Why has the number of dark moths decreased in the last 50 years?
The decrease in the number of dark moths over the last 50 years is primarily attributed to the phenomenon known as industrial melanism. Dark moths, specifically the peppered moth (Biston betularia), were predominantly found in areas with high levels of pollution, such as industrial regions during the 19th and early 20th centuries.
During this period, light-colored peppered moths were more easily spotted by predators against the soot-covered tree trunks, making them more susceptible to predation. However, dark-colored moths had a higher chance of survival as they were better camouflaged against the darkened trees, leading to an increase in the frequency of the dark form.
With the implementation of pollution control measures and improved air quality in many industrial areas since the mid-20th century, the prevalence of soot-covered trees decreased significantly. As a result, the advantage of the dark-colored moths in terms of camouflage decreased, and light-colored moths gained an advantage once again.
Thus, the decrease in industrial pollution and the subsequent reduction in soot-covered environments has led to the decline in the number of dark moths over time, as natural selection favored the re-emergence of the light form.