Why study amphibian responses to noise pollution?
According to recent studies, one-third of amphibian species are threatened with extinction. Suburbanization and associated noise pollution can potentially drive amphibian decline by drowning out their calls.
Drs. Swierk and Tennessen embarked on a multi-year project to understand how suburbanization affects amphibian reproduction and communication using Song Meter SM4 acoustic recorders to compare entire seasons of wood frog (Rana sylvatica) chorusing behavior at suburban and forested ponds.
Six ponds in Connecticut associated with a range of suburbanization cover were selected for monitoring for a period of several years, starting in February 2016. One Song Meter SM4 was placed at each pond before the start of the annual wood frog migration. The recorders collected data throughout each day until the frogs left the ponds.
Song Meters were selected because their programming schedule could be customised to fit the project's specific needs while optimizing battery life and storage capacity. The recorders were set to record for five 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.).
Working in the suburbs came with unique challenges. Most homeowners are excited and willing to have frog biologists conducting research in their backyard. However, when Song Meters had to be put in public areas, they were deployed high in a tree using a ladder and double-locked in place.
The six Song Meter SM4 recorders successfully recorded the wood frog breeding season at ponds in 2016 and 2017, generating more than 1,000 hours of data that was analyzed using Kaleidoscope Pro.
Wood frog breeding was unusual both years, as unexpected late-season blizzards in March divided the season in half by several weeks. There was some mortality of adults and fertilized eggs in the breeding ponds following the blizzard, although most choruses managed to recover after the weather warmed. This extreme weather event provided information on not only examining how wood frog choruses are affected by anthropogenic noise (for example, the large amount of traffic noise that that interfered with some choruses), but also how indirect anthropogenic influences, such as extreme weather events that are predicted to increase in frequency in many global climate change scenarios, affect chorusing behavior of amphibians.
Comparison of the effects of suburbanization on wood frog breeding activity was used to quantify the number of advertisement calls that was performed in the five minutes at the start of every other hour throughout the breeding season. Breeding season durations, start and end times, and a proxy of the number of animals in the chorus at ponds over a suburbanization gradient were analyzed. Different aspects of the weather (temperature, wind speed, water temperature, humidity, etc.) was analyzed to quantify the call rate at ponds in different environments. The 2016 pilot data from the most and least suburbanized ponds showed some interesting trends. Frog choruses peaked during the nighttime hours in the most suburbanized pond but, in the most forested (least suburbanized) pond, this pattern isn't as apparent. If this trend holds true, it could be evidence that frogs are altering their calling behavior in noisier environments.
2017 Song Meter data has provided significant insight about wood frog chorusing behavior across the suburban gradient. Suburban wood frog choruses appear to be more robust to unfavorable weather; colder and windier days appear to be less of an impediment to suburban wood frogs than to those in the forest. Adult male population size, as estimated by call-counting proxy, is not directly related to suburbanization but instead to pond size and habitat. That said, the most suburbanized pond (surrounded by 70% suburban development to 200 m) hosted the smallest population despite its close similarity in size to several of the other ponds in the study. The figure shown here depicts the number of call detections in five minutes collected every two hours over the breeding season in each of six ponds, from "1" (most forested) to "6" (most suburban).