Roles of dispersal mode, recipient environment and disturbance in the secondary spread of the invasive seaweed Codium fragile

Abstract

Secondary spread largely determines the distribution and success of invasive species and depends ultimately on the capacity of the invader to disperse and colonise over multiple scales. Spread of the invasive seaweed Codium fragile ssp. fragile (Codium) can occur through the dispersal of vegetative fragments, which can be buoyant or non-buoyant depending on environmental conditions. This study examined the factors influencing the dispersal, settlement, and establishment of these two types of propagules in eelgrass (Zosteramarina) meadows of iles de la Madeleine, Canada, where Codium lives epiphytically on eelgrass rhizomes. To measure dispersal, ~1,400 Codium fragments were marked, released, and tracked under different hydrodynamic conditions in areas of high and low eelgrass density. Under all conditions, buoyant fragments dispersed one to two orders of magnitude further than non-buoyant fragments. Dispersal distance was positively correlated with wind speed (a proxy for surface currents in this system) for buoyant fragments and with current speed for non-buoyant fragments. For the latter, dispersal distance was also negatively correlated with eelgrass height and density. Natural deposition of drifting fragments in experimentally-manipulated eelgrass meadows was variable in space and time, but was not affected by eelgrass shoot density. Experimental disturbance of eelgrass meadows enhanced the density, biomass, and percent cover of Codium, suggesting that the exposure of eelgrass rhizomes by natural or anthropogenic disturbance promotes invasion by Codium. Our results highlight the importance of small-scale field experiments in determining the local factors affecting the spread of invasive species.