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The species can be the vivid purple of purple loosestrife or the red and yellow bayberry. They crowd out the native species of plants and the birds and the bees and the animals which feed on them. There is also the emerald ash borer which is eating its way through North America's ash trees.
Erie County Forester Ed Kowalski says the damage in Michigan and Wisconsin shows serious problems.
"It's an urban issue, really, in Erie County right now. Lancaster has the most active spots as of yet. We found one emerald ash borer-infected tree in Como Park," Kowalski says.
Kowalski says that's a recent find and it's not clear how widespread the problem is, although he expects it to get worse.
He says the big problem in county parks is Japanese knotweed which can be pulled out but the roots usually remain and the pest will grow back from those. Completely getting rid of the knotweed requires use of herbicides, which is an entirely different problem.