Table S1: Search parameters used in ISI Web of Science. General search terms used for all mechanisms were (non-native* OR nonnative* OR invasi* OR introduc* OR non-indigenous OR nonindigenous OR alien OR exotic OR invade*) AND (estuar* OR marine OR coastal OR ocean* OR sea OR *tidal), followed by mechanism-specific terms outlined below. Search results were then refined using Web of Science tools to those pertaining only to Marine and Freshwater Biology, and all research areas that were clearly not relevant (e.g., not biological) were excluded. All remaining papers were then individually evaluated for relevance. Over 2500 papers were evaluated for negative interactions alone. Mechanism Search terms Negative Interactions AND ("biotic resistance" OR "invasion resistance" OR diversity OR "diversity invasibility" OR "empty niche*" OR "limiting similarity" OR "Darwin's naturalization" OR pre-adaptation OR "enemy release" OR "enemy escape" OR "natural enem*" OR "native enem*" OR allelopath* OR "chemical defense*" OR "novel weapon*" OR "novel chemical*" OR predat* OR herbivor* OR parasit* OR compet* OR consum*) Positive Interactions AND (meltdown OR facilitat* OR mutual* OR "positive interact*" OR commensal* OR "positive species interact*") Invader Traits AND (tolerance* OR tolerate* OR constrain* OR thermal OR heat OR cold OR freez* OR hypoxia OR copper OR "heavy metal" OR cadmium OR salinity OR desiccation OR nutrient* OR sedimentation OR antifouling OR UV OR ultraviolet OR deoxygenation OR dark* OR turbidity OR wave OR "growth rate" OR "rapid growth" OR maturation OR fecundity OR plasticity OR larval OR "body size" OR morphology OR reproduction OR "life history" OR metabolism OR behavior) AND (trait*) Post Introduction Evolution AND (EICA OR evolut* OR "rapid evolut*" OR hybrid* OR bottleneck*) Invasive Taxon Invasive Phylum Type of Study Consumption or Competition Focal Interaction Invasion Enhanced or Limited Citation Marenzelleria arctia Annelida E Competition Single-Species Competition Enhanced (Karlson et al. 2011) Marenzelleria cf. wireni Annelida O Competition Single-Species Competition Enhanced (Essink 1999) Marenzelleria cf. wireni Annelida O Competition Single-Species Competition Enhanced (Essink et al. 1998) Marenzelleria viridis Annelida E Competition Single-Species Competition Enhanced (Kotta & Ólafsson 2003) Artemia franciscana Arthropoda O Consumption Parasitism Enhanced (Georgiev et al. 2007) Balanus glandula Arthropoda O Competition Single-Species Competition Enhanced (Vallarino & Elias 1997) Caprella mutica Arthropoda E Competition Diversity (Multi-species Competition) Enhanced (Shucksmith et al. 2009 Carcinus maenas Arthropoda O Consumption Parasitism Enhanced (Blakeslee et al. 2009) Carcinus maenas Arthropoda E Consumption Predation Limited (deRivera et al. 2005) Carcinus maenas Arthropoda E Consumption Predation Limited (Hunt & Yamada 2003) Carcinus maenas Arthropoda O Consumption & Competition Predation & Single-Species Competition Limited (Jensen et al. 2002) Carcinus maenas Arthropoda E Consump...

Seagrass, an important subtidal marine ecosystem, is being lost at a rate of 110 km year, leading to fragmented seagrass seascapes. Habitat fragmentation is predicted to affect trophic levels differently, with higher trophic levels being more sensitive, stressing the importance of a multi-trophic perspective. Utilizing the trophic relationship between the blue crab (Callinectes sapidus) and hard clam (Mercenaria mercenaria), where adult blue crabs prey on juvenile blue crabs, and juvenile blue crabs prey on small hard clams, we examined whether predation rates, abundance, and behavior of predators and prey differed between continuous and fragmented seagrass in a multi-trophic context at two sites in Barnegat Bay, NJ. We tested the hypothesis that fragmented habitats would differentially affect trophic levels within a tri-trophic system, and our results supported this hypothesis. Densities of adult blue crabs were higher in fragmented than continuous habitats. Densities of juvenile blue crabs, the primary predator of hard clams, were lower in fragmented habitats than continuous, potentially due to increased predation by adult blue crabs. Clams experienced lower predation and burrowed to a shallower depth in fragmented habitats than in continuous habitat, likely due in part to the low densities of juvenile blue crabs, their primary predator. Our results suggest that while trophic levels are differentially affected, the impact of habitat fragmentation may be stronger on intermediate rather than top trophic levels in some marine systems.