  Explaining biodiversity is a central focus in theoretical ecology. A
significant obstacle arises from the Competitive Exclusion Principle (CEP),
which states that two species competing for the same type of resources cannot
coexist at constant population densities, or more generally, the number of
consumer species cannot exceed that of resource species at steady states. The
conflict between CEP and biodiversity is exemplified by the paradox of the
plankton, where a few types of limiting resources support a plethora of
plankton species. In this review, we introduce mechanisms proposed over the
years for promoting biodiversity in ecosystems, with a special focus on those
that alleviate the constraints imposed by the CEP, including mechanisms that
challenge the CEP in well-mixed systems at a steady state or those that
circumvent its limitations through contextual differences.
