Citation: Waser, A. M., W. Splinter, and J. van der Meer. 2015. Indirect effects of invasive species affecting the population structure of an ecosystem engineer. Ecosphere 6(7):109. http://dx.doi.org/10.1890/ES14-00437.1Abstract. Species invasion is of increasing concern as non-native species often have negative impacts on ecosystems that they were introduced to. Invaders negatively affect the abundance of native species due to direct interactions like predation and competition. Additionally, invaders may benefit native biota by imposing indirect effects on resident species interactions. Invaders indirectly affect resident species via both density-mediated indirect interactions (DMIIs) and trait-mediated indirect interactions (TMIIs). Previous studies on these different indirect interactions have largely examined the effects on structuring ecological systems, with paying little attention to the role of body size. Here, we experimentally demonstrate that an invasive habitat modifier of European coastal waters, the Pacific oyster (Crassostrea gigas), alters the population structure of native mussels (Mytilus edulis) by modifying the size specific predator-prey interaction between the mussels and the shore crab (Carcinus maenas). In laboratory split-plot experiments, the presence of Pacific oysters reduced the mortality of unconditioned mussels as well as mussels that were acclimatized in presence of predatory cues, while being exposed to predation by crabs of two different size classes. The reduction in mortality was size-dependent both in terms of the predators and the prey. The presence of oysters notably reduced mussel mortality in presence of small crabs, while the mortality rate in presence of big crabs was less affected. Mussels that benefited the most by the presence of oysters were those of recruitment stages, smaller than 20 mm in shell length. Our results suggest that oysters cause a strong shift in the population structure of M. edulis, reducing particularly the mortality of smaller sized mussels.
26Intertidal mussel beds are important for intertidal ecosystems, because they feature a high 27 taxonomic diversity and abundance of benthic organisms and are important foraging grounds 28 for many avian species. After the introduction of the Pacific oyster (Crassostrea gigas) into 29 the European Wadden Sea, many mussel beds developed into oyster dominated bivalve beds. 30Despite the fact that oysters have been colonizing many European intertidal areas for about 31 two decades, their impact on the ecosystem is still poorly understood. Here, we investigated 32 the impact of oysters on the condition of mussels and on the spatial distribution of birds on 18 33 bivalve beds with different grades of oyster occurrence throughout the Dutch Wadden Sea. 34Moreover, in comparing bird densities on bivalve beds with densities expected on the total 35 intertidal area, we could detect which species exhibit a preference for the structured habitat. 36Overall, 50 different bird species were observed on the beds, of which about half regularly 37 frequent intertidal flats. Most of these species showed a preference for bivalve beds. The 38 condition of mussels decreased with the oyster dominance, whereas the majority of bird 39 species was not affected by the oyster occurrence. However, three of the four species that 40 were negatively affected depend on intertidal mussels as food source. Even though the 41 Pacific oyster is a nonnative species, attempts to fight it may do more harm to avian 42 biodiversity than good.
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