Certainly a case can be made for considering them invasive. The issue is definitional, yet not all agree on a definition of “invasive.” I happen to like The Nature Conservancy’s definition: An invasive species is “any species not native to an ecosystem whose introduction does or is likely to cause economic or environmental harm or harm to human health.” On reflection, the better case might be made to declare the native Canada Goose invasive, rather than the exotic Rock Pigeon (formerly the Rock Dove).
Historically, Canada Geese migrated through Ohio on their way to more northern nesting areas. These geese were wary and shied away from developed areas and human activity. Beginning in the 1950s and 1960s, aggressive and highly successful habitat management led both to both a huge increase in goose nesting in Ohio and, subsequently, to goose colonization of agricultural and, ultimately, suburban and urban habitats where they were safe from hunting. Our Canada Geese have largely lost both their fear of humans and their migratory tendencies, and geese unquestionably have major impacts on ecosystems. (For example, accumulation of goose droppings foul water and may serve as a source of pathogens infecting other waterfowl.) Canada Geese introduced to Europe and New Zealand are developing the same familiarity with humans there, and legitimately qualify as invasive by any definition.
Rock Pigeons are non-native, having been introduced from Europe probably over 200 years ago. (Nobody seems to have kept track.) They are unquestionably offensive in urban and agricultural settings largely on account of their droppings, soiling clothing, vehicles, structures, statues and the like. Because they are so closely tied to human activity, they have not had a major ecosystem impact and they have not displaced any native species. Every now and then one can find a pair nesting in their original habitat, cliffsides, but that is rare. They have adopted us as we have adopted them. So although there are lost of pigeons in cities, towns and farms, their overall environmental impact is slight.
In future issues of the Song Sparrow we will feature some native species, like the Canada Goose, that have features of invasive species because of their overall ecosystem effects.
For more information see the National Invasive Species Council (NISC) Web site.