Spatial variability in habitat temperature may drive patterns of selection between an invasive and native mussel species

To understand the mechanisms of invasions it is necessary to explore how thermal environments affect the distribution of both native and invasive species. We examined patterns of species distribution at several scales to determine how thermal selection affects the distribution of native (Mytilus trossulus) and invading (M. galloprovincialis) mussels inhabiting marine rocky intertidal and subtidal habitats. Previous work on these species has focused on the role of water temperature in setting distribution patterns, neglecting the role of aerial exposure at low tide. We therefore examined patterns of abundance in shaded and sun-exposed intertidal habitats and in intertidal versus subtidal habitats at sites within the San Francisco Bay over 3 yr. At half (2/4) of the sites the abundance of the native mussel was significantly higher in shaded intertidal habitats compared to warmer, sun-exposed intertidal habitats, where the invasive mussels were more abundant. Additionally, when comparisons were made between paired subtidal and intertidal sites, native mussel abundance was higher in subtidal habitats. In general, however, there was an unexplained and steady decrease in M trossulus abundance at all sites. Field transplant experiments showed that the native mussel had lower survivorship than the invading species in all habitats examined (subtidal, shaded, and sun-exposed intertidal). Overall survivorship was lowest for both species in the sun-exposed intertidal treatments. This study shows that physiological stress associated with aerial exposure is likely to contribute to the local and geographic distribution of these mussel species. Moreover, our results suggest that processes that operate over larger geographic scales may be experimentally detectable over much smaller scales.