By Kevin Mack

In any discussion on conservation or the preservation of biodiversity, the concept of “non-native” species is bound to come up. But what does it mean to be “non-native” and what impacts might a non-native animal species have on a given area’s native species? The former question is a fairly easy one to answer, while the latter poses a bit more of a puzzle.

Simply put, a non-native species is one that has been introduced outside of its historic or natural range. It is an animal that, with a little human help, has moved beyond its normal physical boundaries (mountain ranges, oceans, etc.) and arrived in an entirely new location.

If the area is similar enough to the animal’s point of origin, or if the animal is highly adaptable, it may survive and begin to reproduce. When a non-native species begins to colonize an area, it will undoubtedly have repercussions for the area’s native species. Unfortunately, since natural systems are so complex, it is not always readily apparent what those repercussions might be.

There are certainly many historic examples of non-native introductions that have had disastrous effects on native wildlife. In one extreme case, the Australian Brown Tree Snake was accidentally introduced to Guam after World War II and wiped out 17 species of native wildlife on the small Pacific Island. Cane toads, introduced in Australia to control a beetle that was destroying sugar cane crops, have also had far-reaching negative effects on that country’s native animals.

We have no such dramatic examples here in Washington, but we certainly have a number of non-native species that may be having a variety of effects, both positive and negative, on our native wildlife.

Some of the non-native animals in Washington have been intentionally introduced by government agencies for hunting or other purposes. Others have been released into the wild by private citizens that were keeping them as pets. Still others have found their own way here by stowing away on ships or other forms of human transport, or under their own power after being released in other states.

There are literally dozens of species from many different points on the globe that have now become established in the Pacific Northwest. Many of these species are so ubiquitous in cities and towns that we forget that they arrived here only recently.

The Eastern Gray Squirrel is one of the most frequently seen mammals in the Seattle area, but it did not exist in this area 70 years ago. Likewise, the European Starling and the House Sparrow are two of our most commonly seen birds, but they are a long way from their original European homes. Examining the habits of these three very common non-native species gives an idea of where conflicts with native species may arise.

Eastern Gray Squirrels are native to the hardwood forests of the Eastern United States. They don’t fare well in the cedar and fir forests of the Pacific Northwest, but they do live quite happily in the urban and suburban areas around Puget Sound. Cities tend to provide the Eastern Gray Squirrels with plentiful food in the form of bird feeders, ornamental plants and direct human handouts.

Washington has a native Western Gray Squirrel that is listed as a threatened species. It also likes hardwood forests, but does not fare well in developed areas. It is currently only found in significant numbers in an oak forest remnant on the Fort Lewis army base. In recent years, as development has spread closer to that remnant forest, so has the Eastern Gray Squirrel. There is great concern that if the Eastern Gray Squirrel becomes established in that area, it will completely replace the native Western Grays through direct competition.

Both House Sparrows and European Starlings are cavity-nesting birds. They are fairly aggressive and will remove the eggs and chicks of other birds from a nest cavity if they find it to their liking. I have not seen any definitive studies that quantify the impact of sparrow and starling nest competition on native species, but it is widely believed that both species are having a significant impact, especially in areas where native habitat has already been compromised.

While non-native species may out-compete native species for limited resources, they may also become a resource for native species. A case in point would be the non-native Rock Pigeon populations in Seattle and Tacoma. Peregrine Falcons have been reintroduced to both of these cities and the abundant Rock Pigeons provide them with a nearly unlimited food source. Many other non-native species are also preyed upon by native predators, so the bane of one native species may be a boon for another.

The bottom line is, when a non-native species is introduced into an ecosystem in which it did not evolve, there is no way to predict what the ultimate repercussions will be. We do know that adjustments in the local community will have to occur to accommodate the introduced species. If the non-native animal is aggressive and adaptable, it is likely that the effect on native species will be a negative one.

NON-NATIVES
Mammals
Eastern Gray Squirrel
Fox Squirrel
Eastern Cottontail
Virginia Opossum
Nutria
House Mouse
Norway Rat
Black Rat

Birds
House Sparrow
European Starling
Rock Pigeon
Ringed Turtle Dove
Mute Swan
Ring-necked Pheasant
Chukar
California Quail
Northern Bobwhite
Gray Partridge
Wild Turkey

Reptiles
Red-eared Slider
Common Snapping Turtle

Amphibians
Bullfrog