The Effect of Human Population on Biodiversity
by Edward J. Otten, MD

"Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them."

Charles Darwin On the Origin of Species

No work has had more influence on the science of biology than the one quoted above, but there are two works, which predicted the impact of human population on the biodiversity of the earth, that stand out in the history of population biology. In 1798 Thomas Malthus wrote An Essay on the Principle of Population and one hundred seventy years later Paul Ehrich wrote The Population Bomb. Malthus had a profound influence on Darwin and Ehrich has been the voice of reason in the quest for human population control. Both men were correct in their predictions (though not in the exact timing of them) but were initially thought of by the general public and the media as false prophets. We know now that their warnings were well-founded and should be heeded.

Most of the scientific evidence collected since Malthus has supported his theory that even the most slow growing species would cover the earth in a short time if its population growth were unrestrained. War, famine and disease have been minor checks on the growth of human population and indeed are usually indications of areas of overpopulation. Most of the current "problems" on the earth are caused by density-dependent factors related to human population. When the population of an area exceeds the carrying capacity of the land, something has to give. Either one group kills off another (Bosnia, Middle East, Uganda), disease kills off some of the population (AIDS, cholera, malaria) or there is insufficient food and some of the population starves (Central Africa, Afghanistan). Erhlich’s theory is that either humans must voluntary control their numbers or nature will definitely control them (through mechanisms that are very unpleasant for humans).

The exponential growth of the human population, making humans the dominant species on the planet, is having a grave impact on biodiversity. This destruction of species by humans will eventually lead to a destruction of the human species through natural selection. While human beings have had an effect for the last 50,000 years, it has only been since the industrial revolution that the impact has been global rather than regional. This global impact is taking place through five primary processes: over harvesting, alien species introduction, pollution, habitat fragmentation, and outright habit destruction.

It is the nature of human beings to control their environment as much as possible. It allows our species to spread to new habitats and obtain new food sources. Man is a "switching predator" and can use various techniques for obtaining protein from any available animal. During the last Ice Age, great herds of "megafauna" mammals, such as wooly mammoth and giant ground sloth, inhabited North America and Eurasia. As the ice receded, populations of humans moved into areas where these herds existed. While many these megafauna had no prior contact with humans, the humans had thousands of years of experience hunting big game on the plains of Africa. In a geologically short time much of the megafauna was extinct. Paleontologists have unearthed numerous examples of wasteful "mass kills," in which entire herds were driven over precipices or otherwise trapped and slaughtered.

During the 18th century a combination of scientific, technical and industrial innovations enabled humans to overharvest not only land mammals, but fish, whales, birds, and any other species of use to humans. Like the technology of the spear and the arrow did for early humans, the technology of the harpoon, gunpowder, and driftnets allowed modern humans to harvest species far faster than they could replace themselves. This is the first and most obvious method of humans causing the extinction of species. Unfortunately it continues today in the hunting of "bushmeat" for urban populations of formerly rain forest dwelling humans. Wasteful "mass kills" are not a thing of the past – they can still be seen today in the unselective long liners, shrimp trawls and purse seines that mercilessly comb the oceans, discarding the bulk of each catch as unusable "bycatch."

Humans are the most mobile of species and can live anywhere on earth. When they travel from place to place they often transport other species along with them, resulting in alien introductions. While the most drastic devastation occurs on small islands, large land masses have also felt the impact of imported species that have no natural control to their numbers. Prior to the arrival of humans, Hawaii before humans had thousands of species of birds, and invertebrates, and plants found no where else on earth. Since the introduction of mongoose, rats, pigs and dogs and – as well as many species of plants, -- over half the bird species and countless species of snail have gone extinct. The introduction of rabbits into Australia, Asian fish species into Florida, Africanized bees into Brazil, plants such as Kudzu, melaleuca, and Brazilian pepper throughout the US, and rhododendrons into England are obvious examples of introduced species that outcompete and exterminate the native animals and plants.