The Industrial Revolution (18th-19th centuries) marked a profound societal transformation through technological advancements and industrial growth. This era introduced widespread manufacturing, often coal-powered, leading to substantial atmospheric pollution. The release of soot and particulate matter had observable consequences, altering ecosystems and visibly impacting the appearance of trees.
How Industrial Pollution Changed Tree Bark
The burning of vast quantities of coal in factories and homes released immense amounts of black, sooty particulate matter into the atmosphere. This airborne pollution settled on surfaces, including birch tree bark. Before this period, birch tree bark was typically light-colored, often covered by pale lichens, which thrive in clean air environments. However, the sulfur dioxide emissions from burning coal killed these sensitive lichens, leaving the bark exposed and allowing soot to accumulate.
The deposited soot stained the tree bark, transforming light-colored surfaces into a darker, almost black, hue. This drastic change in color meant that the natural camouflage previously offered by the bark for many organisms was severely compromised. This alteration created a new environmental pressure for species relying on tree bark for concealment.
The Peppered Moth’s Adaptation
One of the most widely documented biological consequences of this darkened tree bark was the rapid evolutionary shift observed in the peppered moth, Biston betularia. Before the Industrial Revolution, the majority of peppered moths in Great Britain possessed light-colored wings with dark speckles, which provided effective camouflage against light-colored, lichen-covered trees. A rare, naturally occurring dark-colored (melanic) form also existed, but was uncommon. As industrial pollution darkened the tree trunks, the light-colored moths became highly visible to predatory birds when resting on the soot-covered bark. Conversely, the previously rare dark-colored moths gained a significant survival advantage, as their coloration now blended seamlessly with the blackened trees.
This differential predation pressure led to a dramatic increase in the frequency of the dark form within moth populations in industrial areas. For instance, in Manchester, the percentage of dark moths reportedly rose to 98% by 1895, a stark reversal from their earlier rarity.
The Mechanisms of Evolutionary Change
The rapid change in the peppered moth population exemplifies natural selection, a fundamental process of evolution. Natural selection operates when individuals exhibit trait variations, and certain traits provide a survival or reproductive advantage in a given environment. In the case of the peppered moth, the variation was wing coloration: light versus dark. The darkened tree bark created an environment where dark-colored moths had higher survival rates, being better camouflaged from avian predators. These surviving dark moths were more likely to reproduce, passing their genes for dark coloration to their offspring.
Over successive generations, the frequency of the dark-color gene increased within the moth population, leading to a visible shift in the predominant moth form. This phenomenon, known as industrial melanism, demonstrates how environmental changes can drive rapid genetic shifts and adaptations in species.
Environmental Lessons and Recovery
The story of the birch tree bark and the peppered moth offers profound insights into ecological consequences of human activity and environmental recovery. With the implementation of clean air legislation and pollution controls in the mid-20th century, particularly in the 1950s, air quality in industrial regions gradually improved. As sulfur dioxide levels decreased, lichens began to recolonize tree trunks, and the accumulation of soot diminished, allowing the bark to lighten again. This environmental recovery led to a reversal in the selective pressures on the peppered moth.
As tree bark lightened, dark-colored moths became more conspicuous, while light-colored moths regained a camouflage advantage. Consequently, the proportion of light-colored moths in populations increased, mirroring the pre-Industrial Revolution distribution. This shift underscores how ecosystems respond to pollution changes, showcasing both human impact and the capacity for natural systems to recover.