Marine biologists were the first to continuously eavesdrop on marine mammals using a technique called passive acoustic monitoring (PAM). By simply listening to these animals’ sounds, researchers could collect valuable information about animal population density and distribution, population health, and responses to human disturbance. Given the challenges of studying animal communities as they migrate across the sweep of the world’s oceans, acoustic surveys gained popularity as a tool for gathering data from otherwise inaccessible study sites.
This powerful technology is now being applied to terrestrial environments, with exciting results, as revealed in a new study published in mongabay.com’s open-access Tropical Conservation Science . The study provides the first ever continuous long-term soundscape portrait of tropical forests.
Less than 200 blonde capuchins are believed to survive. They are only found in the Atlantic Forest. Photo by: Miguelrangeljr/Creative Commons 3.0.
Researchers chose three threatened environments in southeastern Brazil, including the Atlantic Forest, the iron-rich outcrops of the Rupestrian fields, and the Cerrado, or savanna — all biodiversity hotspots and high priority conservation areas. In addition to recording sounds, the researchers also evaluated their sound sampling methods for future use by researchers and wildlife managers. Their very promising findings have led them to recommend PAM as a useful tool for studying threatened habitats where researchers must take care not to cause further disturbance.
“Fixed, programmable acoustic recording sensors can sample continuously for 24 hours a day for prolonged periods of time, allowing for the non-invasive assessment of changes in the distribution and acoustic behavior of entire animal communities throughout a variety of habitats simultaneously,” wrote the researchers. “Moreover, all of the recordings can be permanently stored and serve as an everlasting memory of the habitat sounds.”
The researchers recorded each forest’s sounds, what they referred to as its “singing community,” in both the wet and dry seasons for six full-length days during a period of 15 days, totaling 2,592 hours. They found each forest produced its own distinct sound, varying according to climate and time of day.
Wet seasons sounded different than dry seasons, and night sounds were different than daytime sounds. Insect and bat choruses, for example, were more pronounced during wet seasons than other times. This “noisy” wet season trend was consistent for each location, which the researchers credited to the fact that wet seasons, mating seasons — and, therefore, mating calls — coincide for many animals and insects.
Although the researchers did report some methodology limitations, such as the problem of sound-masking weather in the form of wind gusts and tropical rain, they concluded that acoustic recordings provide valuable information about animal dynamics and diversity across space and time.
Cleared Cerrado in Mato Grosso, Brazil. Photo by Rhett Butler.
“Given the urgency of climate change and the loss of habitats, understanding normal levels of variation in acoustic complexity could be fundamental for conservation efforts, enabling managers to decide whether changes in acoustic dynamics warrant further investigation,” the researchers wrote. They also concluded that PAM “could be especially useful for wildlife managers whose choices are limited by economic and staff constrictions.”
The study of soundscape ecology is still in its infancy, but it’s potential for evaluating ecosystem health and animal behaviors is promising. Passive acoustic monitoring gives researchers and wildlife managers a way to monitor fragile and endangered animal communities in a manner that is both non-invasive and cost effective.
- Pieretti, N., Duarte, M.H.L., Sousa-Lima, R.S., Rodrigues, M., Young, R.J. and Farina, A. (2015). Determining temporal sampling schemes for passive acoustic studies in different tropical ecosystems . Mongabay.com Open Access Journal – Tropical Conservation Science Vol.8 (1): 215-234.