Zoology - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemEcological effects of the introduced fanworm, Sabella spallanzanii, on hard substratum epifaunal assemblages( 1999)Observations on the impacts of introduced species abound, but little experimental work bears on the nature and mechanisms of these impacts. The recent invasion of the sabellid polychaete Sabella spallanzanii into several embayments on the south coast of Australia provides an opportunity to test hypotheses about the effects of this exotic species on the invaded assemblage. The large size (30 cm long) and high density S. spallanzanii results in a structurally complex canopy of feeding fans above the substratum. I investigated the effect of the canopy on various life history stages of co-occurring marine invertebrates in epifaunal assemblages on hard substrata. The effects of Sabella on recruitment of sessile taxa were complex, but a number of generalisations could be made. Recruitment of some barnacle, bryozoan, spirorbid and ascidian taxa to large experimentally cleared areas (pilings) was higher than on uncleared areas. On the scale of individual 20 by 20 cm settlement plates, where I controlled for the effect of physical structure using fanworm mimics, most of the effects were caused by the presence of physical structure on the plate. Barnacles responded negatively, and sponges and one bryozoan responded positively to structure in one experiment, while a spirorbid, a bryozoan and three ascidians all responded negatively in another. Recruitment on to tubes increased the abundance of several taxa on plates with fanworms, but not all taxa recruited to tubes. Effects were not consistent within taxonomic groups or between sites. Longer term experiments revealed fewer changes to the structure of the understorey assemblage. A two month experiment showed fewer effects of the fanworms on the smaller plate scale, and a six month experiment found few effects on either plate or piling scales. The apparent disappearance of canopy effects with time could be caused by processes associated with assemblage development such as overgrowth and senescence of early recruits. Alternatively it could simply be caused by differences in the composition of assemblages between experiments. Multivariate analyses showed that spatial variation in recruitment on the scale of 10’s of metres was important in determining the composition of the assemblage in both early and later stages. The planktonic abundances of larval solitary ascidians, spionid polychaetes and pooled larval taxa adjacent to the substratum were significantly reduced by the presence of a fanworm canopy, suggesting that larval flux to these areas may be lower. Little difference in overall recruitment between cleared and uncleared areas suggests that settlement rates may be enhanced or post-settlement mortality rates might be lower beneath canopies. Growth of the arborescent bryozoan Bugula stolonifera and an encrusting bryozoan, Watersipora subtorquata, were affected by the presence of a canopy, but these effects were not consistent between experiments. B. stolonifera growth increased beneath the canopy in one experiment and decreased in another, while W. subtorquata grew less in response to physical structure on the plates. Survivorship of B. stolonifera and a serpulid polychaete were lower beneath the canopy in one experiment each, but not in others, and survivorship of W. subtorquata was not affected. The variable results suggested that canopies may act in different ways at different times. The overall impact of Sabella spallanzanii on other species at the scales I looked at can be explained by its modification of the habitat. The use of tubes as settlement sites, and the predominance of small scale responses to physical structure support this idea. Traditional models of the ecological impacts of exotic species, which focus on competitive species replacement, may not be relevant in this system. Any impacts of Sabella spallanzanii must also be evaluated in the light of observations that the canopy was not constant through time, clue to population crashes that occurred a number of times and places.