A vivarium is an area, usually enclosed, for keeping and raising animals or plants for observation or research. Often, a portion of the ecosystem for a particular species is simulated on a smaller scale, with controls for environmental conditions. A vivarium may be small enough to sit on a desk or table, such as a terrarium or an aquarium, or may be a very large structure, possibly outdoors. Large vivaria, particularly those holding organisms capable of flight, typically include some sort of a dual-door mechanism such as a sally port for entry and exit, so that the outer door can be closed to prevent escape before the inner door is opened.