Biotic resistance to the infiltration of natural benthic habitats: Examining the role of predation in the distribution of the invasive ascidian Botrylloides violaceus

Anthropogenic marine habitats, such as marinas and breakwaters, are frequently colonized by nonindigenous species (NIS). Comparative studies show that few sessile NIS are found in nearby natural habitats, but little is known about the processes affecting the spread of NIS into these habitats. In southern British Columbia, the invasive colonial ascidian Botrylloides violaceus is widespread, but is far more common on man-made structures at marinas and aquaculture facilities than adjacent natural rocky reefs. To determine whether predation by native species, one element of biotic resistance, influences the successful infiltration of B. violaceus into natural benthic habitats, we conducted a series of predator exclusion experiments. Established adult colonies and newly settled juvenile B. violaceus were transplanted onto replicate PVC panels and deployed at marina pilings and adjacent natural rocky reefs at two locations. Panels were assigned to caged, uncaged, and procedural control treatments to test whether predator exclusion changed survivorship. Both juvenile and adult ascidian survival was significantly lower in uncaged treatments, suggesting that predation can limit the abundance and distribution of this species. Results were similar between piling and rocky reef habitats regardless of life stage and our results suggest that adult and juvenile B. violaceus are vulnerable to predation in both habitats studied. If B. violaceus propagules can disperse from dock floats and infiltrate adjacent natural rock, biotic resistance, through predation by native species, may diminish the likelihood of successful colonization in these habitats. Ensuring that natural habitats and native benthic communities remain healthy and intact may reduce the likelihood that invasive ascidians will spread beyond anthropogenic introduction sites where they can dominate the fouling community. (C) 2012 Elsevier B.V. All rights reserved.