Of Noise and Men – Heading Into a Silent World

Wallowing in my own misery of being unable to get a decent night’s sleep two days in a row (being a very light sleeper only exacerbated the impact), I was quite oblivious to the impacts of noise pollution on the natural world. That is, until I went to Kingman & Heritage Island Park (close to the RFK stadium) on the Anacostia River, on a District of Columbia Department of Energy and Environment-led citizen science project to collect data on the District’s frog and toad populations, and associated wetland habitats which was also input to a national database. Even though it was night time in early April, we were straining to hear the vocalizations of the amphibians above the din of Blue, Silver, & Orange Line metro trains rumbling through the Stadium Armory stop, vehicles whizzing by on I-295, not to mention the frequent high pitch police sirens and ambulances. Acoustic communications of these amphibians such as alarm signals and mating calls were completely drowned out by anthropogenic noises. For these species, whose survival depends on vocal communications, noise pollution and habitat loss drive them closer to extinction, given that a lot of them are already threatened or endangered.

While there is ample data on the inverse correlation between noise levels and human health, scientific data on the effects of anthropogenic noise pollution on the flora and fauna are still emerging. Shannon et al. reviewed the scientific studies published between 1990 and 2013 on the effects of pervasive anthropogenic noise (deemed “an evolutionarily novel and global pollutant”) on both terrestrial and aquatic wildlife (songbirds and marine mammals) of Europe and North America (Ref 3). Even though auditory capabilities and biological responses to noise differ among species, they found emerging patterns of diverse threats ranging from alterations of individual behaviors to changes in ecological communities with terrestrial wildlife responding to noise levels of ~40 dBA (noise levels in a library, lowest limit of urban ambient sound, and some bird calls (Ref 4)).

Slabbekoorn (2016) summarized the numerous complex and interrelated biological consequences of noise pollution on terrestrial and aquatic fauna into these 6 parts: 1. physical damage (temporary/permanent hearing loss), 2. physiological stress (increased heartbeat or cortisol levels), 3. auditory masking (reduced detectability/recognizability of environmental/echolocation sounds/communicative signals), 4. spatial deterrence (moving away from potentially favored breeding/feeding areas), 5. behavioral interruption (breakdown of signal-response chains, interruption of life cycle events like schooling/spawning), and 6. signal modification (temporal/spectral alteration of communication signals) (Ref 5).

Noise pollution impacts on marine life (fishes and pinnipeds such as harbor porpoises and gray seals) from point sources such as pile-driving (sinking poles into soil to provide foundation support for structures) and use of seismic airguns (in oil and gas exploration of deep ocean floor) as well as nonpoint sources such as marine traffic have been well documented (Ref 6). This subject is the focus of triennial international conferences on the Effects of Noise on Aquatic Life, with the next one to be held in July 2019 in The Netherlands (Ref 7).

Wildlife in controlled environments such as zoos and aquariums suffer double indignities – being captive with no escape and being in noisy environments with no relief. Effects of chronic aquarium noise on lined seahorse (“considered a “flagship” exhibit animal in promoting marine conservation”) manifested in the organism’s stress responses resulting in changes in behavior and physiology such as reduced immune responses with increased susceptibility to parasites. Scientists from major US aquaria who conducted these studies recommended incorporating acoustics management in aquaculture such as soundproofing the tanks to protect fish/reptilian health (Ref 8). I can attest to the noise pollution near aquaria from a recent trip to the National Zoo with my students where we saw excited and noisy folks near open (top) tanks, and people tapping on glass enclosures trying to elicit reactions from the resident fish/amphibians/reptiles so they could take selfies.

Terrestrial carnivores which are already struggling with climate change impacts, loss of habitat, and lack of prey now have to contend with increasing noise pollution. Heterospecific communication is the use by one species of acoustic information such as predation alarm calls produced by other species. Morris-Drake et al. (2017) conducted field studies of wild dwarf mongoose populations and found that anthropogenic noise interfered with their response to heterospecific alarm signals of tree squirrels resulting in noise-induced distraction or stress-induced maladaptive behavior (Ref 9). While the increased mortality of raptors and bats from wind turbines has been well documented, but mammals living near wind farms are seriously impacted as well. Badgers are shown to be physiologically stressed with enhanced activity of the central stress response system and higher cortisol levels. Moreover, they do not become habituated to turbine disturbance (irrespective of wind farm annual power output, or number of turbines) even after several years of living in such environs. Turbines’ sound likely affects the badgers’ immune system which in turn could increase the risk of infection and disease (Ref 10).

The prey species are also at risk from noise exceedances (the amount of sound above natural levels that anthropogenic noise raises). Studies of stress responses in roe deer to noise disturbance from low-traffic roads (< 1000 vehicles per day) indicated that this was capable of causing higher habitat degradation than previously thought (Ref 11).

Birds fare no better. Mulholland et al. (2018) found that traffic noise exposure of nest boxes of Ash-throated Flycatchers resulted in their overall reduced reproductive success owing to reduced clutch size, brood size, or number of fledglings. This failure in successful nesting relative to quiet nests was likely due to higher rates of abandonment at the egg incubation stage. Those birds that do nest in noisy environments “experience stress hormone dysregulation and fitness costs” (Ref 12).

Anthropogenic noise affects not only specific species but threatens entire ecological systems and their biodiversity and cultural resources. Buxton et al. (2017) found that in the US, anthropogenic noise caused a 2-fold increase in background sound levels in 63% of protected area units (e.g. national parks, wildlife refuges, etc.) and ≥10-fold in 21% of them, “surpassing levels known to interfere with human visitor experience and disrupt wildlife behavior, fitness, and community composition”. Of the critical habitats listed under the Endangered Species Act, noise exceedances (>10db above natural level) were the worst for those of plants followed by invertebrates, fish, and amphibians. Even wilderness areas (the highest protected category) experienced noise pollution (Ref 13).

Getting back to the story of frogs, which made me think about this subject in the first place, Troïanowski et al. (2017) showed that noise exposure of European tree frogs had both physiological and phenotypic consequences. Anthropogenic noise not only increased the frogs’ stress hormone levels leading to an immunosuppressive effect but also negatively impacted their vocal sac coloration. These results have profound consequences on sexual selection processes as “the best quality males with initial attractive vocal sac coloration were the most impacted by noise” (Ref 14).

Simply put, the world is becoming louder due to human activities, endangering the health of all species. Marine biologist Rachel Carson launched the modern environmental movement in 1962 with her book the Silent Spring in which she described the toxic effects of pesticides in silencing songbirds and other wildlife. Now we have caused yet another environmental problem which also silences wildlife, ironically, through noise. Sound policy initiatives in natural resources management with strict enforcement informed by latest scientific studies coupled with personal/collective behavioral changes, such as being mindful of our noise-producing activities, having self-awareness in public places, teaching children about appropriate outdoor behavior, are all essential for the conservation of biodiversity in this increasingly noisy world.