Maintenance of diversity altered by a shift in dominant species: implications for species coexistence

Abstract

Diversity can be maintained by the biological characteristics of species within commu- nities, particularly dominant species. Invasions often result in shifts of dominant species. Despite the number of studies on invasion ecology, few have determined their long-term effect on diversity. In the present study, we coupled short- and long-term studies to examine the relationship between invasive species (colonial ascidians or sea squirts) and diversity and to discuss their implications for species coexistence. Diversity patterns between 2 panel studies (1979 to 1982; 2003 to 2006), one conducted before the establishment of invasive colonial ascidians, revealed an increase in species diversity. Short-term recruitment studies designed to elucidate mechanisms behind the patterns observed in our long-term studies suggest contrasting methods of maintaining diversity. Between 1979 and 1982, Mytilus edulis was a consistent spatial occupant whose hard shell provided secondary space for col- onization by other species. In contrast, the 2003 to 2006 community was dominated by the seasonally abundant invasive colonial ascidians Botrylloides violaceus and to a lesser extent Didemnum vexil- lum, which do not provide secondary substrate. These species either senesce after reproduction or undergo seasonal regression and free space for colonization by other species. These studies suggest that the coexistence of species has shifted from a community in which diversity was maintained by secondary substrates to a community in which diversity is maintained by primary substrates. Addi- tionally, they suggest that the community is undersaturated and could support more species, particu- larly those that seasonally utilize a limited resource (free space).