How population dynamics depend on changes in the environment is a
classic but important question in ecology. We propose a new framework to
understand the context dependence of the mechanism driving two-species
population dynamics, in which we use intrinsic growth rates as a proxy
for environmental suitability, then assess how the strengths of intra-
and interspecific density dependence in a two-species system change
depending on environmental suitability. By using census data for
pairwise sessile species on a rocky intertidal shore, collected over 18
years, we showed that the strength of both intra- and interspecific
density dependence decreased as the environmental suitability of the
focal species increased, but was scarcely affected by the environmental
suitability of the other species. Combining this framework with modern
coexistence theory could provide a deeper understanding of coexistence
mechanisms and context-dependence in two-species systems.