Microtopography mediates competition between an native and an introduced seagrass

Background/Question/Methods

Highly productive seagrass ecosystems are declining worldwide, and may suffer from species introductions in highly invaded estuaries. The Asian seagrass Zostera japonica was likely introduced to the Pacific Coast of North America near the beginning of the 20th century, and now ranges from British Columbia to Humboldt Bay, California.

In its introduced range, Z. japonica tends to inhabit shallower intertidal habitats than the native eelgrass, Z. marina, reducing the likelihood of direct interactions. Where their vertical distributions overlap, Z. marina and Z. japonica often co-occur in a patch mosaic associated with intertidal microtopography. At such sites, Z. marina inhabits depressions that retain water during low tides, and Z. japonica inhabits mounds that are fully exposed during low tides. Topographic surveys have indicated that an index of topographic position is a significant predictor of species presence at one such site in Padilla Bay, WA.

To elucidate the roles of abiotic limitations and biotic interactions in this pattern, we experimentally transplanted each species, in monospecific and mixed patches, to intertidal mounds and pools. For both species, shoot density, standing crop, and shoot morphological responses were analyzed with generalized linear mixed-effects models.

Results/Conclusions

In the first year of study, Z. japonica shoot densities were 60% lower (P<.001) in the presence of Z. marina, regardless of topographic position and Z. marina shoot densities were 40% lower (P<.001) on mounds regardless of Z. japonica presence. After 13 months, Z. marina transplanted on mounds suffered a 95% reduction in aboveground biomass (P<.004), and Z. japonica transplanted with Z. marina suffered a 56% reduction in aboveground biomass (P<.001). By the end of the second growing season, we observed a marginal effect of Z. japonica on Z. marina shoot densities in pools, and a marginal reduction of Z. marina’s impact on Z. japonica shoot densities on mounds.

These results indicate that the introduced Z. japonica is competitively excluded from pools and the native Z. marina is physiologically restricted from mounds. Further, competitive interactions appear to depend on topographic context. Such a pattern is consistent with physiological limitation of upper extents, and biotic limitation of lower extents commonly found in other intertidal systems.