Most of us are acutely aware of the damage that introduced, non-native plant and animal species can do to native ecosystems.
Ophiostoma sp., the hated fungus that causes Dutch Elm Disease, has ravaged urban forests across Canada for years. In Montreal, for example, only a handful of Elm trees remain where 35,000 once
stood.
Purple Loosestrife (Lythrum salicaria), invades wetlands, chokes everything else out and soon creates a biological desert devoid of native plants and animals.
Meanwhile, Zebra Mussels, introduced in the Great Lakes system only 20 years ago, already number in billions. The economic damage caused by this species to power plants and water supplies is now counted in the hundreds of millions of dollars, but the ecological damage to our native mollusc populations is unfathomable.
It is not surprising, therefore, that the World Conservation Union has included each of these species on its list of 100 of the World's Worst Invasive Alien Species.
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The fungus that causes Dutch Elm Disease is ranked 64th among the World's Worst alien species. Purple Loosestrife is ranked 50th. And Zebra Mussels are 31st. Joining them are European House Rats (80), House Mice (58), feral cats (38), Walking Catfish (26) and Eastern Grey Squirrels (85).
That's right. Eastern Grey Squirrels! Those loveable backyard creatures first released into the Calgary ecosystem by an official with the Calgary Zoo in 1938 are now counted among the world's worst invasive species.
In Great Britain, where this species has nearly wiped out the native Red Squirrel population and where property damage is measured in the millions annually, the Eastern Grey Squirrel is ranked second in negative impact only to the infamous Norway Rat.
Like rats, Eastern Grey Squirrels carry diseases harmful to other species. Parapoxvirus, can kill our native Red Squirrels -- who have no immunity to the disease -- in weeks or days. Eastern Grey Squirrels, though, rarely suffer from it.
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This factor combined with a lack of native predators and the ecological principle of competitive exclusion, has enabled Eastern Grey Squirrels to become the overwhelmingly dominant tree squirrel species in Calgary.
Less than 70 after the introduction of Eastern Grey Squirrels, Red Squirrels have already been extirpated from dozens of Calgary neighbourhoods. Unless something is done to reverse this trend, it is likely that our native Western Canadian species will completely disappear before another 70 years pass by.
NOTES: Eastern Grey Squirrels come in three colours -- black, grey and cinnamon. October and November are probably the best months of the year to trap and euthanize them. Squirrel traps are available at most local hardware stores. They can also be rented from the Weaselhead Society. Details are available online at weaselhead.org/traps
© Brent Johner. Originally published in Calgary Gardening, October/November 2006. Reprint rights available. $9.95 CDN. Non-exclusive.
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