The Unexpected Threat of the Canada Goose

Canada Goose, Branta canadensis

Although native to North America, escaped captives from estates both in Ireland and England means that the Canada Goose (Branta canadensis) is now established in Counties Antrim, Armagh, Cork, Cavan, Derry, Fermanagh, Monaghan and Tyrone as a breeding bird (1). It is an attractive species, its white cheeks contrasting with its black neck, and can be seen this time of year feeding in flocks on grasslands (2). As an introduced alien, its impact has been seemingly low, with problems of aggressive nesting habits, fouling of parks by droppings and damage by large flocks to grasslands being reported (1). However such anecdotal evidence does not tell the whole story. To do this, a system must be used to compare the impact of not only different alien species but also species from different taxonomic groups (eg., birds vs. mammals). This is just what has been achieved by Nentwig et al.: a generic scoring system that they applied to alien mammal species in Europe (3). The system divides impact into subcategories of either environmental or economic. Scoring these gives a potential impact which is converted to an actual impact by taking into account the distribution of the species. Using this system to assess the impact of B. canadensis reveals that it has the highest economic and environmental impact of the 26 most common alien birds in Europe. While all of this indicates control of B. canadensis as a priority, its large distribution, especially in norther counties, means this would be a labour and resource intensive step.

References:

1. Browne et al., 1999. Irish Birds 6 pp. 233-236.
2. Sterry, 2004. Collins Complete Guide to Irish Wildlife p. 38
3. Nentwig et al., 2009. Conservation Biology 24 pp. 302-311
4. Kumschick and Nentwig, 2010. Biological Conservation 143 pp. 2757-2762