Amphibian decline and fertilizers used on agricultural land in south-eastern Australia
Introduction
Although agrochemicals are widely used throughout the world, the implications of the impacts of their use on amphibian populations has not been adequately studied (Baker and Waights, 1993, Baker and Waights, 1994, Hecnar, 1995, Oldham et al., 1997, Oldham, 1999). Waterbodies situated within agricultural areas receive run off from surrounding land where fertilizers and pesticides are applied, and concentrations of nitrogen and phosphorus in these often exceed levels encountered in non-agricultural areas. Bishop et al. (1999) recorded higher levels of phosphorus and nitrogen in wetlands associated with agricultural areas of the Holland River watershed in southern Ontario, compared to those upstream of the crop growing area. Because amphibians rely on waterbodies to reproduce, high concentrations of these chemicals may affect local populations and community structure. Reduced survivorship, altered feeding activity, and mobility, and decreased growth and development of amphibian larvae exposed to varying concentrations of nitrate fertilizers have recently been reported (e.g. Baker and Waights, 1993, Baker and Waights, 1994, Hecnar, 1995, Xu and Oldham, 1997, Marco et al., 1999).
The potential impact of agrochemicals on three species of frogs was examined in south-eastern Australia by assessing their survivorship in different concentrations of nitrate and phosphate fertilizers. The common eastern froglet Crinia signifera Girard and the striped marsh frog Limnodynastes peronii Duméril and Bibron, are common in New South Wales (NSW), whereas the green and golden bell frog Litoria aurea Lesson has disappeared in this area (White and Pyke, 1999).
The primary aim of the study was, based on sightings records of the three species in NSW (including the Australian Capital Territory), to elucidate whether there had been a contraction in their distribution and at what time. Agricultural records were then examined to determine if the amounts of fertilizers used (ammonium nitrate and calcium phosphate) in the State had changed, and corresponded with trends in annual rainfall, to explain any range contraction. In addition, experiments were performed using different fertilizer concentrations to determine if C. signifera, L. peronii, and L. aurea differed in their sensitivity to ammonium nitrate and calcium phosphate.
Section snippets
Materials and methods
C. signifera (18–28 mm) develops in 21 days (Lane and Mahony, 2002) and occurs in almost every freshwater habitat of south-eastern Australia (Barker et al., 1995, Cogger, 2000). L. peronii (48–73 mm) inhabits most freshwater bodies along the coast and ranges of eastern Australia (Barker et al., 1995). L. aurea (57–108 mm) inhabits a wide variety of waterbodies, but is listed as endangered in NSW (Threatened Species Conservation Act 1995) and as vulnerable nationally (Environment Protection and
Results
The databases used contained 773 records of L. peronii in New South Wales for 1940–1996. The curve for the cumulative number of records increased steadily from the mid-1970s to around 1990, where it rose sharply (Fig. 1). No change in the species’ range was evident. For the 1583 records of C. signifera, the curve increased sharply from the mid-1970s to around 1980, and only gradually to 1990 (Fig. 1). For the 893 records of L. aurea, the curve increased steadily and levelled in 1975 when the
Discussion
It was possible to show that the range of L. aurea contracted in 1975, whereas no range contraction was evident for C. signifera and L. peronii. Temporal patterns in fertilizer application and rainfall suggested that many waterbodies in agricultural NSW had elevated nitrate and phosphate concentrations immediately prior to 1975. The experimental evidence showed differential sensitivity to fertilizers between L. aurea and two non-declining frog species.
The number of records for L. aurea falls
Acknowledgements
This work was funded by Port Waratah Coal Services through the Hunter Catchment Management Trust. We thank Jack Trezise for assistance with experimental work. The New South Wales National Parks and Wildlife Service kindly provided records in the Atlas of NSW Wildlife and records of the Australian Museum herpetological database (Licence no. 535, released 31 July 2001). Permission to use animals in the experiments was given by the Animal Care and Ethics Committee, The University of Newcastle
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