  Dispersal of species is a fundamental ecological process in the evolution and
maintenance of biodiversity. Limited control over ecological parameters has
hindered progress in understanding of what enables species to colonise new
area, as well as the importance of inter-species interactions. Such control is
necessary to construct reliable mathematical models of ecosystems. In our work,
we studied dispersal in the context of bacterial range expansions and
identified the major determinants of species coexistence for a bacterial model
system of three Escherichia coli strains (toxin producing, sensitive, and
resistant). Genetic engineering allowed us to tune strain growth rates and to
design different ecological scenarios (cyclic and hierarchical). We found that
coexistence of all strains depended on three strongly interdependent factors:
composition of inoculum, relative strain growth rates, and effective toxin
range. Robust agreement between our experiments and a thoroughly calibrated
computational model enabled us to extrapolate these intricate interdependencies
in terms of phenomenological biodiversity laws. Our mathematical analysis also
suggested that cyclic dominance between strains is not a prerequisite for
coexistence in competitive range expansions. Instead, robust three-strain
coexistence required a balance between growth rates and either a reduced
initial ratio of the toxin-producing strain, or a sufficiently short toxin
range.
