Double-breasted cormorant(Painting by Barry Kent MacKay)
1 Another alien fish species who had a profound effect on the Great Lakes is the sea lamprey. This eel-like fish entered the Great Lakes sometime around 1830, through canals that allowed it to move inland from the Atlantic Ocean, but is probably not native. It is a jawless, parasitic fish who attaches a sucker-like mouth to the sides of fish, draining them of bodily fluids, thus slowly killing them. It is believed to have taken a particular toll on the native lake trout.
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2 In a naturally evolved predator-prey relationship the population size of the prey size tends to determine the population size of the predator. But when a new species is introduced, it can overwhelm some prey species. This is particularly true on islands, when specialized endemic species — species found nowhere else in the world — are especially vulnerable to a new predator, against which they have no defense. A classic example might be the Stephen Island Wren (which was not really a wren), a small, flightless songbird that reportedly was exterminated by a single cat, although apparently there were several feral cats on the island, brought there by humans. The birds didn’t have a chance.
But the features that make such a species vulnerable to a predator as efficient as a cat would never have evolved in the first place had the cat always been there.
There are numerous other examples of exotic species who have not wiped out any species where they are native, but do so when introduced to new localities. In North America, all native species of fish consumed by cormorants co-evolved with cormorants and other native predators, and prey. In short, they belong. But introduced species did not co-evolve with the species found new environs, which is why they can create such havoc, at least initially. In the fullness of time other predator-prey balances will work themselves out, but native species may be endangered or exterminated in the process, especially if they are highly specialized and of limited distribution. There is great concern not only about the ultimate impact of the round goby, but also of other non-native fish species, such as several Asian carp species, now established in North American watersheds.
3 CDI (http://www.zoocheck.com/cormorant/ ) is something of an ad-hoc, Canadian based group of organizations, of which Born Free USA is a founding member, dedicated to protecting cormorants and other colonial waterbirds from persecution and to documenting such prosecution and educating the public.
4 It is possible to conceive of the odd adult double-crested cormorant being eaten by, say, a shark or an alligator or perhaps killed by an eagle, all of which arguably exist in lower numbers than in primal times. But no such predator ever has had a statistically significant impact on cormorants and to suggest otherwise would be like arguing that bread is a significant factor in human population sizes because every so often a person chokes to death on a piece of sandwich. Cormorant eggs and young are obviously vulnerable to nest predators, and there is no reason to think that any of them (crows, for example, or ravens or raccoons) are either at lower than primal numbers or ever had any statistically significant influence on the number of cormorants whatsoever.
5 This is not to suggest that humans had no influence on the population size of cormorants (or other native wildlife species) prior to the arrival of colonizing Europeans. Modern estimates of the human population size are significantly greater than those postulated by earlier authors who seemed to assume that aboriginal humans were too few and too devoid of technology to have any significant discernable influence on the ecosystems they inhabited. On the contrary, before Europeans arrived people were abundant in the Americans, had sophisticated societies that interacted with each other, formed trade routes, coalitions and nations and certainly also “managed” the environment with a view of providing them with their needs. There is no evidence we know of to allow us to know, one way or the other, if First Nations in the Great Lakes region contributed to the destruction of colonies of cormorants but certainly no reason to assume a priori that they were any more favourably disposed toward cormorants then than are many people currently. Cormorant remains from earlier centuries have so far shown up in two middens (accumulations of bones from animals eaten) in southern Ontario, but cormorants, because of their fish diet, are not very palatable and thus unlikely to show up in such places. There is no reason to assume that their feathers or other parts had any special value that would cause them to be incorporated into lasting artefacts. We know that cormorants colonies are very vulnerable to disturbance and it is certainly possible that through nest destruction, egg collecting and target practice that such colonies as existed could have been largely eliminated from the Great Lakes and other parts of North America by the time Europeans began to take note of them, or their absence.
Introduction: Why Cormorants Can’t Kill All Fish
Wildlife Management North and South of the Border
Energy Transference, Basic Physics and the Technology Prosthesis
Cormorants: Food Chains and Basic Ecological Principles
The Alewife, Alien Salmon and Trout, and the Double-Crested Cormorant
The Difference Between Science and Management