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.