Organ systems or organism level. In animals, organs are organized into systems that perform particular body functions, such as the circulatory
system, whose heart and blood vessels transport blood. Body systems help each other and work together. For
example, the respiratory system takes in oxygen and removes carbon dioxide from the body, but it can not
do this without the help of the circulatory, nervous and other body systems.
Ecological level Population is a group of organisms of the same species living in the same geo-graphical area at the same
time, such as all students in the class. The basic unit of ecological organization is population. Several
populations form a community. A community is all the population in a given area. A community and its
physical environment form the ecosystem. All of the ecosystem form the biosphere.
