The ecology of native Australian frogs

Abstract

Amphibians play a major ecological role in many ecosystems. Despite their ecological significance, however, many native amphibians remain undescribed, underrepresented in the scientific literature, and unmanaged. Globally, a quarter of amphibian species have insufficient data for assessment of conservation status, but we know that amphibians are threatened by many processes, including introduced fish species and climate change. The family Hylidae is widespread throughout the Americas, Europe, Asia and Oceania. Litoria, the most speciose genus of Hylid frogs in the Australasian region, exhibits a diversity of ecology and life history strategies. Despite the abundance and diversity of the genus, many of its species are poorly known. In particular, important information on reproductive ecology, developmental biology, and response to environmental stressors is lacking for much of the group. To address these key knowledge gaps, this thesis aims to: 1) investigate the invasion biology of the Litoria genus, 2) determine the reproductive phenology of Litoria frogs, and 3) examine the responses of Litoria tadpoles to predation risk when under stress. I provide context for such investigations with a comprehensive literature review on the ecology, life history, and conservation of Litoria frogs. Following its accidental introduction via boat cargo from mainland Australia, Litoria dentata became invasive on Lord Howe Island: an example that demonstrates the failure of existing biosecurity measures. This finding highlights the risks illustrated by other invasions (e.g. Litoria fallax on the island of Guam), and which will likely lead to future introductions if biosecurity practices are not improved. From detailed analysis of calling patterns of a diverse anuran assemblage at Karawatha Forest Park, Queensland Australia, I identified four discrete modes of reproduction: explosive summer breeders, prolonged breeders, opportunistic breeders and winter breeders. It is likely that all Litoria can be divided into these groups, but for most species, there is not enough published data to be able to do so. Humidity and temperature were the main influences on calling, but surprisingly, rainfall was not a significant driver of calling in the community. In the laboratory, both the presence of predators and warmer water caused Litoria ewingii tadpoles to grow faster, and reach a larger size before metamorphosing, than the cold treatments without predators. Temperature might be more important than predation risk in determining developmental rates of tadpoles due to the relationship between metabolic rates and water temperature, and potential phylogenetic constraints on phenotypically plastic responses to a predatory cue. A parallel result in Litoria spenceri tadpoles revealed that both predator absence and lower population densities yielded faster growth and larger size before metamorphosis. Tadpoles moved less often and reduced feeding effort under both conditions. However, although predation risk appeared to be most important when tadpoles also experienced increased competition, prioritising competitiveness over predator avoidance may leave tadpoles more vulnerable to the lethal threat of predators. This is particularly important in species such as L. spenceri, which, like many Litoria species, now co-occurs with introduced fish predators.