Coastal marine invasions spanning the US/Canada border

Background/Question/Methods

Invasive marine species have confounded both science and management across international borders because of rapid and episodic spread and highly variable populations with consequently unpredictable ecological and economic impacts.  Among the most important invaders in coastal marine systems in western North America are salt marsh cord grass Spartina spp. and the European green crab Carcinus maenas, which have both spread across the U.S.-Canada border and have resulted in significant ecological and economic impacts.  I will use these species to ask the following questions:  1) What are the patterns and causes of range expansion? 2) What are the ecological and economic impacts across this border? 3) How has management across the U.S.-Canada border responded to these invasions?  I will synthesize data from my own collaborative studies of cord grass and green crabs as well as other recent work on these species.

Results/Conclusions

For both species, the data show a broadly directional process of range expansion from south to north with the U.S. being the source of the Canadian invasions.  With green crabs, population genetic data strongly support a single invasion in San Francisco Bay with northward range expansion as far as Vancouver Island.  This contrasts evidence from eastern North America that shows clear evidence of independent invasions in the U.S. and Canada.  Data on economic impacts of green crabs shows similar contrasts with historically significant economic impacts in eastern North America with comparatively little economic impact to date in western North America.   Ecological impacts, while more difficult to compare, are significant on both coasts and as well as across borders.  The cord grass invasions have resulted from multiple introductions along the west coast and has resulted in significant ecological impacts along the U.S. coast with similar changes predicted for Canada if populations become widely established.   Both U.S. and Canadian agencies have invested in managing these invasions including local eradication of cord grass and monitoring of population impacts and spread for green crabs.  However, efforts to coordinate management activities across borders have not been well developed.  I conclude by discussing the future of cross-border invasions in marine systems based on these examples.