Dynamics of Ascidian-Invaded Communities over Time

Human-mediated transport of non-native species can negatively impact ecosystem services, and the scale of these impacts is predicted to increase over time. However, the manner in which invaded marine communities on artificial substrates change over time has rarely been investigated. Several marinas in Tampa Bay and Miami (Florida, USA) were surveyed to document changes in ascidian community composition of native and introduced species between 2003–2004 and 2017. Seventeen species found in 2017 were not recorded in 2003–2004; all of these 17 were rare in both Tampa Bay and Miami. Five species were found in 2003–2004 but not in 2017, with four of these five being rare. In Tampa Bay, the abundances of both native and introduced species did not change over time, though the number of native species increased while the number of introduced species was unchanged. In Miami, the number of native species increased but their abundances declined; however, introduced species number and abundances did not change. Our study showed that although populations of introduced species were dynamic, they appeared to reach carrying capacity, where overall species number and abundance remained mostly constant over time independently of recurrent introductions and the presence of native species. However, since invasion meltdown and invasion debt can happen at any time, periodic surveys and knowledge of species’ biological cycles and environmental tolerance thresholds remain critical.