Hypothesis: 
The progression of the spiny waterflea (Bythotrephes) through inland lakes in Ontario can be tracked using a combination of 'gravity' and GIS models.  This Geographical Information System (GIS) model will be based upon information gained through surveying boaters in Ontario (gravity model).

Procedure:
According to past studies, the spiny waterflea is mainly spread by boaters and anglers attached to equipment such as fouled fishing lines, boat anchor lines and downrigger cables.  Other possible reservoirs for the species which could facilitate its spread include infected bilge water and live well water, live minnow bait that have consumed waterfleas and hold their eggs in the digestive system, fouled waterski or tubing lines, and SCUBA diving equipment.  All these factors were taken into consideration, and a short boater survey was devised.  Questions asked on the survey pertained to the habits of boaters (i.e. do they clean their fishing line), where they travel (i.e. the lakes they normally visit), and if individuals have ever encountered the spiny waterflea. 

After devising the survey, marinas, lake associations and hunter and angler clubs, in and around areas containing lakes invaded by Bythotrephes all over Ontario, were contacted.   For locations far from Windsor, we mailed copies of the survey, along with self-addressed stamped envelopes, to the various clubs.  For locations within driving distance (i.e. within 8-9 hours away from Windsor), a  team of researchers visited various marinas and docks and performed surveys with boaters in person.  Once surveys were returned to the University, the information was entered into a database.  Tables based on this information were then constructed for input into a GIS.

Results:
The gravity model predicted spread of Bythotrephes based upon seven identified risk factors (e.g. use of contaminated fishing or boat anchor line) that may allow dispersal of either live individuals or their resting eggs from invaded to noninvaded lakes, as well as based on the spatial arrangement of invaded and noninvaded lakes in Ontario.  Discriminant analysis of lake gravity scores successfully identified invasion status for 74% of 170 inland lakes.  A retrospective analysis of 31 invaded lakes revealed that the order in which lakes were invaded was directly related to the magnitude of vector inflows from invaded sources.  Analysis of the dominant vector inflow to each invaded lake revealed a ‘stepping stone’ pattern in which at least five lakes were sequentially invaded from the source population in Lake Huron.  One invaded lake (Muskoka) apparently served as an invasion 'hub', resulting in up to 18 additional direct and 17 indirect invasions.  Species spread occurred via a combination of dominant, local diffusion (median distance 12.5km) and rare, long-distance (>100km) dispersal.  Eleven of 131 lakes that were not invaded in 2000 were reported invaded in 2001.  Gravity scores of these lakes were significantly higher than those of other noninvaded systems, indicating that susceptibility to invasion can be related to the magnitude of vector inflows. A GIS model based on gravity scores indicated that Bythotrephes’ distribution is expected to expand to eastern and northwestern Ontario, although most new invasions are expected to occur in the central region of the province.  Our results indicate that quantitative analysis of human dispersal vectors provides a robust starting point with which to assess vulnerability of discrete ecosystems to invasion.  Management efforts focused on reducing the number and magnitude of human-mediated dispersal vectors may reduce the rate of invasion of new ecosystems.  This work is now in press (2003) in the journal Ecological Applications. 

Jim Muirhead, a PhD student, is currently expanding our work on Bythotrephes to include new lakes that have been invaded, and to assess spread of this species against patterns of other species (e.g. zebra mussel).  Jim has utilized our survey data to model individual invaded lakes as threats to other lakes.  His Michaelis-Menton type assessments of vector outflows from invaded lakes to other invaded lakes or to other noninvaded lakes allow usto determine which invaded lakes will be most important in future spread of the species.  For example, while Lake Muskoka was very important to initial spread of the wateflea in Ontario, most of the lakes with which this lake has contact have been invaded (i.e. most vector outflow is to other invaded lakes).  On the other hand, Kash Lake, Lake Simcoe and Lake Nipigon all have outfloes predominately to noninvaded lakes, and vector flow is substantial in each case.  So, we anticipate that these three lakes will be very important to future spread.  During 2003, three of the newly invaded lakes have their strongest vector inflows from Kash Lake, consistent with our model. 

Red dots indicate pre-2001 invasions, blue dots represent reports of new invasions in 2001. 
Green isobars indicate vulnerability to invasion based upon vector outflows from invaded lakes. 

The sequence by which we think lakes in Ontario were invaded based upon the dominant 
vector only.  Lake Muskoka was central to the initial spread of the species, while vector 
data indicate Lakes Simcoe and Nipigon will be central to future spread. 

Model of Bythotrephes spread based upon vector flows (all vectors included).  The pattern is much more 
complicated than that of the dominant-only vector model.