About 80% of 177 species studied show a change in at least one trait of the body, growth or behaviour when exposed to human noise. This is the finding of a review in the journal Science by Hansjoerg Kunc and Rouven Schmidt of Queen's University Belfast, which sums up 214 experimental studies.
The authors gathered studies that compared animals exposed to human noise with a control group kept in quiet. The studies measured how the body works, growth, behaviour, survival, breeding, numbers and the link between predator and prey. In the analysis, the authors accounted for which species are close relatives, so that species from one family would not skew the result.
A share of species, not of individual animals
The percentage shows in how many species scientists measured at least one effect. If noise raised stress hormones in a species, a change was recorded, even if no other trait changed. According to the article, this applies to about 140 of the 177 species. Noise can raise cell stress and stress hormones and change physical development.
Another measure is numbers: for every 100 animals in quiet places, about 63 live in noisy ones, or 37% fewer.
A 2019 review by the same authors in Biology Letters covered 108 studies of 109 species: amphibians, arthropods, birds, fish, mammals, molluscs and reptiles. The groups did not differ from each other in how they reacted to noise, so the authors concluded that most species react, not just a few highly sensitive ones.
Predators and prey under noise
Under noise, predators put in about two and a half times more effort but catch about 25% less prey, and the prey is more alert. Many predators find victims by sound, and noise drowns it out.
This showed in an experiment in Sweden. Thirty-eight captive-bred young tawny owls were tested in aviaries, each bird in quiet, in traffic noise, in artificial light, and in light with noise together. Noise cut the chance of a bird reacting to a prey signal by about 10%. It had the strongest effect on rustling, which the bird has to hear. The birds also found it harder to tell where a sound came from.
Noise does not affect all species in the same way. The review authors write that it creates winners and losers among species and changes ecosystems. One predator may lose out because of noise, while another gains when noise drives prey into the open. In the 2019 review, less hunting by bats was noted as something that could indirectly help the animals they prey on. The authors do not claim they can say whether the effect is harmful or helpful in each single case. The overall result in the new review is negative, although some species gain.
Limits of the data and quiet zones
The data come from experiments, not from a count of all wildlife. The Swedish study, for example, used captive birds and no real hunting, so it does not measure how successful hunting is. The birds' responses weakened with repetition, partly from getting used to the noise. The article on the new review does not discuss its limits.
The authors suggest targeted sound management, for example low-noise zones that could serve as acoustic refuges.
An earlier review in Science from 2021 on the ocean noted that, unlike chemical pollution, the effect of noise stops once the noise falls. As evidence, the authors point to the quick response of animals during Covid restrictions.