Air Quality and Atmospheric Deposition. Effects Deposition of acid anions and heavy metals have most often been considered to be the main air- quality related threats to boreal toads. Although not strictly a problem of deposition, thinning of the atmospheric ozone layer and resulting increased incidence of ultraviolet (UV) radiation has also been investigated as a potential cause of declining populations of boreal toads. It has been proposed that acid and heavy metal deposition and increased UV radiation all may cause developmental abnormalities or increased mortality of boreal toad embryos (Xxxxxx and Xxxxxxxx 1976; Xxxx et al. 1989; Xxxxxxxxx et al. 1994). Deposition or UV radiation sufficient to kill adult toads would have detectable human health effects. Most research on atmospheric deposition and UV radiation has focused on effects on embryos (Corn and Xxxxxxxx 1992; Xxxxxxxxx et al. 1994; Xxxxxxxx and Corn 1996), which are the most vulnerable life stages. Studies to date indicate that UV radiation does not appear to have direct lethal effects on any life stages of the boreal toads in the southern Rocky Mountains. In two studies in Oregon, ambient levels of UV radiation has caused increased mortality of amphibian embryos, including boreal toads (Xxxxxxxxx et al. 1994, 1995). However, replication of this experiment in Colorado failed to obtain the same results (Corn 1998). Increased UV radiation cannot yet be entirely dismissed as a cause of the decline of the boreal toad in the southern Rocky Mountains. There may be sublethal effects on adult toads, and these have not been adequately studied. Heavy metals and UV radiation, possibly acting synergistically with other stressors, may act to depress the immune system, which may allow infection and death from common pathogens (Xxxxx 1993). Also, the possible effects of UV radiation on primary food production for boreal toad larvae warrants further study (X. Xxxxxx in Boreal Toad Recovery Team 1998). Acid deposition is not thought to be a significant problem for boreal toads in the Rocky Mountains. The LC50 pH, the hydrogen ion concentration at which 50% of boreal toad embryos die, was 4.4–4.5 in one laboratory study (Xxxx and Xxxxxxxx 1992). Breeding habitats of boreal toads rarely have pH less than 6.0. However, about one-half of known boreal toad breeding sites are sensitive to damage from acid deposition (acid neutralizing capacity [ANC] < 200 µequivalents per liter). Sulfate deposition > 10 kg per hectare per year has the potential to reduce ...
