Frogs as bioindicators for chemical use in irrigation-based agriculture

Preliminary evidence suggests that chemical-use may affect frog populations within irrigation areas. The endangered Southern Bell Frog (SBF), Litoria raniformis, is widespread and reasonably common within the Coleambally Irrigation Area (CIA), where rice is the dominant crop, but has disappeared from most areas of natural habitat. Within the CIA, this species appears to have declined as growing of crops other than rice (soybeans, sorghum, maize) increased. This project tested the hypothesis that the Southern Bell Frog is favoured by irrigated rice-growing and not adversely affected by the chemicals commonly associated with this crop, but suffers negative effects from chemicals that are commonly used with other summer crops.

Project Objectives

• Review existing information in terms of known or likely effects of chemicals used in irrigation-based agriculture on Australian frogs.
• Determine, within rice-growing areas of the CIA, whether contamination of water with chemicals associated with other crops has any apparent effects on the SBF and other frogs. 
• Develop the application of histopathology biomarkers for monitoring frog populations.

Outcomes

We now know, as a result of the research funded in this project and completed by Dr Ross Hyne Department of Environment and Conservation (NSW) and others that:

• Southern bell frogs were sensitive to more intensive agricultural practices that reduce habitat, increase agricultural chemicals and reduce water quality.
• Detailed statistically robust comparative analysis of Coleambally Irrigation Area Southern bell frogs (L. raniformis) shows that their distribution is related to agricultural practices that included agricultural chemicals. Metolachlor (used as a herbicide in the production of corn) appears to be an indicator associated with other factors that lower the probability of detecting southern bell frogs in northern areas.
• Two types of solvent-based passive sampling devices were used and 50% detected residues of herbicides and fipronil, whereas the insecticides chlorpyrifos and endosulfan were detected in more than 85% of the samplers. Concentrations that exceed the water quality guideline triggers were measured at some locations, although endosulfan and fipronil residues were very low.
• Southern bell frogs did not display the impact of endocrine disruption effects from atrazine such as hermaphroditism or intersex gonads as expected from a previous study.

We also found that the 14-d average metolachlor concentrations that the L. raniformis tadpoles were exposed to at the study sites (0.05-0.73 μg/L) in the Coleambally Irrigation Area were four to five orders of magnitude lower than its 96-h LC50 of 13.6 mg/L for embyrotoxicity to the frog, Xenopus laevis (Osano et al. 2002). Assuming there is no much difference in sensitivity between these species, this suggests that metolachlor (used as a herbicide in the production of corn) may not cause direct adverse effects on L. raniformis, but rather it is an indirect indicator of agricultural practices such as pesticide usage, which are in turn a result from the different crop types being grown in the area. Detailed statistically robust comparative analysis of Coleambally Irrigation Area Southern bell frogs shows that their distribution is related to agricultural practices that included agricultural chemicals.

Sites in the southern region of the Coleambally Irrigation Area are dominated by traditional rice farmers with small land holdings, which utilise minimal machinery for the construction of rice bays and levees. This results in well-vegetated rice bay levees on the farms and terrestrial environment adjacent to the irrigation supply channels from which the frogs disperse. In contrast, a feature of the northern irrigation region is the preponderance of larger land holdings, which in summer grow other crops such as corn and sorghum in addition to rice. This has lead to more mechanised farm practices on the larger holdings including more water recycling which results in higher conductivity. Associated with the water recycling are less vegetated on-farm water channels particularly at the beginning on the rice season, which may also discourage frog recruitment. Limited recruitment early in the rice growing season would be particularly disadvantageous for L. raniformis because of its more restricted breeding period, compared to the other anuran species. Southern bell frogs were sensitive to intensive agricultural practices that reduce habitat, increase agricultural chemicals and reduce water quality.

The figure above shows where L. raniformis is present (CP) and absent (CA) on corn and rice farms and present (RP) and absent (RA) on rice only farms. Sites with solid symbols show that L. raniformis was present. The results of the statistical analysis (Discriminant Analysis) arranges the data and identifies the most powerful explanations for the pattern. The sites are shown arranged across the most powerful explanatory variables. In summary L. raniformisis is more likely to be found in sites with low metolachlor concentrations, high pH, low conductivity and high temperatures. This is a powerful relationship with 20/28 sites being correctly classified and the correlation coefficients are 0.84 for the first function and 0.60 for the second, and together account for 94% of the variance.

Hermaphroditism or intersex gonads occurred in both Limnodynastes species, but only at a very low frequency (below 2%) which is considered the natural frequency of hermaphroditism in wild frog populations (Pettersson and Berg 2007). Furthermore, the study found no evidence of increased intersex gonads in late stage tadpoles or juvenile Limnodynastes species and L. raniformis frogs at sites with relatively high atrazine concentrations (0.16─1.67 μg/L) over 6 weeks during the period they would be undergoing metamorphosis. This contrasts with the study by Hayes et al. (2003) that found an increased occurrence of intersex gonads of male frogs in agricultural areas growing corn that was attributed to their exposure to concentrations of atrazine higher than 0.1 μg/L. Southern bell frogs did not display the impact of endocrine disruption effects from atrazine such as hermaphroditism or intersex gonads as expected from a previous study.
  

Publications and Resources

None listed

Citation

Land & Water Australia. 2009. Frogs as bioindicators for chemical use in irrigation-based agriculture. [Online] (Updated June 25th, 2009)
Available at: http://lwa.gov.au/node/2991 [Accessed Wednesday 29th of February 2012 07:48:41 PM ].