Non-native seaweeds on urban southern California rocky shores: Effects on native community structure and trophic dynamics

Background/Question/Methods

The introduction of non-indigenous species is among the greatest threats to marine biodiversity and native ecosystem functioning. Non-native seaweeds are perhaps the least documented and studied of marine exotic species, particularly in regards to their ecological impacts. In southern California, several non-native seaweeds have become integrated in the community structure of coastal habitats and are important contributors to community primary productivity. To evaluate the ecological impacts of non-indigenous seaweeds on southern California shores, we examined 1) the effects of a non-native seaweed on high shore community structure; and 2) how native macroalgal consumers are responding to non-native seaweeds. For community structure examination, we compared species composition and diversity of seaweeds and macro- and meiofaunal assemblages in the upper-intertidal zone in patches dominated by the non-native red alga Caulacanthus ustulatus with native barnacle patches. To examine how non-indigenous seaweeds fit into trophic-dynamics of coastal ecosystems, we paired four sets of native and non-native seaweeds that were similar morphologically and/or taxonomically and fed them to four native macroinvertebrate herbivores in laboratory controlled single food consumption rate experiments and two-choice feeding preference experiments.

Results/Conclusions

For community composition comparisons, macro- and meiofaunal assemblages differed between patches of Caulacanthus turf and neighboring areas that lacked the introduced seaweed. Additionally, significantly more seaweed species were found in patches where Caulacanthus was present. This dissimilarity in the upper-intertidal invertebrate and algal communities between native and non-native patches was likely attributed to increased moisture maintenance and sediment accumulation in the Caulacanthus algal turf. For trophic dynamic experiments, seaweed consumption rates and feeding preferences varied greatly among herbivore taxa and among seaweed pairs. Overall, there was no consistent pattern of higher consumption or preferences for native over non-native seaweed foods, or vice versa. Results suggest that consumption rates or preference may not solely be related to location of origin of the seaweed, as theorized by the enemy release hypothesis or the biotic resistance hypothesis, but possibly also attributable to a match between feeding mechanics of the herbivore and the seaweed characteristics. In application to potential future seaweed introductions, feeding patterns suggest that the response by native herbivores to a must be evaluated on a case-by-case basis.