Invasive ascidians have significant impacts on marine communities, aquaculture and human economies. Ecologically, they can alter biodiversity and species richness and affect the growth and survivorship of aquacultured organisms. Economic impacts of invasive ascidians can be severe and at times may threaten aquaculture industries. Despite these problems, invasive ascidians occasionally provide ecological or economic benefits.
Invasive ascidians greatly affect community ecology. Due to their strong competitive abilities, invasive ascidians frequently overgrow and outcompete resident species and reduce the space available for settlement of resident larvae (Osman & Whitlatch 1995, Osman & Whitlatch 1999, Osman & Whitlatch 2007, Rajbanshi & Pederson 2007). As a result, invasive ascidians can decrease species richness and change the biodiversity of invaded habitats (but see Whitlatch et al. 1995). For example, in San Francisco Bay, invertebrate species richness was negatively correlated with Ciona intestinalis abundance and many species were missing or rare from C. intestinalis dominated substrates (Blum et al. 2007). In New England, invasive ascidians affected the biodiversity of subtidal systems because fewer species settled on the surface of invasive colonial ascidians compared to the surfaces of other organisms (Dijkstra et al. 2007b, but see Bullard et al. 2004). Invasive ascidians may also affect fundamental ecological processes by reducing the strength of benthic-pelagic linkages. For example, dense mats of Didemnum vexillum in New England prevent pelagic predators from foraging effectively from the benthos (Lengyel et al. 2008, Mercer & Whitlatch 2008). Changing environmental conditions, such as increasing surface sea water temperatures associated with climate change, can facilitate the growth and spread of invasive ascidians and may exacerbate these problems (Stachowicz et al. 2002, Agius 2007, Djikstra et al. 2007a).
Heavy fouling by invasive ascidians can harm aquaculture organisms (mainly bivalves) by competing with them for food or physically attaching to their shells. Both ascidians and bivalves are highly efficient filter-feeders. While ascidians and bivalves share the same food source, efficiency of particle removal differs; bivalves are selective, whereas ascidians are incapable of particle selection (Petersen & Riisgard 1992, Riisgard & Larsen 2000 & 2001, Ward & Shumway 2004). In addition, the retention efficiency of particles < 4 ^m is greatly reduced in bivalves, whereas it is almost 100% in ascidians due to the indiscriminate nature of the mucus net feeding structure (Riisgard & Larsen 2000 & 2001, Ward & Shumway 2004). Thus, the intensity of food competition between ascidians and cultured bivalves can vary depending on conditions (Lesser et al. 1992, LeBlanc et al. 2003, Daigle & Herbinger 2008). The physical presence of dense ascidian aggregations on aquaculture gear may harm cultured bivalves by reducing seawater exchange between cages and the environment and limiting food availability. As a result of several of these processes, cultured mussels in Atlantic Canada grew less and decreased in condition as ascidian density on gear increased (Daigle & Herbinger 2008). Very heavy ascidian fouling on mussel lines (2 kg ascidian m-1) also resulted in ~50% mussel mortality (Daigle & Herbinger 2008, Ramsay et al. 2008).
Invasive ascidians can cause extensive economic damage. Many costs are associated with losses to aquaculture from reduced harvests and increased production expenditures. For example, in Nova Scotia Ciona intestinalis fouling causes a loss of $1.86 ($C) m-1 of mussel line due to decreased mussel growth and increased mortality (Daigle & Herbinger 2008). In an effort to reduce these losses, shellfish farmers typically remove ascidians from their cultures. However, cleaning fouling from aquaculture gear is labor intensive and costly and may contribute 30% (or more) to the operational expenses of shellfish farmers (e.g., Claereboudt et al. 1994). For example, in Saldanha Bay, South Africa, mussel farmers spend the equivalent of ~$13,000 ($US) per year to keep their mussel ropes ascidian-free (Robinson et al. 2005). In 2003 it was estimated that control of ascidians on PEI (at the time predominantly Styela clava) cost ~$0.115 ($C) per pound of shellfish (ACRDP 2003). Given these economic impacts, invasive ascidian infestations can threaten regional aquaculture industries.
In addition to losses to aquaculture, large scale ascidian control efforts can also incur significant costs. When Didemnum vexillum invaded Shakespeare Bay, New Zealand cost-benefit analyses were conducted to assess the costs of different management strategies (Sinner & Coutts 2003, Coutts & Sinner 2004). Strategies ranged from doing nothing to attempting a complete, immediate eradication with costs ranging from ~$175,000-810,000 ($NZD) (Sinner & Coutts 2003). In the end, the actual costs incurred by an attempted eradication campaign were ~$650,000 ($NZD), but the efforts failed to eliminate the species (Coutts & Forrest 2007). Costs of invasive ascidians can also be incurred from monitoring and research efforts. For example, in 2005 the Canadian Government and PEI Aquaculture Industry allocated $1,000,000 ($C) to research and monitoring of invasive ascidians in Atlantic Canada (Department of Fisheries & Oceans Canada 2005).
Despite their potential problems, invasive ascidians can have positive ecological and economic impacts (e.g., Rodriguez 2006). In the intertidal zone in Chile the invasive ascidian Pyura praeputialis acts as an ecosystem engineer and increases species richness by forming dense intertidal aggregations (Castilla et al. 2000, Castilla et al. 2002, Castilla et al. 2004). On Georges Bank, New England two species of polychaetes use the invasive ascidian Didemnum vexillum as a predator refuge and have increased population sizes in sediments under D. vexillum mats (Lengyel et al. 2008). Filtering by large populations of invasive ascidians reduce phytoplankton abundances and improve turbidity levels in shallow bays (Petersen & Riisgard 1992). Invasive ascidians also provide direct benefits to humans. A few species, including Styela clava, are consumed by humans and might be profitably harvested (Bullard et al. 2006, Karney & Rhee 2008), potentially beneficial natural products have been isolated from many species (Haefner 2003, Newman & Cragg 2004, Blunt et al. 2006), and invasive ascidians may serve as bio-indicators of pollution because ascidian diversity appears to correlate with pollution levels (Carman et al. 2007).
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