The role of recreational activities in creating fragments of invasive Caulerpa taxifolia
Once non-indigenous species are introduced to a new area, secondary spread is important in determining their ecological and economic impacts. Recreational activities may facilitate the secondary spread of invasive species by creating and transporting viable propagules, however to date there are few manipulative experiments that demonstrate the mechanistic basis of their impact. We examined the effect of two of the main aquatic recreational activities, boating and swimming, on the abundance of propagules (fragments) of the invasive green alga, Caulerpa taxifolia in southeastern Australia. Surveys of two infested estuaries each with locations of different levels of recreational activity demonstrated that locations with high recreational activity had a significantly greater abundance and biomass of fragments than locations with low recreational activity. Manipulative experiments using beyond BACI type designs showed that both boating and swimming created fragments. A single pass of a motor-boat over C taxifolia beds significantly increased the biomass of fragments. but only in shallow water. Similarly, swimming in C. taxifolia beds for 5 min significantly increased the biomass of fragments, but only when background levels of fragments were low. This study has provided the first manipulative experiments confirming that recreational activities such as boating and swimming can create propagules of aquatic invasive species. These findings provide scientific justification for targeting management efforts to minimize secondary spread of aquatic invasive species by restricting recreational activities in invaded areas.