The red cockaded woodpecker is an endangered species that occupies longleaf pine forests of the southeastern United States. Heavy demand for the enormous pines in the 19th and early 20th century has left only 2% of the longleaf's historic range. The vast forests and the understory ecosystem, once defined by natural fires, were soon replaced by faster growing pines like loblolly and virginia species. This had tremendous effects on red cockaded woodpecker populations. Their biology and behavior limits them to longleaf pine groves because they nest in live tree cavities in order to use the sap which pours from their nest openings as deterrents from potential predators. RCWs are also keystone species as over 27 species of animals use their cavities for refuge and nesting, including flying squirrels. Massive efforts to restore longleaf pine forests are underway through controlled burns and replanting. The US Fish and Wildlife biologists and their partners are in charge of monitoring nesting populations.