Invader traits and community context contribute to the recent invasion success of the macroalga Heterosiphonia japonica on New England rocky reefs

Abstract

The successful establishment and spread of invasive species may be determined by intrinsic traits of the invader, characteristics of the invaded community, and interactions between the two. The red filamentous macroalga Heterosiphonia japonica has recently invaded and expanded its range in the northwestern Atlantic. We surveyed and compared macroalgal communities at sites with different levels of invasion in Nahant, Massachusetts, and examined potential factors contributing to Heterosiphonia’s successful invasion by assessing its growth rates and rates of herbivory by native grazers in the laboratory. We evaluated growth and herbivory for isolated individuals and for individuals within a macroalgal assemblage comprised of the most abundant species in the native community. We also measured macroalgal nitrate uptake rates to examine a potential mechanism underlying observed differences in growth rate. High abundances of Heterosiphonia were associated with lower native macroalgal species richness, evenness, and diversity and with differences in species composition in the field. Experiments showed that rates of growth and grazing were context dependent. In isolation, Heterosiphonia grew comparably and was grazed similarly to the two native filamentous macroalgae. Within the context of the native macroalgal assemblage, it grew much faster than all common native species, experienced reduced herbivory from one native grazer, and increased herbivory from another. Consistent with its faster growth, Heterosiphonia was the most efficient at nitrate uptake. Our results suggest that multiple interactions between intrinsic traits (e.g., competitiveness for nutrients) and extrinsic factors (e.g., presence of native macroalgal species, identity and relative abundances of native grazers) contribute to Heterosiphonia’s invasion success.