The aquarium pathway for non-indigenous plants to the Saint Lawrence Seaway in Canada
The Saint Lawrence River is an entry point into the Great Lakes Basin, in which substantial resources are spent each year to control plant invaders (29 USD million). The aquarium trade is a major source of introduction of invasive alien plants into the Saint Lawrence River. A new method has been developed to quantify propagule pressure by analyzing each step in the pathway of introduction, then synthesizing this information to calculate propagule pressure for each species. The propagule pressure is a single value that expresses the number of individuals of a species introduced into a given area. Propagule pressure is therefore considered by the authors as a mean for predicting invasion success.
The study was performed by first collecting copies of invoices of aquatic plants from all 16 aquarium and related product stores in Montreal. A total of 138 species of aquarium plants is sold in Montreal per year. It was assumed that all plants ordered from distributors were sold. The proportion of plants entering each disposal sub-pathway (sub-pathways consist in direct release, waste, escape due to a storm, and other ways of escape) was estimated for each species by customers (75 respondents to a questionnaire left in stores) and these results were analyzed through Bayesian statistics, taking into account uncertainty. The proportion of each species entering each disposal sub-pathway was then multiplied by the probability of introduction associated with that sub-pathway, summed across sub-pathways, and multiplied by the total number for each plant sold per year to determine the final propagule pressure in the Saint Lawrence Seaway.
The estimated propagule pressures (in plant/year) for the top 10 species are: 247 for Microsorium pteropus (Polypodiaceae), 201 for Cladophora aegagropila (Cladophoraceae), 187 for Egeria densa (Hydrocharitaceae, EPPO List of Invasive Alien Plants), 145 for Vallisneria gigantea (Hydrocharitaceae), 131 for Hygrophila corymbosa (Acanthaceae), 124 for Bacopa caroliniana (Plantaginaceae), 116 for Cabomba caroliniana (Cabombaceae, EPPO List of IAP), 105 for Egeria najas (Hydrocharitaceae), 92 for Anubias barteri (Aroideae) and 87 for Vallisneria spiralis (Hydrocharitaceae). Although this study focused on the Montreal aquarium trade, the authors recommend that the estimate of the propagule pressure could be used for other pathway analyses and other organisms.
The list of 138 species recorded with their relative frequency and propagule pressure has nevertheless not been checked against the list of invasive species in Canada. Such a comparison could effectively test the hypothesis that propagule pressure is the most important factor in determining whether a species will become invasive.
Cohen J, Mirotchnick N, Leung B (2007) Thousands introduced annually: the aquarium pathway for non-indigenous plants to the St Lawrence Seaway. Frontiers in Ecology and the Environment 5, 528-532.