Microtopography promotes coexistence of an invasive seagrass and its native congener

Environmental heterogeneity can promote coexistence, and is therefore predicted to increase invasibility of communities, but decrease invasion impacts. We examined the role of local-scale environmental heterogeneity in promoting the coexistence of the invasive seagrass, Zostera japonica, and its native congener Zostera marina in a patch mosaic. In its introduced range, Z. japonica often co-occurs with the native Z. marina in a patch mosaic associated with intertidal microtopography (centimeter to decimeter relief over meter to decameter distances). Here, Z. marina inhabits depressions that retain water during low tides, and Z. japonica inhabits well-drained mounds. Transplant experiments revealed that Z. marina suppressed Z. japonica shoot densities, more so in pools than on mounds. Z. marina suppressed Z. japonica above-ground and below-ground biomass by 47 and 19 % respectively, on mounds, and by over 60 % in pools. Z. marina shoot densities and biomass were 40 and 95 % lower, respectively, on mounds, regardless of Z. japonica presence. Topographic context remained the most influential predictor of Z. marina responses, even when we transplanted Z. marina into higher densities of Z. japonica. These results indicate that the native Z. marina is physiologically restricted from mounds and competitively excludes the introduced Z. japonica from pools. We provide empirical evidence of local-scale heterogeneity promoting coexistence of an invasive and a native macrophyte, supporting the hypothesis that environmental heterogeneity increases invasibility and decreases invasion impacts. Furthermore, active control of the invader in mixed beds is unlikely to benefit the native.