Gas exchange is the process by which oxygen and carbon dioxide (the respiratory gases) move in opposite directions across an organism's respiratory membranes, between the air or water of the external environment and the body fluids of the internal environment.
niche. The function or position of a species within an ecological community. A species's niche includes the physical environment to which it has become adapted as well as its role as producer and consumer of food resources.
A fundamental niche of a species is the habitat that is natural for it while a realized niche is the result of many factors, such as human activities, presence of predators, and availability of its natural food resources. There are many differences between a fundamental niche and a realized niche that will be talked about in this article.