You might think that wild frogs would always fare better in wild, natural environments. New research from the University of Florida shows that this isn’t entirely true, much to the surprise of Brett Scheffers, lead researcher and assistant professor in the Department of Wildlife Ecology and Conservation at University of Florida. When he first started researching humanmade ponds and their effect on wood frogs (Lithobates sylvaticu) in Alberta, Canada, his expectation was to find fewer frogs but also frogs of poor condition. “This is because urban wetlands tend to have high levels of chemical run-off, siltation and introduced predatory fish — conditions that are harmful to the health and survival of frogs.”
As humans increase their urban development year on year, the amount of natural landscapes dwindle as they make way for residential and commercial infrastructure. Importantly, these vanishing landscapes include natural wetlands that store rainwater and form an important part of the ecosystem. Without them, urban landscapes can be prone to flooding. In order to counteract the missing water-storage systems, artificial ‘stormwater ponds’ can be installed near these urban expansions. These are designed to collect and maintain the water that is no longer being managed by the missing natural systems, preventing floods during storms and heavy downpours by collecting the water and releasing it slowly. It was these humanmade stormwater ponds that Dr Scheffers wanted to investigate, because not only do they provide a service for humans, but they are also utilised by wildlife in the absence of the natural equivalent.
Despite expecting to find that the frogs he found in these stormwater ponds would be of lower quality, Scheffers discovered something different. “To our surprise we did find frogs at urban ponds and their offspring were noticeably larger than those in more natural settings,” he said.
However, this doesn’t necessarily make stormwater ponds a better environment than their natural counterparts. The number of eggs, larvae and tadpoles was fewer in the humanmade stormwater wetlands in comparison with the natural wetlands. This reduced-competition in the environment allows the individuals that are there to grow larger.
For growing frogs, “more isn’t merrier” co-author, Cindy Paszkowkdi, said. “We observed that fewer tadpoles existed in the urban wetlands and so there was likely less competition.” Some frog species lay thousands of eggs at a single time and this means there is a lot of competition for resources. “But instead of splitting their resources a thousand ways, tadpoles in urban wetlands just had a few individuals to compete with,” Paszkowski said. This allows them to grow bigger in the stormwater wetlands, which also offer advantageous cool and stable water temperatures.”
The researchers are keen to encourage decision makers to pay attention to the amount of suitable habitat that surrounds stormwater ponds, so that the frogs that do exist in the area have the best chance of success after leaving the wetland. “The city life of giant frogs offers hope that humans may offset some losses in nature by creating animal-friendly habitats,” Scheffers said. “To do this, we suggest that urban parks and recreational areas should try to maximize the amount of natural vegetation in parks and in areas surrounding urban stormwater ponds.”
Brett R. Scheffers, Cynthia A. Paszkowski. Large body size for metamorphic wood frogs in urban stormwater wetlands. Urban Ecosystems, 2015; DOI: 10.1007/s11252-015-0495-z