Abstract
Non-indigenous species can dominate fouling assemblages on artificial structures in marine environments; however, the extent to which infected structures act as reservoirs for subsequent spread to natural habitats is poorly understood. Didemnum vexillum is one of few colonial ascidian species that is widely reported to be highly invasive in natural ecosystems, but which in New Zealand proliferates only on suspended structures. Experimental work revealed that D. vexillum established equally well on suspended artificial and natural substrata, and was able to overgrow suspended settlement plates that were completely covered in other cosmopolitan fouling species. Fragmentation led to a level of D. vexillum cover that was significantly greater than was achieved as a result of ambient larval recruitment. The species failed to establish following fragment transplants onto seabed cobbles and into beds of macroalgae. The establishment success of D. vexillum was greatest in summer compared with autumn, and on the underside of experimental settlement plates that were suspended off the seabed to avoid benthic predators. Where benthic predation pressure was reduced by caging, D. vexillum establishment success was broadly comparable to suspended treatments; by contrast, the species did not establish on the face-up aspect of uncaged plates. This study provides compelling evidence that benthic predation was a key mechanism that prevented D. vexillum’s establishment in the cobble habitats of the study region. The widespread occurrence of D. vexillum on suspended anthropogenic structures is consistent with evidence for other sessile invertebrates that such habitats provide a refuge from benthic predation. For invasive species generally, anthropogenic structures are likely to be most important as propagule reservoirs for spread to natural habitats in situations where predation and other mechanisms do not limit their subsequent proliferation.
Highlights
IntroductionHuman activities in the marine environment have led to the creation of extensive areas of artificial habitat (e.g. floating pontoons, wharf piles, aquaculture structures) along coastal margins [1,2]
Human activities in the marine environment have led to the creation of extensive areas of artificial habitat along coastal margins [1,2]
The community composition and dominant species inhabiting artificial structures often differs greatly to that on the adjacent natural seabed [7,8,9,10,11], there are a number of non-indigenous fouling organisms that are highly invasive in natural systems [3,12,13,14,15,16]
Summary
Human activities in the marine environment have led to the creation of extensive areas of artificial habitat (e.g. floating pontoons, wharf piles, aquaculture structures) along coastal margins [1,2]. The community composition and dominant species inhabiting artificial structures often differs greatly to that on the adjacent natural seabed [7,8,9,10,11], there are a number of non-indigenous fouling organisms that are highly invasive in natural systems [3,12,13,14,15,16]. The ascidians are a group that often dominate the fouling biomass on suspended artificial structures, but whose invasiveness into natural habitats is highly variable among species, and within species among different geographic regions [7,17,18]. Didemnum is most prevalent on artificial structures in these locations, it has been described from natural rocky substrata, macroalgal beds, seagrass habitats, tide pools, estuaries, lagoons, and open coastal areas [27,28,29]
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