Globally, the growth rate of suburbanization is greater than the human population growth rate, and suburban cover now exceeds 25% of land in most developed countries. Suburban sprawl, essentially landscape conversion, is a major issue effecting biodiversity loss. Amphibians are known to be the most rapidly declining taxa and landscape conversion is recognized as a driving factor in this decline. However, wood frogs (Rana sylvatica) have been studied as a suburban -tolerant species. In fact, they can be found breeding in ponds surrounded by over 70% suburban cover.
This species of frog is distributed across North America and utilizes a wide range of habitat types in both undisturbed areas and suburban habitats. Dr. Lindsey Swierk, Postdoctoral Fellow at Yale University, and Dr. Jennifer Tennessen, Research Associate at Western Washington University, are studying wood frogs' remarkable adaptability to their changing environment. They find that "understanding how amphibians, such as the wood frog, persist despite suburban challenges can help us understand their capacity to adapt to a rapidly changing world."
Dr. Swierk's and Dr. Tennessen's overall goal in studying wood frogs is to better understand how suburbanization affects amphibian reproduction and communication. Wood frogs are known to tolerate a variety of habitats which makes them an ideal species to study when examining the broader question of how amphibians cope with human-altered and noisy environments. Suburban traffic noise is one such condition that can potentially affect and mask wood frog vocalizations. This project aims to understand how wood frogs respond to anthropogenic noise, considering the large role that acoustic communication plays in their biology.
Wildlife Acoustics Song Meters have been placed near six ponds over a suburbanization gradient in southern Connecticut. The ponds ranged from being surrounded by more than 70% suburban cover (i.e. houses, lawns, paved surfaces) to dense forested areas. Song Meters were selected because of their customizability that allowed for programing a schedule that was specific to the project's needs, while optimizing battery life and storage capacity. Wood frog breeding season can last anywhere between a few days and a few weeks. The recorders were set to record for 5 minutes every two hours and for 20 minutes during high and low road traffic periods (5 a.m., 11 a.m., 5 p.m. and 11 p.m.). Dr. Swierk and Tennessen found that the 20-minute samples enabled them to understand how chorus frequency changed with, or acclimated to, traffic noise across pond types. However, "knowing when to deploy the meters is a careful art…as wood frogs overwinter away from ponds but migrate to the ponds each spring to breed" Dr. Swierk asserts. This migration occurs on one of the first few warm days of spring, usually after or during warm rain. The hope is to deploy 1-2 weeks before there is any chance of frog breeding.
Drs. Swierk and Tennessen find that working in the suburbs comes with unique challenges. Dr. Swierk advises, "Working in the suburbs requires excellent local collaborators and strong relationships with local organizations, landowners, and the community. This is a major key to success in projects involving urban/suburban ecology and conservation. Making your research as accessible and relatable as possible greatly helps in gaining acceptance in the community." Homeowner buy-in is usually easy. Dr. Swierk continues, "Most homeowners are excited and willing to have frog biologists conducting research in their backyards, and we're very grateful for their enthusiasm."
However, there are times Song Meters must be put in public areas. When this is the case, Dr. Swierk and Dr. Tennessen find that they "deploy all of our Song Meters high in trees using a ladder, double-locking them in place. Usually the trees we select are located a few feet into the ponds, which serves as an additional deterrent for curious hands. For the ponds that are not easily accessed by the road, we take the same precautions – we hike the ladder a couple miles into the forest to deploy the Song Meters."
The project is in its second year of monitoring wood frog vocalizations and has generated approximately 1,000 hours of recordings. Using acoustic analysis software from Wildlife Acoustics (Kaleidoscope Pro/Song Sleuth) and Cornell (Raven), the differences between suburban and forested frog calls (with a focus on call duration, frequency and daily/ seasonal timing) are being documented. Call rate is also being looked at, with the help of the statistical software package R to estimate the size of the breeding population. The data will help answer questions of whether wood frogs are acclimating/adapting to noise and the stresses of suburbanization. The six ponds that were originally chosen to be acoustically monitored will continue to be monitored in the future in order to document long-term change.
Long-term monitoring will also shed light on the effects of climate change. The beginning of the project happened to coincide with unusually warm weather in late winter followed by a harsh winter storm in 2016. This pattern repeated itself in 2017. In both years the wood frogs began to breed and then stopped, causing many frogs and eggs to die. Warmer winters, earlier springs and severe storms in the northeastern United States are all predicted to increase due to climate change. By tracking breeding activity in the coming years Dr. Swierk and Dr. Tennessen will be able to monitor the effects of climate change on wood frog proliferation.
Dr. Swierk and Dr. Tennessen hope to continue the project in its current form for several more years. Data will be continually collected, analyzed and shared. "Success of our project will be defined as sharing our results with the public and with the scientific community, hopefully to increase awareness of suburban amphibians and our understanding of the needs of suburban wildlife populations. It is a long-term goal that these data will be used to influence policy, and we endeavor to make our work as accessible as possible to non-scientists."
As told to Wildlife Acoustics
Thanks to Dr. Lindsey Swierk and Dr. Jennifer Tennessen who provided the content for this case study