The peppered moth, Biston betularia, is a common insect found across temperate regions of the Northern Hemisphere, including parts of Asia, Europe, and North America. This moth typically inhabits woodlands, scrublands, hedgerows, parks, and gardens. Adult moths spend their days resting on trees. These moths serve as a food source for various animals.
Common Predators
Insectivorous birds are the primary predators of peppered moths. During daylight hours, adult moths resting on tree trunks and branches become vulnerable to detection. Common bird species like robins and nuthatches prey on moths, foraging on tree bark. Birds typically locate these moths visually before capturing them.
Other animals also prey on peppered moths. Bats hunt adult moths at night. Spiders and small mammals can also prey on moths, especially if they encounter resting individuals. Birds remain the most significant predatory force due to their consistent and widespread foraging behavior.
The Role of Camouflage in Predation
Camouflage is a crucial defense mechanism for peppered moths, as predators locate them primarily through sight. The peppered moth exists in two main forms, or morphs, which are visually distinct. The light-colored typica form is white with black speckles, providing effective concealment against lichen-covered tree trunks. The dark-colored carbonaria form is nearly black, offering better camouflage against dark, soot-stained tree bark.
Each morph is adapted to blend into specific backgrounds, making it difficult for visual predators to spot them. When resting on appropriately colored surfaces, the moths effectively blend in, reducing their chances of being detected. This selective advantage of blending in directly influences survival, as better-camouflaged moths are less likely to be eaten by visually hunting predators.
Environmental Change and Predation Dynamics
The relationship between peppered moth camouflage and predation shifted during the Industrial Revolution. Widespread coal burning during the Industrial Revolution, particularly in Great Britain, released soot and pollutants into the atmosphere. This pollution coated tree trunks, turning them dark and killing lichens.
This environmental change impacted the moths’ camouflage effectiveness. The light-colored typica form, previously well-hidden against lichen-covered trees, became highly visible on the darkened, sooty bark. Conversely, the dark carbonaria form, once conspicuous in pristine environments, now blended into the polluted, dark trees. This shift in background color altered the predation pressure on the two morphs.
Birds preyed on the more easily seen moths. The light typica form suffered higher predation in polluted areas, while the dark carbonaria form survived more often. Over time, this differential predation led to a rapid increase in the proportion of dark-colored moths in industrial regions, a phenomenon known as industrial melanism. As environmental regulations improved air quality in later decades, tree trunks began to lighten and lichens returned, causing the predation dynamics to reverse and the lighter morphs to become more common again.