Introduction
What are alien invasives?
Zebra mussels
In its most basic definition, invasive species are those which do not occur naturally in the geographic regions. In other words, they are non-indigenous. These can belong to any group of organisms, including plants, birds, reptiles, invertebrates and microorganisms. Varying definitions exist that allow some leeway in identifying a species as invasive, but for simplicity's sake species that we are concerned of have characteristics which allow them to heavily colonize, outcompete and alter the ecological attributes of the geographic area with which they are present.
Aquatic invasives are such invasive species that dominate in marine regions. They can be found in freshwater lakes as well as saltwater coastlines and estuaries. Some of the most famous examples of this group include zebra mussels, the european green crab and more recently the tunicates.
Club tunicate - Styela clava
What causes their propagation?
Essentially, alien invasives are usually brought to an area that is "unnatural" for their ordinary habitat through tranportation methods that move them faster than they can move themselves. Sometimes in the case of certain plants, animals can act as a vector that transports a plant from one area to another through eating and expelling its seeds. In the case of aquatic invasives however, transportation usually occurs through human-based ship vessels and marine traffic.
It is difficult to trace the origins of where many of these species may have originated from, but general interpretation is that species that were transported across the Atlantic to the East Coast of the North America likely came from European sources, and species that have travelled to the western coast across the Pacific likely were sourced from Asia (Hinterland: Invasive Alien Species in Canada. link).
Invasive tunicates in particular are thought to have been brought to the west coast through all the international trade across the Pacific from Asian sourses. However, recent studies may indicate that once they are initially established, their propagation along the western coastline may be occurring from the small-vessel traffic that occurs on the many bays, harbors and private marinas that line the region.
Why does this concern us?
Invasive species are damaging both economically and ecologically. Control costs in the United States for these types of species have amounted to $138 billion over time (Pimentel, D et al. 2005), affecting agriculture, forestry, tourism and human health.
With regards to the aquatic species, they can cause immense damage to to fishery stocks and ship hulls. Tunicates, or sea squirts in particular have high environmental resilience and an impressive ability to reproduce and colonize many habitats. Unfortunately, they can be so efficient at outcompeting other species for nutrients and space that they can eradicate groups of bottom dwelling fish, jeopardizing the ecosystem. As of 2006, state officials of the Puget Sound of Washington have allocated $750,000 ("Sea Squirts invade Puget Sound", link) to target the problem of these non-native tunicates.
Colonial tunicate - Didemnum sp.
Tunicates are receiving increasing attention locally in British Columbia, as sighting records are implying a gradual advancement of these species northward from the Californian coast. A previous study that animates the progression of these species up along the coast can be viewed here. The three species that are focused upon in this study are Ciona savignyi (transparent tunicate), Styela clava (club tunicate) and Didemnum sp. (colonial tunicate). These are generally regarded as the most prominent of the invasive tunicate species with the most literature and sighting record data, and hence is both the most important and easiest group of tunicates to spatially analyze.
Transparent tunicate - Ciona savignyi