Range expansion by invasive marine algae: rates and patterns of spread at a regional scale

Aim Introduced macroalgae are widespread in the world's oceans and, despite increasing awareness and attempts to limit the phenomenon, the number of species introductions in coastal waters has increased exponentially over time. Little is known about the rates and mechanisms of spread, even among species that have received the most attention. We compare patterns of range expansion for nine species of invasive algae across eight geographic regions. Location World-wide. Methods We compiled records of introduced algae from the scientific literature, herbaria, and by contacting experts to reconstruct chronologies for 22 algal invasions. These were used to map patterns of spread at a regional scale (thousands of km). Results Range size tended to increase linearly with time, often after an initial lag. Range expansion occurred at rates of tens to hundreds of kilometres per year, often with large infrequent increases. Rates of range expansion differed significantly between species within the same region, and between regions for the same species. Main conclusions Our results suggest that anthropogenic vectors likely play a key role in the spread of introduced macroalgae at a regional scale, although natural long-distance dispersal also may be important for some species. The lack of consistency in the rates within individual species and regions suggests that multiple interacting factors (e.g. algal traits, characteristics of invaded communities, environmental conditions and anthropogenic activities) determine where propagules of introduced algae are delivered and whether they become established.