages, was low or declined, while the SMF population was high or increased. This threshold yearly average temperature translates into a required temperature at onset of spring breeding for the GGBF of about 20°C. The GGBF population on the island has declined since 1998, when our study began, due to increased aquatic vegetation and consequent decline in water temperature in major breeding ponds. As ponds are small, manual vegetation removal should reverse the trend. A comprehensive understanding of the biological effects of climate change, past and future, can now be pursued.
