Understanding species coexistence under environmental variation is crucial for biodiversity conservation, especially amid climate change. Modern Coexistence Theory (MCT) integrates environmental variation and stabilization mechanisms, defining coexistence through niche and fitness differences. Stable coexistence occurs when niche differences enhance intraspecific density dependence enough to counterbalance fitness differences, enabling mutual invasion and persistence.

Using 18 years of long-term census data on two sessile intertidal species, Chthamalus dalli and Gloiopeltis furcata, from stable (both species persist) and transient (focal species has no continuous existence) habitats, We quantified niche differences, fitness differences, and invasion growth rates (IGR) and assessed their variation across habitat types. Additionally, we evaluated the relative importance of IGR components and applied a hierarchical causal structure to analyze environmental effects.

Results showed no significant differences in IGR or the combined effects of fitness and niche differences across habitat types. Environmental suitability and fitness differences played key roles in spatial variation of IGR. Bare space availability directly influenced IGR, while effective shore level had an indirect effect mediated by bare space availability.

These findings suggest that spatial variation of coexistence between C. dalli and G. furcata mainly caused by equalizing mechanism rather than niche differentiation maintaining. Resource availability and environmental stress were the main drivers for spatial variation of IGR in  C. dalli -G. furcata system. Moreover, the difficulty in detecting significant difference of coexistence outcome across habitat types highlights the limitations of two-species systems in assessing coexistence within more complex natural communities.