Acid rain does not directly burn wildlife, but instead triggers chemical reactions that alter the fundamental composition of ecosystems, particularly water and soil. This degradation starts with the emission of sulfur dioxide (SO2) and nitrogen oxides (NOx) from burning fossil fuels. These compounds form sulfuric and nitric acids, which return to Earth as acid deposition. The resulting changes in habitats make them toxic and nutritionally deficient, causing biological harm to animals.
Altering Ecosystem Chemistry: The Role of pH and Aluminum
Acid deposition changes the chemistry of the environment by introducing excess hydrogen ions (H+) into water and soil. Normal rain has a pH of about 5.6, but acid rain often measures below 4.5. This decrease in pH, known as acidification, is damaging in regions where the soil and bedrock have a limited ability to neutralize the acid, known as “buffering capacity.”
The influx of hydrogen ions displaces essential nutrient cations like calcium (Ca2+) and magnesium (Mg2+) from the soil. Increased acidity mobilizes aluminum (Al) naturally bound within the soil structure. While aluminum is harmless in its bound form, when the pH drops below 6, it transforms into the highly soluble and toxic ionic form (Al3+).
This mobilized aluminum leaches out of the soil and into streams and lakes, where it becomes a potent poison for aquatic life. The more acidic the water becomes, the higher the concentration of toxic aluminum released. This chemical mobilization translates atmospheric pollution into direct biological harm for animals.
Direct Impacts on Aquatic Habitats and Species
The most visible effects of acid rain occur in aquatic ecosystems, where low pH and high aluminum concentrations directly harm fish, amphibians, and invertebrates. Fish populations are sensitive to these changes, with many species unable to survive in water below a pH of 5.
The primary mechanism of toxicity involves mobilized aluminum adhering to delicate gill structures. As water passes over the gills, the aluminum forms a thick mucous layer that clogs the gill filaments. This prevents the fish from absorbing oxygen, causing respiratory distress and leading to suffocation.
Reproductive cycles are also severely disrupted, often before adult fish show distress. At a pH of 5, most fish eggs fail to hatch, and higher acidity can cause skeletal deformities in developing fry and juveniles. Amphibians, such as frogs and salamanders, are also susceptible to reproductive failure because their eggs and larvae are directly exposed to the acidic water.
Acidification also affects the base of the food web by eliminating acid-intolerant invertebrates, such as certain species of mayflies and stoneflies. Even if a fish species can tolerate moderate acidity, the loss of its primary food source can lead to chronic stress, reduced growth rates, and eventual population collapse, simplifying the entire aquatic ecosystem.
Indirect Effects on Terrestrial Wildlife
While aquatic life faces direct chemical poisoning, terrestrial animals are harmed through indirect pathways that degrade their habitat and food supply. Acid rain leaches essential nutrients, particularly calcium, from the soil. This nutrient depletion weakens the forest canopy, making trees susceptible to disease and frost, which reduces the quality of cover and foraging grounds for mammals and birds.
Wildlife depending on calcium-rich prey for reproduction suffers consequences. Songbirds relying on invertebrates like snails for calcium during egg-laying may produce eggs with thin shells if the snails are calcium-deficient due to acidified soil. This reproductive failure is linked to declines in bird populations in acid-stressed forests.
The mobilization of heavy metals, particularly mercury, is a concern for terrestrial food chains. Acidification enhances the conversion of inorganic mercury into methylmercury, a highly toxic form that bioaccumulates up the food chain. Predators like mink, otters, and certain bird species consuming contaminated fish and invertebrates accumulate high levels of methylmercury. This leads to neurological damage and reproductive issues. The overall impact is a slow degradation of the terrestrial environment, combining reduced food quality and increased exposure to toxins.