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Cormorants: Limiting Factors

Published 01/25/11
By Barry Kent MacKay, Senior Program Associate

Double-breasted cormorant
(Painting by Barry Kent MacKay)

Cormorants are dependent on accessible fish biomass, and there’s no doubt that it has increased in some places at the local level. A field that once grew cotton and now grows fish obviously provides more accessible food for cormorants, and as indicated above, it seems quite possible to think that while the Great Lakes — the largest body of fresh water on the continent — may have fewer fish overall than the vast numbers who met the first European colonists (indeed, several of those species that were once common now endangered or extinct), the arrival of alewives and round gobies may have enhanced the ability of the lower Great Lakes to feed cormorants over what existed centuries earlier.

There is no way to know for sure, but the paucity of records of breeding cormorants in the Great Lakes (which does not mean there were none; some records do exist) suggests the possibility.

The amount of food and suitable nesting sites are what real biologists call “limiting factors” in determining population sizes of many species of bird. In other words, the limits to how much food and suitable nesting sites are available are, and always have, been major factors in determining how many cormorants there are. For millions of years they were by far the major limiting factors in the absence of human influence and thus can be called “natural” limiting factors.

What are not natural are the factors that determined the number of double-crested cormorants since Europeans began to inhabit North America.5 The first and most important of these was firearms. With guns blazing there was a truly devastating destruction of eastern North American wildlife, both for profit and “sport.” Nothing big enough to be a target was spared. Losses in eastern North America include the extinction of the distinctive “heath hen” subspecies of the greater prairie-chicken, the extirpation of the Canadian lynx, cougar and the gray wolf, the extinction of the eastern race of elk, bison in the east and the ivory-billed woodpecker, the extirpation of the wild turkey, the extinction of the Carolina parakeet, the extinction of the sea mink, Labrador duck and the Eskimo curlew and, most spectacularly, the extinction of what had been by far the most abundant bird species in North America, the passenger pigeon, once numbering in the billions, but totally gone by 1914 when the last known bird died, in a zoo.

Numerous other species were reduced to remnants of current population sizes, many never recovering. During the Victorian era, stuffed birds or parts of them were sewn into large hats that were the fashion of the day. In 1866 the great ornithologist, Frank Chapman, conducted an odd kind of bird census. He took a couple of afternoon walks through an uptown shopping district of New York City, and counted the birds he found on 700 ladies' hats. Three out of four contained feathers, and Chapman was able to identify no fewer than 160 North American species, including the American robin, scarlet tanager, blackburnian warbler, cedar waxing, bobolink, blue jay, scissor-tailed flycatcher, red-headed woodpecker, northern saw-whet owl, snow bunting and pine grosbeak. Cormorants also were killed for their plumage, with 19th century accounts from Ohio mentioning “boatloads” of dead cormorants.

Gunfire was not the only factor in this vast slaughter. There was nest destruction, disturbances and displacement and a massive destruction of habitat, but direct persecution was enough to account for most of the losses in larger species of fauna in eastern North America. The wheel and axel, draft animals, metallurgy, use of sails and, ultimately, the internal combustion engine rendered nothing out of reach of men with guns.

But as surviving species struggled to recover from the slaughter, other threats emerged.

It was during World War II that scientists invented a completely synthetic toxin called dichlorodiphenyltrichloroethane, better known as DDT. It was a powerful pesticide that killed insects and saved many human lives otherwise lost to insect-transmitted disease, including malaria, and thus was seen as a miracle substance and was put into wide use.

But this chemical has a dark side. It is fat-soluble, meaning it is absorbed in fats, and it “bioaccumulates.” When put into the environment it may be directly absorbed by small organisms, such as minnow-sized fish, in trace amounts that do no harm to the animal. But as those small fish, each with a trace amount of the chemical in its body, are consumed by larger fish those amounts of chemicals increase as they accumulate, moving up the food chain. Species at the top of the food chain thus have much more of the chemical absorbed into their fat than those at the bottom. And the chemical, in larger amounts, can cause harm. In birds such as peregrine falcons, ospreys, brown pelicans and cormorants, the chemical inhibits the ability of the birds to produce normal eggs. In ways not understood the ability to produce calcium is compromised, the shells of the eggs become too thin, the egg collapses and this leads to massive reproductive failure.

There is still a great deal of controversy over exactly what happened, since some experiments seemed to exonerate DDT as the causative agent in well-documented egg shell thinning, some experiments seem to indicate that it was a metabolite of DDT called DDE that caused the problem, and some research suggests that some types of birds were susceptible while others were not. But the bottom line is that cormorant numbers, and those of several other top-of-the-food-chain predators — including brown pelicans and ospreys — went into freefall decline with the appearance of DDT in the environment, and the numbers recovered when use of DDT was curtailed or eliminated.

Thus, loss of cormorants not only to DDT but to various other toxins such as lead and mercury that were increasing in the environment occurred as they were recovering from a much earlier reduction that saw precipitous declines in cormorants long before the invention of DDT. But those earlier declines mostly happened so early in the history of European (not to mention Asian and African) colonization of the Americas that they went largely or completely unrecorded. Wildlife managers, and even many naturalists, assume that the “normal” numbers of cormorants were those that appeared once accurate record keeping began, and even then they often ignore such information that does exist from earlier eras, including Wires and Cuthbert’s 2006 paper.

Introduction: Why Cormorants Can't Kill All Fish

Wildlife Management North and South of the Border

Energy Transference, Basic Physics and the Technology Prosthesis

Back in the Real World

Cormorants: Food Chains and Basic Ecological Principles

The Alewife, Alien Salmon and Trout, and the Double-Crested Cormorant

The Round Goby

The Difference Between Science and Management

The Agricultural Subsidy

The Missing Predator Argument

NEXT » Semi-Science and Wildlife Management

So ... What Do Real Scientists Say?

Footnotes

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