Secondary spread of invasive species: historic patterns and underlying mechanisms of the continuing invasion of the European rockweed Fucus serratus in eastern North America

Abstract

Post-establishment spread of invasive species is a major determinant of their impact, but the spatial pattern and temporal rhythm of secondary spread are often poorly known or understood. Here we examine the spread of the European rockweed Fucus serratus over 1,500 km of shoreline after its initial discovery in Pictou, Nova Scotia (Canada) in 1868. Building upon earlier periodic surveys, we document the current distribution of this invader and provide a historic analysis of the invasion, including the integration of the recently-detected multiple introductions from Europe. The initial spread was rapid in the southern Gulf of St. Lawrence (6.0 km year -1 ), likely due to both natural spread in a favorable environment and the extensive regional shipping traffic at the time which linked Pictou to several key ports in eastern Canada and northeast USA. Later spread, especially along the Atlantic coast of Nova Scotia, was slower (recently 0.9 km year -1 ), but included several major jumps (*50-500 km), undoubtedly due to human-mediated transport. Although the spatial extent of the invasion has increased over the past 140 years, it has experienced several remarkable events: apparent local extinctions of northern satellite populations, a major retraction (100-150 km) along the northwestern limits of its continuous range, and stalled invasions at several points and times during its southward progression. Distributional data suggest that this invader is excluding congeners in shallow zones and possibly kelp species in deeper zones. Genetic data from one contact zone showed hybridization with native F. distichus but no evidence for introgression in migrating F. serratus. Hybridization and several other reproductive traits likely contribute to the competitive dominance of the invader in this environment.

Habitat, dispersal and propagule pressure control exotic plant infilling within an invaded range

“…Localized dispersal infills uninvaded patches behind advancing invasion fronts, but it is the least-studied dispersal scale for invasive species (Johnson et al 2012). Infilling likely results in the highest landscape coverage by invasive species, and hence the greatest impact on communities and ecosystems (e.g., Zhu et al 2007).…”

“…Patchiness within invaded ranges can occur when disparate mechanisms control longdistance and local dispersal so that invasion occurs in ''jumps'' (Zhu et al 2007). Local dispersal (infilling) reaches the gaps left by patchy advancing invasion fronts and, notably, is the least-studied dispersal scale for invasive species (Johnson et al 2012). Yet, post-invasion infilling likely results in the highest landscape coverage by invasive species, as well as the greatest impacts on communities and ecosystems (Wangen and Webster 2006, Zhu et al 2007, Miller and Matlack 2010.…”

Climate change impact on seaweed meadow distribution in the North Atlantic rocky intertidal

“…serratus along Northwest‐Atlantic shores (Johnson et al. ). Although modern ships use water instead of rocks as ballast, they still can facilitate dispersal of macroalgae through hull‐fouling, accidental entanglement in anchors or fishing gear, or deliberate use as packing material (Hewitt et al.…”

Phymatolithon (Melobesioideae, Hapalidiales) in the Boreal–Subarctic Transition Zone of the North Atlantic

“…It is highly abundant in the warmer, lower-saline waters of the southern Norwegian coast and in the Skagerrak, and the holotype derives from Helgoland. It would be particularly interesting to examine the genetics of P. laevigatum populations from the eastern and western North Atlantic, to determine when the cross-ocean transit took place, as has been done with the rockweed Fucus serratus (Johnson et al, 2012). Phymatolithon rugulosum occurs in deeper and colder full coastal salinity waters where summer temperatures generally do not exceed 15°C; it is abundant only in southwestern Iceland and the central western North Atlantic.…”