The effects of increased environmental salinity on the ecology, physiology and behaviour of Australian frogs

Increasing environmental salinisation is rapidly becoming one of the greatest causes of freshwater habitat degradation. Elevated salinity levels are a substantial threat to freshwater biodiversity, but the effects of increasing salinity on anuran populations remain poorly understood. Global anuran populations are in decline, and therefore studies that specifically examine anuran responses to environmental stressors are vital for effective conservation management of an already at-risk animal group. This PhD project has been designed to expand our understanding of the effects of increasing environmental salinity on anuran populations by investigating the salinity tolerances of native Australian anurans, as well as exploring the non-lethal effects of salinity on anuran development, physiology, ecology and behaviour. I examined the effects of elevated salinity on larval growth, metamorphosis and survival in three Australian frog species. The results indicated that salinity tolerances are species-specific, with two of the species studied (Limnodynastes tasmaniensis and Neobatrachus sudelli) showing lower survival in high, but ecologically relevant salinities (5.6ppt NaCl equivalent). A higher survival in high salinity treatments compared to freshwater treatments indicated the potential for saline specialisation in one species, Litoria aurea. Differences in larval duration in L. tasmaniensis and L. aurea between salinity treatments indicated that the magnitude of salinity stress can alter the developmental responses of tadpoles. I surveyed the effects of environmental salinisation on anuran distributions and species richness in south-east Australia. Salinity concentration was a significant predictor of anuran diversity and species distributions at study sites. No anuran species were discovered at sites with salinity levels above 13.9ppt, and embryos and tadpoles were absent from sites with salinities above 5ppt, suggesting that current salinity levels are negatively influencing anuran population structure throughout the region surveyed. When examining how developmental plasticity differed in response to salinity between populations of Litoria ewingii, I confirmed that salinity exposure can affect post-metamorphic morphology, and determined that tadpoles sourced from ecologically distinct populations differed in their developmental responses to environmental stress. Tadpoles sourced from a constantly brackish wetland exhibited no differences in metamorphic timing or morphology between salinity treatments, potentially indicating that tadpoles from this population are no longer able to maximise fitness in freshwater environments. I explored how the magnitude, timing and frequency of temporary salinity fluctuations affected survival, growth and maturation in L. ewingii tadpoles. I confirmed that late-stage tadpoles show higher survival rates in elevated salinity compared to tadpoles exposed to salinity at a younger age, and discovered that higher magnitude salinities suppressed the potential for compensatory growth in L. ewingii. I found that L. ewingii tadpoles exposed to elevated salinity early in development did not significantly differ in their developmental responses to subsequent salinity stress compared to tadpoles that had not been pre-exposed to salinity. Finally I examined the potential short and long-term consequences of developmental saline stress on two species of Australian anuran (L. ewingii and Limnodynastes peronii) by examining changes in tadpole respiration, escape response behaviour (pre- and post-metamorphosis) and foraging ability post-metamorphosis. Elevated salinity treatment did not alter tadpole respiration rates relative to controls. Non-lethal salinity exposure during development impaired larval escape-response behaviour in both early-stage and late-stage tadpoles. Differences in adult escape response and foraging performance were species-specific. Higher salinity during development reduced L. ewingii escape response behaviour immediately after metamorphosis, but significant differences in locomotor performance between salinity treatments were no longer detectable three weeks post-metamorphosis. In addition, L. ewingii reared in higher salinity were significantly faster to catch pray than freshwater controls. The results from this thesis show that salinity levels in Australia are already impacting on anuran survival, and potentially limiting anuran species richness and distributions. Current levels of environmental salinity are also expected to alter developmental processes, physiology and behaviour in a number of native frog species. Considering that salinity levels are predicted to continue to rise, immediate action will be required to mitigate the deleterious effects of elevated salinity on freshwater ecosystems. I emphasise the need for continued studies that examine anuran responses to environmental stress in order to better combat the decline of anuran populations and maintain ecosystem stability.