<P> Throughout its history, the status of the principle has oscillated between a priori (' two species coexisting must have different niches') and experimental truth (' we find that species coexisting do have different niches'). </P> <P> Based on field observations, Joseph Grinnell formulated the principle of competitive exclusion in 1904: "Two species of approximately the same food habits are not likely to remain long evenly balanced in numbers in the same region . One will crowd out the other". Russian ecologist Georgy Gause formulated the law of competitive exclusion based on laboratory competition experiments using two species of Paramecium, P. aurelia and P. caudatum . The conditions were to add fresh water every day and input a constant flow of food . Although P. caudatum initially dominated, P. aurelia recovered and subsequently drove P. caudatum extinct via exploitative resource competition . However, Gause was able to let the P. caudatum survive by differing the environmental parameters (food, water). Thus, Gause's law is valid only if the ecological factors are constant . </P> <P> Gause also studied competition between two species of yeast, finding that Saccharomyces cerevisiae consistently outcompeted Schizosaccharomyces kefir by producing a higher concentration of ethyl alcohol . </P> <P> Competitive exclusion is predicted by mathematical and theoretical models such as the Lotka - Volterra models of competition . However, for poorly understood reasons, competitive exclusion is rarely observed in natural ecosystems, and many biological communities appear to violate Gause's law . The best - known example is the so - called "paradox of the plankton". All plankton species live on a very limited number of resources, primarily solar energy and minerals dissolved in the water . According to the competitive exclusion principle, only a small number of plankton species should be able to coexist on these resources . Nevertheless, large numbers of plankton species coexist within small regions of open sea . </P>

When one species cannot live without another we use the term