Although context dependence of species interaction has been unveiled by assessing how intrinsic growth rate and strengths of intra- and interspecific density dependences vary along a single specific environmental gradient in two-species systems, this approach ignores environmental suitability that is a function of various environmental factors. We offered a new approach incorporating influence of environmental suitability with environmental gradient, in which intrinsic growth rates are used as the proxy for environmental suitability, then assess how strengths of intra- and interspecific density dependence change depending on environmental suitability along a single specific environmental gradient. We use this approach in rocky intertidal two-species system to explore relative importance among environmental gradient and environmental suitability for species interaction.

By using 18-years census data, we showed that strengths of both intra- and interspecific density dependence decreased as environmental suitability of focal species increased. Meanwhile, the shore level has no significant effect on environmental suitability or strengths of density dependence. Our results suggest that the effect of environmental suitability of focal species is more important than system specific environmental gradient for strengths of density dependence. Combining this framework with modern coexistence theory could provide a deeper understanding of context-dependence and coexistence mechanisms in two-species systems.