The spread of organisms to new environments is one of the major drivers of speciation, since it exposes them to novel suites of environmental pressures (Mayr 1942). Recently, however, human activity has greatly accelerated the spread of organisms to new environments and altered ecosystem conditions in ways that often benefit these newcomers at the expense of pre-existing biota (Palumbi 2001). Although most research has focused on terrestrial species introductions, a recent meta-analysis determined that marine species introductions currently outpace them tenfold (Sorte et al. 2010).
Numbers of introduced marine species have increased exponentially in the last 200 years, and rates of introductions continue to increase due to trade, climate change and various other anthropogenic disturbances (Ruiz et al. 2000, Raitsos et al. 2010). It is harder to quantify the number or rate of marine algal introductions, as they are often overlooked or even encouraged (Inderjit et al. 2006). Despite representing a minority of reported marine species introductions, alien macroalgae can have profound economic and ecological impacts on existing systems (Schaffelke et al. 2006, Schaffelke & Hewitt 2007). Prevention of these impacts necessarily involves identification not only of algal species likely to become invaders but also of attributes that contribute to the invasiveness of potential nuisance algae (Anderson 2007). It is worth noting that many successful introduced algae have not been observed to negatively impact their new environments and so are not considered invasive (Boudouresque & Verlaque 2002); these cases are not pertinent to this review.
