Geographic limits of distribution and species interdependencies.
Correlations are often through uncontrolled variable effects.
Ecosystems are open, within the biosphere. Ecologists and biogeographers are both concerned to understand the distribution and abundance of plants and animals. They often see different species as interchangeable units in trophic systems. Biogeographers study the geographic distributions and evolutionary changes of organisms in space and time. Different species are likely to be regarded as unique.