Invasive ecosystem engineers both positively and negatively affect their recipient ecosystems by generating novel habitats. Many studies have focused on alterations to ecosystem properties and to native species diversity and abundance caused by invasive engineers. However, relatively few studies have documented the extent to which behaviors of native species are affected. The red seaweed Gracilaria vermiculophylla (Rhodophyta) invaded estuaries of the southeastern United States within the last few decades and now provides abundant aboveground vegetative cover on intertidal mudflats that were historically devoid of seaweeds. We hypothesized that G. vermiculophylla would affect the foraging behavior of native shorebirds positively for birds that target seaweed‐associated invertebrates or negatively for birds that target prey on or within the sediment now covered with seaweed. Visual surveys of mudflats >1 ha in size revealed that more shorebirds occurred on mudflats with G. vermiculophylla relative to mudflats without G. vermiculophylla. This increased density was consistent across 7 of 8 species, with the one exception being the semipalmated plover Charadrius semipalmatus. A regression‐based analysis indicated that while algal presence predicted shorebird density, densities of some bird species depended on sediment composition and infaunal invertebrate densities. At smaller spatial scales (200 m2 and <1 m2), experimental removals and additions of G. vermiculophylla and focal observations showed strong variation in behavioral response to G. vermiculophylla among bird species. Birds preferentially foraged in bare mud (e.g., C. semipalmatus), in G. vermiculophylla (e.g., Arenaria interpres), or displayed no preference for either habitat (e.g., Tringa semipalmata). Thus, while the presence of the invasive ecosystem engineer on a mudflat appeared to attract greater numbers of these predators, shorebird species differed in their behavioral responses at the smaller spatial scales that affect their foraging. Our research illuminates the need to account for species identity, individual behavior, and scale when predicting the impacts of invasive species on native communities.

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Discoveries of persistent coastal species in the open ocean shift our understanding of biogeographic barriers. Floating plastic debris from pollution now supports a novel sea surface community composed of coastal and oceanic species at sea that might portend significant ecological shifts in the marine environment.The ubiquitous spread and increase of plastic pollution have galvanized international attention and attracted rapidly growing research, focused on diverse effects. In the coming decades, the effects of plastic pollution in the marine environment are expected to expand, with plastic production and waste predicted to exponentially increase, reaching an estimated total of 25,000 million metric tons of waste generation by 2050 1 . Here, we discuss an additional and overlooked cascading consequence of plastic pollution, triggered by the introduction of an immense floating plastic habitat in the open ocean 2,3 -the unpredicted establishment of coastal species in high seas ocean gyres.We define this emergent novel ecosystem as the neopelagic community (Fig. 1). Although the transport of coastal species across oceans and along coasts on floating debris, also known as ocean rafting, has long been known to occur on natural rafts, including seeds, trees, seaweeds and pumice, past documented occurrences were assumed to be ephemeral 4 . The extent and frequency of coastal species on rafts in the ocean were likely historically constrained, due to the biodegradable nature and, therefore, relatively short longevity of natural materials 5,6 , as well as a likely limited and highly episodic raft supply. Nevertheless, the presumed ability of coastal species to survive ocean transits has been a fundamental tenet of island biogeography and thought to explain the presence of continentally derived species on oceanic islands 7 . Thus, until now, consideration of rafting of coastal species focused primarily on transient passage through

When non-native primary producers become successful, the structure and function of native detrital food webs can be fundamentally altered. Salt marsh estuaries of the southeastern USA are in part detritus-based ecosystems and rely on the annual production of detritus from a single native species, the smooth cordgrass Spartina alterniflora. Over the last several decades, the success of a novel primary producer, the red macroalga Agarophyton vermiculophyllum (formerly Gracilaria vermiculophylla), in a system historically devoid of macroalgae provides the opportunity to measure the effect of non-native basal resources on native detrital pathways. We conducted 2 in situ experiments to compare (1) decomposition rates of A. vermiculophyllum and S. alterniflora and (2) invertebrate colonization rates onto dead A. vermiculophyllum and S. alterniflora. Relative to S. alterniflora, we found that A. vermiculophyllum decomposes more rapidly, losing 80% or more of its biomass within 3 wk, while S. alterniflora lost ~50%. Experimental litterbags with decomposed A. vermiculophyllum and S. alterniflora harbored similar highly abundant invertebrate communities that differed greatly from denuded areas. Our results demonstrate that A. vermiculophyllum provides a complementary source of labile organic matter relative to S. alterniflora, boosting the amount of food and available habitat for small invertebrates of intertidal salt marshes and mudflats. Thus, non-native macrophytes may differentially affect community and ecosystem properties just as much when dead as alive, especially when they are biologically distinct from native species.