from the book, "Our Fascinating Earth"
If animals overgrazed, overhunted, and exploited sources of food as man does, all living things would have vanished from the earth a long time ago. Worse yet, they could overpopulate, as man does, and accelerate the process of extinction.
Most animals, however, prevent overpopulation by engaging in some type of instinctive behavior that will keep their numbers within the limits of available food.
Scent figures prominently as a birth control device among many animal species. A most impressive example is that of the meal beetles. These insects inhabit mills and granaries and usually reproduce rapidly. But as soon as their population exceeds two beetles to a gram of flour, the female reacts rapidly by devouring her own eggs the moment they are laid. The trigger for this unmotherly behavior lies in a chemical substance contained in the beetle's excrement. As the substance increases in concentration, its scent first causes the fertility of the female to diminish and then prolongs the duration of larval development. The final stage includes the aforementioned egg cannibalism!
In 1859 Thomas Austin turned twenty—four rabbits loose in Australia as a cheap and easy source of food. Within six years the rabbit population grew to twenty—two million, and that was only the beginning of the rabbit explosion. They rapidly overran the country, reducing farmland to a dust bowl and driving native fauna to remote corners as they occupied every nook and niche. Having conquered the continent, they were destined to share the food famine they had created.
As they neared the point of collapse, the rabbits stopped "breeding like rabbits." Their fertility depends on the weather; they seem to be aware that offspring will not survive in unfavorable conditions. In times of extreme drought the males resolutely refrain from approaching the female. If the female is already pregnant when the drought sets in, she will miscarry during the hot, dry days that follow. All is not lost, for when the first major rainfall occurs, signaling the end of the drought, the rabbits react almost instantly, returning to their favorite pastime of producing offspring.
In the early twentieth century African big—game hunters were amazed that even after years of shooting large numbers of elephants in a particular area the animal populations had not diminished. It seems that, just as some fish produce more offspring when they are preyed on by larger fish, the elephants keep their numbers similarly stabilized. They leave a region if the population seems in danger of becoming exterminated. Many times they invade an area not equipped to handle them. This becomes obvious when trees are knocked down, uprooted, or otherwise destroyed, grasslands are denuded, and the water supply is inadequate. It is then that the elephants take measures to limit their numbers. For them birth control means abstinence, so the female will double or triple the number of years between pregnancies.
Animals exhibit a variety of methods of taking census of their population and keeping it within the limits of the food supply. Some make an audible demonstration: when the nightly concert of frogs, the chirping of cicadas, the caroling of birds, the sounds of spawning fish reach a critical noise intensity, the fertility level of the species diminishes. Other animals make an optical display such as flights of ducks at twilight, dances of clouds of gnats, and the lights of tropical fireflies to show their numbers. These, as well as scent, observed among guppies, fruit flies, mice, the above—mentioned meal beetles, and numerous other species, trigger the phenomenon that causes them to slow down the reproduction rate.
These are just a few examples of the ways animals prevent overpopulation; numerous species use many other self—regulatory controls. But what happens when a species has no such controls?
Such groups as lemmings and locusts, and others in which the regulatory system is ineffective, reproduce to a point where other, less palatable systems intervene. In many animals, when population density exceeds a prescribed limit, their social behavior disintegrates. Quarrels break out within groups, offspring are neglected, polygamy and adultery replace monogamous relationships, and the entire society degenerates. With this comes a high mortality rate, and only when the population is stabilized at a reasonable density will the social order return to normal. As man may discover, from overcrowding to total nothingness is really only a step.
Stone Age man of 30,000 years ago lived in widely separated clannish groups, each consisting of about twenty—five people. In all of England there were probably no more than 500 humans. Scientists estimate that in the entire world the population of early man was only around 3.34 million, fewer than the number of people living in the present—day Los Angeles!
It took from the dawn of human history until 1830 for humankind to produce a billion people. The second billion arrived about one hundred years later; the third billion took thirty years, the fouth only fourteen years, and the fifth was added in twelve years. We are now anticipating an additional billion every ten years. With seven billion projected for 2010. At present the human population grows daily by over 260,000 people, about the population of Las Vegas, Nevada. By the end of a single week the human numbers worldwide will have swelled by 1.82 million, equivalent to the population of West Virginia.
Compared with other species, the human female is rather unproductive — usually fewer than five offspring in a lifetime. Contrast this to the common toad, which produces over 7,000 eggs at a single laying; or the herring, with 50,000 eggs at one time; or the queen termite, which delivers eleven million eggs annually for fifteen years.
There is a tragic ending to the story of animal population controls, because the self—regulatory method does have its dark side. In many places on our globe animals are exposed defenselessly to the depredations of man. When hunting, agricultural expansion, development, or plain unadulterated extermination causes a species to fall below a critical limit in population density, disaster falls suddenly on the remaining stock. The will to live and propagate the species becomes disrupted, and the small number of individuals that remain will die of their own accord. This was observed in the last years of the passenger pigeon and is currently a grave concern with the Galapagos tortoise. The species will voluntarily vanish from the face of the earth. And extinction is forever!
Unfortunately, very few species vanish along; their absence is felt by other species further up or down the food chain, and a chain reaction of extinctions begins. Perhaps the rabbits that were deliberately introduced on Laysan atoll in 1903 will serve as an example of a species that couldn't plan ahead. With no natural enemies and plenty of food, they had nothing to do but enjoy the tropical paradise and multiply. Within ten years they were out of control, and vegetation was seriously depleted. Attempts were made to reduce their numbers, but they were unstoppable.
Not surprisingly, the vegetation gradually died off and topsoil blew away. The three species of birds unique to the island became extinct. By 1927, in less than twenty—five years, Laysan had become a barren strip of sand, and the remaining rabbits had died of starvation.
While humans reproach the rabbit that failed to adapt and destroyed an entire ecosystem (including, of course, itself), we may wish to look closely at ourselves. As long as the human population remains unstabilized, and we continue to deforest, erode, pollute earth, air, and water, poison, and consume, we come closer to making a planet that can support neither plant nor animal, including, of course, mankind.
A thoughtful study of global human carrying capacity, released in 1994, calculates that the world can permanently support a human population of two billion people at a lifestyle that resembles middle—class life in industrialized nations. This is one—third of the current world population, so the effort to reduce to a sustainable number will make great demands of the next five generations. Actually, within a hundred years we could return to two billion without having to experience a catastrophic collapse caused by starvation , plague, or war. It would require that every family on earth limit itself to an average of 1.5 children.
Because all alternatives are unthinkable, it is important to think about this solution. The time is now!