ABSTRACT
Biological units (BU) do not exist in isolation. There are relations with a physical environment, with achemical environment, and with other BU. These other BU may be atthe same hierarchicallevel as the target BU, as between avegetable plant and aweed in agarden, or as between two hepatic cells in the liver. They may also be atdifferent hierarchical levels, as with an ape in aforest ecosystem.
