Structural engineering by an invasive macroalga alters habitat usage, low tide retention, and survival of native mudflat fauna

Background/Question/Methods

Exotic producers can have far-reaching effects on invaded ecosystems, especially where invasion transforms the physical structure of affected environments. One invader that may have such effects is the macroalga Gracilaria vermiculophylla, which has significantly altered the structure and productivity of the estuarine mudflats of Georgia. These intertidal habitats were previously largely devoid of macrophytes, so these changes are likely to affect native fauna living within these systems. However, the nature of these effects will be influenced by whether resident organisms select for or against invaded patches, as well as how the presence of Gracilaria alters their performance. In intertidal habitats, the consequences of these choices are more complex, as both habitat preferences as well as the invader’s effect on species performance may vary depending on whether the mudflat is exposed or inundated. We determined how Gracilaria affects habitat usage of benthic invertebrates during both high and low tide by seasonally manipulating the presence of this invasive macroalga within study plots both above and below mean water level at four sites. Samples were collected the following day through the use of lift baskets (high tide) and by scraping exposed plots (low tide). Field and lab trials then tested the effect of Gracilaria on the survival of resident species during each tidal stage.

Results/Conclusions

The immigration of invertebrates such as amphipods, shrimp, and crabs into Gracilaria plots exceeded immigration into uninvaded plots. The retention of fauna on the exposed mudflat during low tide was also increased by the presence of Gracilaria. Structural mimics increased immigration and retention, as well, emphasizing the importance of engineering effects as a component of this invasion. Subsequent field and lab trials revealed that Gracilaria can influence the survival of these organisms by providing refuge from predation during high tide as well as shelter from desiccation during low tide. Gracilaria therefore influences resident species both through effects arising from small scale habitat selection (i.e. invaded vs. uninvaded patches) as well as more large scale alterations in habitat usage (i.e. increased retention on the mudflat during tidal drop).