The natural control of populations

StoryImage( ‘/Images/Story//Auto-img-112248404421698.jpg’, ‘Photo courtesy of’, ‘The blue crab is an aggressive hunter, and a fast swimmer. For centuries, fishermen on the East Coast have supplied generations with soft-shell crabs and crab boils. The blue crab is found in the low salinities (fresh and salt water mix) of the Chesapeake Bay and are caught by the millions.’);
StoryImage( ‘/Images/Story//Auto-img-112248410121703.jpg’, ‘Photo by Robert A. Hedeen’, ‘Of the some 2 million eggs in this sponge crab's egg mass, only a few will develop to maturity.’);

In 1798, the Rev. Thomas Malthus in England published a small book. In this sociological treatise, the good Reverend warned, as others have warned ever since, that the human population was increasing so rapidly that in a few generations, there would not be enough food to go around.

Years after Malthus published his ideas, Charles Darwin just happened to read his book, and he thought that if this was true for humans, then why not for all species? More are produced than can possibly survive, so Darwin reasoned the ones that did survive to reproduce had some advantage over the others in the struggle for survival and were naturally selected by nature. Thus, Malthus’s idea became the driving force behind Darwin’s theory of the origin of species by natural selection.

This brings to mind a story related by William Warner in his Pulitzer Prize winning book, Beautiful Swimmers, the story of the life and lore of the blue crab. Warner was conversing with a veteran waterman on the Chesapeake Bay when the old man netted a female crab with her mass of eggs on the underside. Such a crab is called a sponge crab as the egg mass resembles a sponge.

The waterman knew it was a sponge crab laden with eggs, and when Warner asked him if he knew how many eggs made up the sponge, he replied, “maybe a couple hundred.” He was stunned when informed the number was closer to a couple of million. The old man thought for a while and then mused: “What if all of them lived?—They would soon fill up the Bay—and, then they would come out on the land—and just think how mean those bastards are.”

The waterman need not have worried because, of those some 2 million eggs that female blue crab would have laid, only a few would have developed to maturity. And so it is with all species, with the possible exception of man. As a general biological rule, the more primitive the animal, the more offspring it will produce due primarily to lack of parental care, predation, the carrying capacity of the environment, and a host of other natural factors.

In regard to higher animals, please consider this: Darwin calculated that a pair of elephants, among the slowest of breeders, would, if all of their offspring lived to reproduce, produce a population of 19 million elephants in 750 years. Yet, the average number of elephants remains the same over the years.

Each species has its own ecological niche in the natural world that it can claim as its own, and that piece of real estate can only support a definite number of individuals. Food supply is an important limiting factor as well as actual space itself. One of the requirements for the success of a newly evolved species is that it has an ecological niche of its own to occupy. If the newly formed species would try to move in on a niche already “leased” by a successful form, it would probably be quickly eliminated.

Predation is also an important population limiting factor. In the complex web of life, definite food chains are formed, with the more primitive animals being at the bottom of the chain, which is often depicted as a pyramid. The ones nearer the base produce great numbers of offspring to feed others higher in position. As one progresses toward the apex of the pyramid, fewer and fewer offspring are produced.

Parental care of offspring is also of great significance. Animals that give no attention to their young produce many more offspring than those that give at least some parental care. A female oyster may lay up to a million eggs during the course of one spawning season, but they are abandoned to the tides immediately after being laid and provide food for a host of other marine forms. A female elephant carries her young inside her uterus until it completes a significant stage of development before it is born. The mother, as do all female mammals, has a built-in food supply for the newborn in the form of nutritious milk from her mammary glands. For these reasons, a female elephant produces only one baby every two years.

Overcrowding of an environment tends to reduce population numbers by the encouragement of disease. Wildlife specialists agree that sooner or later, the increasing populations of resident Canada geese that are a nuisance in many areas will be brought under control by some disease that will kill off most of the weaker ones. By extension, this concept can be applied to other animals.

Sometimes it is best to let “Mother Nature” act on her own when it comes to reducing the populations of species we humans consider to be obnoxious.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

From the July 27-Aug. 2, 2005, issue

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