The peppered moth (Biston betularia) is a widely recognized insect, notable for its distinct color variations. Found across the Northern Hemisphere, including Europe and North America, this species inhabits diverse environments like forests, parklands, and shrublands. Understanding the animals that prey on peppered moths is important for comprehending their population dynamics and how species adapt to their environment.
Primary Predators
Birds are significant predators of peppered moths. Many common insect-eating birds, such as robins, great tits, flycatchers, and nuthatches, include these moths in their diet. These avian hunters are visual predators, actively searching for moths resting on tree trunks and branches during daylight hours.
Bats also prey on peppered moths, primarily as nocturnal hunters. They employ echolocation to locate moths in the dark, emitting high-frequency sound pulses and interpreting the returning echoes to create a “sound map” of their environment. Moths, in turn, have developed specialized hearing organs to detect these ultrasonic sounds, enabling evasive maneuvers to avoid capture. While birds and bats are the most consistent predators, other opportunistic animals like spiders, predatory insects, and small mammals may also consume peppered moths.
Camouflage and Natural Selection
Camouflage is a primary defense mechanism for peppered moths, allowing them to blend into their surroundings and evade predators. The species has two main forms: the light-colored typica form, which is speckled, and the dark-colored carbonaria form. Their coloration helps them merge with different environmental backgrounds, such as lichen-covered trees or soot-darkened surfaces.
Predators act as a selective force, favoring moths that are better camouflaged in their particular environment. If a moth’s coloration does not match its background, it becomes more visible and is more likely to be eaten. This differential survival illustrates natural selection, where advantageous traits increase an individual’s likelihood of reproduction and passing on those traits. Studies have shown that pale moths have a survival advantage against bird predation when resting on lichen-covered bark, while dark moths are less visible on plain, darker bark.
Predation’s Role in Moth Evolution
The long-term influence of predation on peppered moth populations is famously demonstrated by industrial melanism. Before the Industrial Revolution, the light-colored typica form was common in Britain, blending well with lichen-covered trees. The dark carbonaria form was rare, as it was easily spotted by birds against the light bark.
During the Industrial Revolution, soot from coal-burning factories blackened trees and killed lichens, particularly in urban and industrial areas. This environmental shift meant that light-colored moths were no longer camouflaged and became more visible to predators, leading to increased predation. Conversely, the dark carbonaria moths, once conspicuous, now blended effectively with the soot-darkened trees. As a result, the dark form rapidly increased in frequency, reaching up to 98% in some industrial regions by the late 19th century.
Following clean air legislation in the mid-20th century, pollution levels decreased, and lichens began to regrow, making tree bark lighter again. This reversal in environmental conditions led to a decline in the dark moth population and a resurgence of the light form, as the selective pressure from bird predation shifted. This demonstrates how predator pressure, combined with environmental changes, can drive rapid evolutionary shifts in a species.