David,
Canada Geese are not in the came category as the other vagrants that you
mention. They were introduced into New Zealand and therefore are quite
different to the Northern Pintail, Northern Shoveler etc. These vagrants
are mostly northern hemisphere breeding species and are unlikely to breed in
Australia, even if more than one was to arrive. However if they did
establish it would be a natural event and not a human induced introduction.
If they stow away on ship, like the House Crow, then that is a different
situation again, as they are human assisted. A native New Zealand bird that
found its way without human intervention to Australia would not be treated
as feral. The Kelp Gull has done just that and I don't hear anyone
suggesting that the Kelp Gulls in Australia are feral. The Cattle Egret,
Monarch (butterfly) and Asian House Gecko are in a different category
again - self introduced species that only occur because of human
modification to the natural environment in Australia. They probably don't
pose the same sort of threat to natural ecosystems as do the Canada Geese as
they have exploited human made, and otherwise vacant, niches. In fact
Cattle Egrets may provide some protection to nesting Great, Intermediate and
Little Egrets by increasing colony size and therefore improving security
from predation. Cattle Egrets feed in different microhabitats to the other
egrets and therefore don't compete directly for food.
Greg Clancy
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