Frogs at contaminated wetlands along the Merri Creek

Mavi Rasa is an environmental scientist, BSc (Hons) from the University of Melbourne, researching ecotoxicology, contaminated land and risk assessment. This article is a summary of Mavi's honours research project supervised by Prof. Kirsten Parris.

Increased urbanisation has led to significant environmental change. Heavy-metal pollution, through urban runoff from the impervious surfaces in urban areas to receiving waters, poses a significant threat to the health of aquatic ecosystems and animal communities. Stormwater-treatment wetlands are a common feature in many urban environments. These are incorporated into urban design to capture polluted urban runoff and filter it before it enters streams and rivers. These wetlands are often quickly colonised by fauna, such as frogs, that use them as habitat.

In my research, I assessed the relationships between sediment concentrations of eight heavy metals and frog species detection, richness and community composition at 33 stormwater-treatment wetlands along the Merri Creek corridor, across the spring-summer breeding seasons of 2021-2022.

Frogs were present at all study sites. A total of eight species were detected across all sites. Six species were detected at the wetland that holds the most frog species. My results suggest that many frogs indiscriminately inhabit polluted wetlands. This is consistent with other studies in the region, that found polluted wetlands could function as ecological traps for some frog species.

Urban wetlands function as ecological traps when frogs are unable to detect and avoid polluted wetlands that harm them and compromise breeding levels. This could be due to the fact that polluted wetlands are so prevalent in the urban environment, that it becomes harder for frogs to avoid them. Combinations of pollutants can also impair the ability of frogs to detect and avoid harmful habitat. This pattern of occupying ecological traps has the potential to inflict serious consequences on breeding patterns of frog populations.

The Common eastern froglet (Crinia signifera) was less likely to be detected at sites with high levels of copper and heavy-metal pollution. Crinia signifera is a widespread and common species, with its aquatic stage being key to its survival and population persistence. Coupled with the fact that the grazing habits of tadpoles are linked to the bioaccumulation of heavy metals, occupying polluted habitat could lead to reduced breeding success.
 

    
Growling grass frog, Litoria raniformis; Common eastern froglet, Crinia signifera

My results also suggest that some frogs are discriminately inhabiting wetland habitat based on higher nickel pollution. The Spotted marsh frog (Limnodynastes tasmaniensis) and the Southern brown tree frog (Litoria ewingii) are two.  This relationship may be attributed to a release from competition faced by these two species when more sensitive frog species, such as Crinia signifera, have reduced occurrence due to heavy-metal pollution.

Since the European colonisation of Australia, the Merri Creek corridor has had a diverse history of human influence. While my results point to stormwater-treatment wetlands potentially acting as ecological traps, in the contemporary urban landscape, where the numbers of artificial wetlands vastly outnumber their natural counterparts, unmodified aquatic habitats may not exist. Polluted habitats may serve as supplementary, or the only available, habitat for breeding.

There is ample opportunity for managers to create a landscape where wetlands that accumulate lower levels of pollutants are made more attractive to frogs. These can connect in the landscape to other good-quality habitat, such as source habitats, with abundant vegetation, to promote and sustain healthy frog populations. Similarly, lower-quality filtration wetlands, with high levels of heavy-metal pollution and predatory fish, could be made less attractive to frogs (e.g., steeper banks) and more isolated in the urban setting, thus protecting frogs from occupying polluted wetlands.

Spotted marsh frog, Limnodynastes tasmaniensis.

Images: Mavi Rasa