Declines in populations of Australia's endemic tropical rainforest frogs

Abstract

Comparisons of present and past occurrences suggest that populations of six frog species endemic to the tropical rainforests of northern Queensland have declined during the past ten years. Most declines have occurred at high altitudes in the southern portions of the tropical rainforest. An extensive survey conducted during the summer of 1991-1992 did not locate any individuals of two upland species, Litoria nyakalensis and Taudactylus rheophilus. Another upland species, T. acutirostris, which formerly was widely distributed, appears to have declined in rainforests south of the Daintree River. Three species (Litoria nannotis, L. rheocola and Nyctimystes dayi) were absent from most upland sites south of the Daintree River, but were common at lowland sites and at all sites north of the Daintree River. Aspects of water chemistry, including inorganic ions, heavy metals, and pesticide residues, were analysed for many sites. These analyses failed to identify any abnormalities that might have contributed to frog declines. Declines appear to be unrelated to the history of forestry or mining at sites, or to low rainfall in wet seasons. Levels of habitat disturbance by feral pigs appear to have increased at some sites in recent years and, either by this disturbance or through direct predation, feral pigs may have contributed to declines in some populations. However, pigs are unlikely to be the sole cause of frog population declines. Once declines have occurred, fragmentation of rainforest habitats may prevent recolonization from adjacent sites. Until causal agents associated with declines can be identified, management strategies to ensure the long-term survival of these species must involve protection of the riparian habitats in which they occur.