temperatures increased by 2.2°C. Their study shows that southern invertebrate species increased 
in abundance and expanded their ranges while northern species that were not tolerant of warmer 
waters declined. Some of this response, however, may be due to cyclical tidal fluctuations that 
influence water temperatures (Denny and Paine, 1998). Overall, the change in species 
composition may be more species- and location-specific than a simple latitudinal response to 
temperature (Helmuth et al., 2006). 
An example of a species that could move farther north as a result of warming waters is 
the invasive mussel, Mytilus galloprovincialis. This blue mussel species has a higher tolerance 
for warm water temperatures and increased salinity levels than the native blue mussel, Mytilus 
trossulus, in California. Thus, M. galloprovincialis has replaced the native mussel along much 
of the southern and central California coastline (Braby and Somero, 2006). The native mussel 
still dominates the northern coast of California as the invasive mussel is less tolerant of cold 
water temperatures (Braby and Somero, 2006). However, as temperatures warm in the North 
Pacific, M. galloprovincialis could have the potential to expand its range northward. 
Dukes and Mooney (1999) also discuss increasing temperature in the context of climate 
change and find that it enables species invasions under certain circumstances. Mandrak (1989) 
examined 58 species to determine their potential to establish in the Great Lakes by comparing 
characteristics of these potential invaders to 11 recent invaders. Of the 58 species studied, 27 
could potentially establish if temperatures warm from climate change, while the others could not. 
McFarland and Barko (1999) examined the effects of increased water temperature on a 
monoecious hydrilla, finding that the species is better adapted to higher temperatures than 
previously shown in the scientific literature. Populations of the common reed, Phragmites 
australis , also increase with higher-than-average ambient air temperatures (Wilcox et al., 2003). 
Another effect of warming temperatures may be an increase in the number of sexual versus 
asexual reproductive periods for plant species, resulting in increased rates of spread. Diaz- 
Amela et al. (2007) linked the flowering cycles of a Mediterranean seagrass ( Posidonia 
oceanica ) to warming water temperatures. If these types of changes occur in AIS, they may lead 
to further expansion and impacts. 
1.6. INTERACTING GLOBAL CHANGE STRESSORS 
Invasive species can be major ecosystem stressors, and their interaction with other global 
change stressors is not fully understood. Kolar and Lodge (2000) identify global change and 
other anthropogenic stressors that increase the number or the impact of freshwater invasive 
species: globalization of commerce (including shipping; bait, aquarium, and pond trade; and 
aquaculture); waterway engineering (including canals and dams); land use changes (including 
siltation, eutrophication, and water withdrawal); climate and atmospheric changes; and 
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