Sensitive Species of the Big Thicket - Big Thicket National Preserve (U.S. National Park Service) Sensitive species are plant and animal species that are given special management considerations or legal protections due to their vulnerability to population declines or extinction. The most vulnerable species have been listed under the Endangered Species Act or by the State of Texas as endangered or threatened, while other species of concern may receive special monitoring. As a national preserve designated by the U.S. Congress, Big Thicket takes an active role in re-establishing and protecting any sensitive species that may occur within its boundaries. The following are just a few examples of sensitive species that currently or historically occurred in the Big Thicket.