Several years ago, as a measure to reduce the population of gypsy moths, which depend on oak leaves for food, entomologists introduced into many oak forests a species of fungus that is poisonous to gypsy moth caterpillars  Since then, the population of both caterpillars and adult moths has significantly declined in those areas
Entomologists have concluded that the decline is attributable to the presence of the poisonous fungus
In the past several years, air pollution and acid rain have been responsible for a substantial decline in oak tree populations.