Effects of Non-Native Species on Two Life Stages of the Olympia Oyster, Ostrea Lurida, in the Elkhorn Slough Estuary
Marine and estuarine systems worldwide have been altered due to introductions of nonnative species. A common mode of non-native species transport is in ship ballast water that, when released, deposits larvae in new locations. Once established, non-native species compete for resources, reduce native species populations and alter ecosystem services. Some non-native species have been found to be ecosystem engineers, encouraging additional non-native species settlement. Many studies have looked at the competitive interaction between non-native and native species, but few have examined the possibility of a facilitative effect or how additional environmental stressors, such as desiccation, may modify species interactions. This study examined the effect of non-native species on two life stages (adult and juvenile) of the Olympia oyster, Ostrea lurida, a species in decline at two different tidal heights in a central California estuary. In separate experiments, adult and juvenile O. lurida were outplanted on settlement plates in Elkhorn Slough, and two treatments were maintained: (1) manual removal of non-natives every two weeks from half the plates or (2) control treatments that allowed non-native species to colonize and persist. Every two weeks, photographs were taken of each plate in order to calculate changes in percent cover of non-native species and to measure growth of O. lurida in response to the experimental treatment conditions. Results indicated that in the presence of the Australian tubeworm, Ficopomatus enigmaticus, and other non-native species, adult O. lurida exhibited an ~50% increase in area (cm2) on average across the plates. The presence of non-natives consistently resulted in a neutral or positive facilitative effect on oyster growth at two different life stages. The results of this study show that adult O. lurida are able to survive and even benefit from the presence of other species currently found in Elkhorn Slough both above and below MLLW. Juvenile O. lurida demonstrated high initial growth regardless of treatment, with increased growth below MLLW. With this increased understanding, there is much that is unknown about the long-term affect of introduced species on ecosystems and it is important to continue to study their impacts. In particular, the effect of nonnatives on oyster recruitment requires further investigation because the earliest life stages are most likely to be adversely affects by a rapid growing non-native that can monopolize available bare space for settlement. If O. lurida recruitment and juvenile survivorship is further reduced, the local Elkhorn Slough population may be lost.