Amistad National Recreation Area supports flora and fauna in a combination not found in any other National Park Service site. This biodiversity is a result of the park's location in a transition zone between major life and climate zones: it lies between (1) three major plant communities: Tamaulipan Shrubland, Chihuahuan Desert, and Edwards Plateau; (2) major climate zones: in the transition between temperate and tropics; and (3) in the transition zone from dry arid zones in the western half of the US to the more humid and wet eastern half of the US. This mix of habitats is combined with a huge expanse of the clear, clean waters of Amistad Reservoir.
People use Amistad Reservoir year round. In contrast, some animals utilize the area only during seasonal migrations. In the fall, thousands of Monarch butterflies roost on park lands before continuing their journey south to wintering sites in Mexico. Many waterfowl species spend the winter on the lake before returning north in the spring. Even threatened and endangered species, such as the Interior Least Tern, use the lake to nest and raise young. The river canyons provide oases of warmth and moisture and harbor unique communities found in few other places.
The National Park Service and state agencies have implemented several projects in order to monitor the natural resources of the lake and surrounding areas. Research on threatened and endangered species, cave and karst resources, yearly Monarch butterfly monitoring, a prevention program to avoid the accidental introduction of invasive zebra and quagga mussels, and water quality testing provide invaluable data. By monitoring changes, park staff can quickly take measures to conserve the natural resources and beauty of Amistad National Recreation Area.