Ecologists Jim Muirhead and Professor Hugh MacIsaac of the University of Windsor, Ontario have been studying the spread of the spiny water flea, Bythotrephes longimanus, through Canadian lakes. Using network theory, they built up a picture of the lakes as nodes in a network connected by human traffic, including boat trailers and anglers. Like internet viruses, which spread fastest when they attack the most widely-used email programmes, Muirhead and MacIsaac examined patterns of human vector movement to see whether some invaded systems have the potential to develop into invasion hubs.

According to Muirhead and MacIsaac: "Some lakes invaded by the spiny water flea may serve as invasion hubs if departing boaters and anglers travel to large numbers of non-invaded destination lakes." They found that two of the five lakes they studied, Lake Simcoe and Lake Kashagwigamog, are likely to develop as invasion hubs because most boaters and anglers leaving these lakes travel to lakes that are still free from the spiny water flea.

Earlier work by the pair found that another lake in the network, Lake Muskoka in central Ontario, has served as the hub from which 39 other lakes have become infected since 1989. "It quickly developed into a regional hub for two reasons. First, all of its outbound traffic was to non-invaded lakes and second, the total amount of traffic leaving this source was high," they say.

The findings are important because they allow the limited resources available to control invasive species to be targeted at points on the network where they will have most impact. "Outbound vector traffic from hubs with large flows to non-invaded destinations should be targeted for management efforts to restrict the transportation of propagules across the network and to reduce the rate at which non-indigenous species disperse to novel sites," Muirhead and MacIsaac conclude.

The spiny water flea was first found in Lake Ontario in 1982 and by 2003 it had invaded at least 57 inland lakes and lake systems in Canada. Its spread has been facilitated because it can produce resting eggs able to survive adverse environmental conditions (such as drying out or being eaten by fish), and because humans transport the eggs on their fishing gear and pleasure boats.

Professor Norman Yan of Ontario's York University has demonstrated that predation by the spiny water flea causes an average loss of three zooplankton species. Thus, as the species spreads, it could cause loss of thousands of populations of zooplankton species in Ontario alone.

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