Manipulating larval supply in the field: a controlled study of marine invasibility

In order to gather experimental data to test theories of invasion biology, ecologists have recently adopted a rigorous experimental approach in which community traits and propagule supply are manipulated independently. However, these studies have been limited to terrestrial systems and their generality is not well understood. We conducted a manipulative field experiment to test the relationship between common human disturbances and invasibility in hard-substrate marine epifaunal communities. A larval dosing technique was developed to allow marine experimental units to be dosed with a controlled supply of propagules under field conditions. Six month old assemblages were exposed to 3 fully crossed factors of (1) physical disturbance, (2) chemical disturbance, and (3) timing of larval dose relative to the disturbances. Physical disturbance was shown to increase the invasion success of the introduced species, Bugula neritina, up to 30 d following the disturbance. This was mainly through an increase of recruitment onto primary space, however physical impacts with an antifoulant surface also led to increased recruitment on secondary space. After 15 d, B. neritina recruits on primary space were found to be larger than recruits on secondary space, suggesting an elevated threat of invasion through primary recruitment. Natural recruitment of barnacles, ascidians, and sedentary polychaetes was also increased by physical disturbance, although these effects were weaker and varied between taxa and sampling times. This study supports the theory that disturbance facilitates invasions in marine epifaunal communities, and contributes a new technique of manipulating propagule supply in the marine environment.