Individual watersheds are the organizational units for the volumes comprising the Atlas. As the name indicates, a watershed is a catch-basin or drainage basin for rain and condensate funneled into stream beds that either join other stream beds or terminate at the edge of the sea. Only a few years ago, the terms “ecosystem approach” and “watershed approach” were used interchangeably for broad-based studies of the animals living in island streams. More recently, the term “ahupua'a” or “ahupua'a-approach” is employed in the literature and in discussions describing broad-scale stream studies in Hawaii. As used by the ancient Hawaiians, an ahupua'a includes the entire watershed and also tide pools and ponds, near-shore waters along the beach, and the sea out to and including the coral reef. An ahupua'a provided everything needed by way of food, clothing, and shelter for Hawaiians who, with all the other animals and plants in the ahupua'a, conformed to the classic definition of a functioning ecosystem. For stream-dwelling animals in Hawaii, an ahupua'a approach is broader, more encompassing, and certainly more appropriate because all the fishes, mollusks, and crustaceans which make up the freshwater stream fauna have a life cycle that includes a period of time in the ocean.