Antagonistic ecosystem engineering effects differ by seagrass life stage and density of bioturbating shrimp

Abstract

Because ecosystem engineers shape environmental conditions, interactions between ecosystem engineers can depend not only on the external environment but on “which species arrives when” within habitats. Yet, while endpoint outcomes for adults at high density have often been investigated, few studies have examined how these interactions change across density and life history stages. We tested for antagonistic engineering effects of the burrowing shrimp Neotrypaea californiensis (Dana, 1852) at a range of densities on eelgrass Zostera marina L., 1753, including seedlings as well as vegetative shoots. In an observational study, abrupt borders of eelgrass beds were not mirrored by shrimp, and shrimp were never excluded across the full range of observed eelgrass densities, patterns that are inconsistent with alternative stable states. However, eelgrass density declined with increasing shrimp density, and no eelgrass occurred at >336 shrimp m⁻². Survival of eelgrass transplants also declined with increasing shrimp density, and in a manipulative experiment, seedlings declined more rapidly than vegetative shoots within a shrimp bed. Thus, shrimp have strong antagonistic engineering effects on eelgrass that increase with shrimp density and can preclude successful seedling establishment and persistence of vegetative shoots.