Habitat structural complexity determines the importance of landscape setting on survivorship of intertidal oysters

Background/Question/Methods:

Predation is a driving mechanism in regulating species distributions. Habitat arrangements within a landscape mosaic influence predator foraging in habitat patches. Predators may avoid isolated habitats due to enhanced risk, but access similar habitats with adjacent corridors, resulting in greater predation than in isolated habitats. Predation pressure is also affected by the predator’s ability to forage within the habitat. Areas with high structural complexity and increased aerial exposure may limit the predators foraging ability. In euhaline regions of estuaries, Eastern oyster reefs develop in a variety of landscape settings and vary greatly in structural complexity, but only persist in the intertidal zone. Our study examined if predation of oyster spat limits oyster reefs from persisting in the subtidal zone and how predation rates may vary across different reef complexities and landscape settings. We quantified oyster predation rates across an aerial exposure gradient (mid-intertidal to subtidal) at two landscape settings (isolated patch reefs, saltmarsh fringing reefs) and examined how structural complexity (low, high) affects oyster survivorship.  Additionally, we determined if predator identity changes across reef complexity, landscape setting or aerial exposure across the intertidal.  

Results/Conclusions

Habitat complexity determined the identity of the primary predator and the corresponding effects of aerial exposure and landscape setting on oyster survivorship. At high complexity, mud crabs were the primary predator that consumed oyster spat and the greatest predation rates occurred at intermediate aerial exposures. Unlike the patterns of mud crab predation, crab densities increased with increasing aerial exposure, indicating that foraging may only occur while mud crabs are inundated, producing the maximum predation at intermediate aerial exposures. At low habitat complexity, nekton (predominately sheepshead) were the dominant consumers of oyster spat, and oyster survivorship decreased with increasing inundation time, indicating that predation rates were directly related to the time nekton can access each aerial exposure. Landscape setting only had an effect on predation rates at low complexity, where oyster survivorship was greater on fringing saltmarsh reefs than on isolated patch reefs. Our results indicate that reduced complexity due to continued habitat degradation may cause a shift in predator identity and increase predation rates in the low intertidal, further limiting the vertical distribution of euhaline oyster reefs.