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Abstract Ocean warming caused by global climate change is driving range expansions and shifts in marine species. The lady crab Ovalipes ocellatus (Herbst, 1799) is generally found south of Cape Cod, Massachusetts, USA with a disjunct population in the southern Gulf of St. Lawrence, Canada, but absent in the Gulf of Maine and Bay of Fundy. Here we present trawl survey data, recent crowd-sourced observations, and temperature data that suggest a range expansion of O. ocellatus north of Cape Cod into the Gulf of Maine and Bay of Fundy after a marine heat wave in 2012. Crowd-sourced observations of lady crabs increased in the Gulf of Maine at the same time that abundances surged after 2000. In the Gulf of Maine, O. ocellatus was found as far north as Freeport, Maine (43°48′17.136″N, 70°6′30.9594″W) and in the Bay of Fundy as far north as Alma, New Brunswick, Canada (45°36′ 13.6794″N, 64°56′29.184″W). We also extend the southern limit of O. ocellatus to St. Augustine, Florida, USA (29°42′9.432″N, 81°13′56.028″ W). The recent observations of O. ocellatus in the northwestern Atlantic and higher abundances combined with continued warming in this area may signal a permanent expansion of this species. If so, a key goal for ecologists and managers will be to understand the effects of O. ocellatus on food webs and fisheries in the Gulf of Maine and Bay of Fundy. 

ABSTRACT Herbivore fronts can alter plant traits (chemical and/or morphological features) and performance via grazing. Yet, herbivore‐driven trait alterations are rarely considered when assessing how these fronts shape ecosystems, despite the critical role that plant performance plays in ecosystem functioning. We evaluated herbivore fronts created by the purple marsh crab,Sesarma reticulatum, as it consumes the cordgrass,Spartina alterniflora, in Virginian salt marshes.Sesarmafronts form at the head of tidal creeks and move inland, creating a denuded mudflat between the tall‐formSpartinalow marsh (trailing edge) and the short‐formSpartinahigh marsh (leading edge). We quantifiedSesarmafront migration rate, tested ifSesarmaherbivory altered geomorphic processes andSpartinatraits at the trailing and leading edges, and examined how these trait changes persisted through the final 8 weeks of the growing season.Sesarmafront migration in our region is two times slower than fronts in the Southeast United States, andSpartinaretreat rate at the leading edge is greater than the revegetation rate at the trailing edge.Sesarmafronts lowered elevation and decreased sediment shear strength at the trailing edge while having no impact on soil organic matter and bulk density at either edge. At the leading edge,Sesarmagrazing reducedSpartinagrowth traits and defensive ability, and trait changes persisted through the remaining growing season. At the trailing edge, however,Sesarmagrazing promoted belowground biomass production and had limited to no effect on growth or defensive traits. We show that herbivore fronts negatively impact saltmarsh plant traits at their leading edge, potentially contributing to front propagation. In contrast, plants at the trailing edge were more resistant to herbivore grazing and may enhance resilience through elevated belowground biomass production. Future work should consider herbivore‐driven plant trait alterations in the context of herbivore fronts to better predict ecosystem response and recovery. 

Abstract Global change is impacting plant community composition, but the mechanisms underlying these changes are unclear. Using a dataset of 58 global change experiments, we tested the five fundamental mechanisms of community change: changes in evenness and richness, reordering, species gains and losses. We found 71% of communities were impacted by global change treatments, and 88% of communities that were exposed to two or more global change drivers were impacted. Further, all mechanisms of change were equally likely to be affected by global change treatments—species losses and changes in richness were just as common as species gains and reordering. We also found no evidence of a progression of community changes, for example, reordering and changes in evenness did not precede species gains and losses. We demonstrate that all processes underlying plant community composition changes are equally affected by treatments and often occur simultaneously, necessitating a wholistic approach to quantifying community changes.