Consumer- and seaweed-specific impacts of invasion-mediated changes to detrital subsidies on rocky shores

Abstract

Detrital subsidies such as leaf litter, animal carcasses, and marine wrack can profoundly shape recipient habitats by influencing resiliency and productivity. Species introductions and climate-driven range shifts alter the quantity and quality of these subsidies in donor habitats, thereby potentially influencing recipient communities. Such impacts might be particularly important when detrital shifts alter detritivore feeding and performance. Attempts to identify a general theory predicting the consequences of invasive species on detritivores have been challenging, in part because most theories have been based on the study of microbes or consumers of living prey. Further, two recent meta-analyses disagree about the impacts of invasive plants on detritivore populations. Here, we examined the potential impact of a human-mediated shift in macroalgal detrital subsidy from native giant kelp (Macrocystis pyrifera) to invasive devilweed (Sargassum horneri) on recipient rocky shores, with an emphasis on exploring species-specific impacts. We assessed consumer performance on single species diets or on a mixture in no-choice assays, and we assessed feeding preference in choice assays. Additionally, we examined the impacts of this shift on grazing of native benthic seaweeds by an intertidal consumer assemblage. Replacing native Macrocystis with invasive Sargassum had consumer-specific impacts on performance—suppressing growth of red abalone (Haliotis rufescens) but enhancing growth of black turban snails (Tegula funebralis). The effect of mixed diets on consumer growth also displayed consumer specificity. Also, replacing Macrocystis with invasive Sargassum increased grazing of native benthic seaweeds by a realistic detritivore assemblage, but only on the habitat-forming brown seaweed, Silvetia compressa. Thus, invasion-mediated changes in detrital wrack composition had consumer- and seaweed-specific impacts. Such species specificity could underlie disagreements about the impact of species invasions on detritivore populations and could impede our ability to identify a general theory about how species invasions will impact recipient communities via detrital pathways.