Cygnus olor (mute swans) impact native waterfowl habitat. They consume great quantities of submerged aquatic vegetation (SAV) and aquatic invertebrates and cause competition for space and food, attacking and driving off native waterfowl and potentially reducing the carrying capacity of breeding, staging and wintering habitats for native species of migratory waterfowl. Mute swans occupy and defend large territories (up to 6ha) of wetland habitat during nesting, brood rearing and foraging. Studies have shown that in Europe mute swans have successfully eliminated individual plant species from some wetlands and it is feared that in North America, similar impacts on food resources will occur and effect migrant and wintering waterfowl populations. They have been reported to kill adult and juvenile ducks and geese as well as other wetland-dependent birds (Allin and Husband, 2003; Petrie and Francis, 2003).
At high densities, mute swans can overgraze an area with the potential to eliminate some plant species from an ecosystem. During winter, mute swans may also consume nutrient storage and over wintering structures such as tubers, which could reduce the future availability of perennial species such as wild celery (Vallisneria americana) and American bulrush (Scirpus americanus), both important food sources for native waterfowl (Petrie and Francis, 2003).
Location Specific Impacts:
Herbivory: In closed waterways in Europe, Cygnus olor have been documented as removing entire species of submerged aquatic vegetation (Chesapeake Bay Program, 2002).
England (United Kingdom (UK))
Reduction in native biodiversity: Cygnus olor are also believed to have an adverse effect on brown trout (Salmo trutta) fisheries in southern England, through overgrazing of aquatic macrophytes, especially Ranunculus spp. (water-crowfoot) (Watola et al 2003).
Chesapeake Bay (United States (USA))
Competition: Unlike the tundra swan (Cygnus columbianus) that migrate to the Bay for the winter, Cygnus olor is a year-long resident and, therefore, reports of conflicts with nesting native waterbirds and the consumption of submerged aquatic vegetation (SAV) have raised concerns among resource managers (Perry et al 2002).
Mute swans are highly territorial during the breeding season, which leads to localised depletions of SAV during the growing period. The cumulative effect of their year-round foraging on aquatic plants and associated implications to SAV restoration efforts are unknown (Perry et al 2002).
Tundra swans lose mass during the winter and depart from the wintering grounds at their lowest mass. Harassment by mute swans may cause tundra swans to lose mass more rapidly, which could affect subsequent reproduction (Chesapeake Bay Program, 2002).
Inter-specific aggression during the nesting season has been documented in the Chesapeake Bay between Canada geese, mallard and black ducks (Chesapeake Bay Program, 2002).
Human nuisance: The Chesapeake 2000 Agreement includes a commitment to restoring 114,000 acres of submerged aquatic vegetation; however, restoration efforts, particularly in the mid-Bay where submerged aquatic vegetation (SAV) decline is most severe, are frequently obstructed by feeding Cygnus olor (mute swans) (Chesapeake Bay Program, 2002).
Reduction in native biodiversity: Populations of black skimmers (Rynchops niger) and terns (Sterna antillarum) nesting on beaches and oyster shell bars have been eliminated by molting Cygnus olor (mute swans) (Perry et al 2002).
Washington (United States (USA))
Competition: In the Northeast, many consider the Cygnus olor a pest as it overgrazes ponds and excludes native waterfowl. The populations in the Pacific Northwest do not seem to be growing or expanding quickly and thus have not yet become a management issue (Seattle Audubon Society, 2005).
Other: Highly territorial, male Cygnus olor will aggressively defend their large territories against their own and other species, including humans; displaying, hissing and attacking when provoked (Seattle Audubon Society, 2005).