  In apparent contradiction to competition theory, the number of known,
co-existing plankton species far exceeds their explicable biodiversity - a
discrepancy termed the Paradox of the Plankton. We introduce a new
game-theoretic model for competing micro-organisms in which one player consists
of all organisms of one species. The stable points for the population dynamics
in our model, known as strategic behavior distributions (SBDs), are probability
distributions of behaviors across all organisms which imply a stable population
of the species as a whole. We find that intra-specific variability is the key
characteristic that ultimately allows co-existence because the outcomes of
competitions between individuals with variable competitive abilities is
unpredictable. Our simulations based on the theoretical model show that up to
100 species can coexist for at least 10000 generations, and that even small
population sizes or species with inferior competitive ability can survive when
there is intra-specific variability. In nature, this variability can be
observed as niche differentiation, variability in environmental and ecological
factors, and variability of individual behaviors or physiology. Therefore
previous specific explanations of the paradox are consistent with and provide
specific examples of our suggestion that individual variability is the
mechanism which solves the paradox.
