Human Population and the Future of Diversity

Early Population Growth

For the vast majority of human history, the number of people on earth has been surprisingly small. In fact, early on in our roughly 100,000 year history, our species was perilously close to extinction; genetic studies suggest humans underwent a dramatic population bottleneck around 70,000 years ago, with global numbers falling to around 15,000 people. Subsequently the population of humans rebounded and grew, but at a relatively slow pace. Not until the invention of agriculture did humans pass the 1 million mark, and it was not until around 1800 A.D. that the total number of humans reached 1 billion; today, there are two countries, China and India, that have more than 1 billion citizens, and it projected that the 7th billion human will join us on October 31, 2011.

How Did We Get To 7 Billion?

So how did we go from a paltry 15,000 individuals to 7 billion? The early demographic history of humans did not see remarkable growth. Rather, it was characterized by a relatively gradual increase over time, albeit with a few setbacks, such as the Black Death which devastated the population of Europe in the 14th century. Overall, however, birth rates slightly outpaced death rates over the first 98 percent of our history, a trend that continued until the 19th century. Then things began to change. The trajectory of human population growth morphed frighteningly fast, from one of gradual increase to one of unprecedented, exponential growth. The fuse of human population explosion had been ignited.

It had taken our species more than 100,000 years of persistence, luck and ingenuity, to reach the 1 billion mark. Then, in a blink of an eye, the population doubled to 2 billion in a mere 130 years. What was the root cause of this never before seen population explosion in humans? Most likely it was a combination of factors linked to the industrial revolution, such as advances in sanitation, healthcare, disease prevention and food availability, to name a few. Infant mortality dropped, and lifespans increased. As a result, more infants were living long enough to become adults and have children themselves, and people were being added to the planet at a much faster rate than ever before. However, as striking as the population growth of the 19th century was, it would pale in comparison to what was to come. The human population was about to truly explode.

From 1930-1974, the world population added not 1, but 2 billion more people; that is, in just 44 years humans had doubled in number from 2 billion to 4 billion! Since 1974, we have almost doubled the human population again, adding 1 billion people to the planet every 12-14 years! To provide some perspective, this is the equivalent of adding the entire population of the United States to the planet every 4 years (faster than most of us can make it through college) or the entire population of China every 15 years. It was only in 1999 that we welcomed our 6th billionth person to the planet—1 billion new fellow humans have joined us since then. It is worth pausing to consider that people who are now in their late 70s and early 80s have seen the population of planet earth increase from around 2 billion to 7 billion in just their lifetime.

Lowering death rates, better healthcare, increased food production and major medical and technological breakthroughs all contributed to the meteoric population growth of humans in the 20th century, and they continue to do so today.

The good news is the rate of human population increase has slowed; the bad news is that the overall population size continues to grow, it is just growing at a slower rate (we are still on the proverbial runaway train, but it’s now moving a little slower). Predictions vary, but forecasts for 2050 are typically around 9 billion (give or take a billion or two). Birth rates are expected to continue to drop globally, and at some point in the 21st or 22nd century, the human population is predicted to stop growing, and perhaps even begin to shrink. In the interim, we face many serious challenges, and the choices we make will ultimately determine not just our fate, but that of the many other species with which we share the planet.

The Next Land Rush?

Typically, the challenges and potential consequences of overpopulation are cast in human terms, such as “can we effectively deal with decreasing resources essential to human civilization– fresh water, oil, food, etc.—while adding several more billion people to the planet, and if so, how?” Humans have repeatedly overcome seemingly insurmountable odds to succeed, and indeed, we may be savvy enough to deal with continued population growth via breakthroughs in technology, especially in the areas of energy production and conservation, water use, and agriculture. So yes, it may be possible to support a few more billion people. But this is almost certainly to come at a huge cost to the planet.

In order to greatly increase the availability of energy, clean water and food, humans will need to utilize more of the earth’s landmass. More land will be needed for housing, more land for crops, more land for raising cattle, more land to serve as reservoirs and watersheds, more land used for extracting fossil fuels and more land to house nuclear reactors. Even “clean” energy such as wind farms take space. The demand for additional food will not be restricted to converting more land for farming itself; increasing crop-yields has relied heavily on fertilizers, which are made with petroleum (an often-overlooked cost). Furthermore, many of our most productive farm regions rely heavily on irrigation and already face water shortages. Together, the additional land used by, and for, people will almost certainly exceed predictions. This is because humans have already colonized much of the most productive land, and more land per capita will need to be “domesticated” in the future. As the following examples illustrate, this trend does not bode well for our fellow species.

A Matter of Scale

For most species, habitat loss is the greatest threat they face, and for species such as large mammals, which need proportionally greater space, this threat is amplified. Large mammals often face yet another challenge – overexploitation by humans, either for meat or for body parts. In Monster of God: The Man-Eating Predator in the Jungles of History and the Mind, author David Quammen masterfully explores the question of whether any room remains in the modern world for “maneaters” (in this context, he is referring specifically to lions, tigers and bears and crocodiles). The essence of this question is whether humans will “allow” the continued existence of these species in the wild. After millennia of living together with large carnivorous animals (and not always peacefully), will humans make the necessary sacrifices to ensure the long-term survival of these magnificent beasts in their native habitats? Or will humans, faced with increasing population pressure, choose to marginalize these species and commit them to the same fate as their ice-age brethren, the saber tooth cat, giant short-faced bear, giant ground sloths and wooly mammoth?

Making the Commitment

Only the committed and enduring protection of large tracts of high-quality, interconnected natural habitats can sustain viable populations of large mammals. And while large-scale habitat destruction tends to receive most of the attention, smaller-scale habitat loss and fragmentation (the breaking up of a large, contiguous habitat into many small patches of habitat with large gaps between them) can be dangerously insidious. A little deforestation here, one highway there, and over time the landscape becomes smaller and smaller and more subdivided. In perhaps his finest work, Song of the Dodo, Quammen likens this scenario to systematically cutting a beautiful 12 by 18 foot Persian rug into into thirty-six equal pieces, each one a rectangle, two feet by three. The area of these 36 individual “carpet like” pieces, when added up, will still be nearly 216 square feet—the same size as the original rug. But we do not have thirty-six nice Persian throw rugs. Rather, we are left with a pile of “three dozen ragged fragments, each one worthless and commencing to come apart.”  Small ragged fragments of habitat are not the same as large, intact natural areas. For many species, especially large mammals, these habitat patches are nearly as worthless as Quammen’s 36 scraps of unraveling carpet. What lions, tigers, bears, and their brethren need is the Persian rug.

Icons on the Brink

At the beginning of the last century, an estimated 100,000 tigers (Panthera tigris) were alive throughout the wilds of Asia. Nine subspecies ranged from Siberia to Sumatra. Today, only six of those subspecies remain; the other three subspecies, the Bali, Javan and Caspian tigers all faded into extinction by the mid 1900’s. Tigers have lost 93 percent of their historic range and are completely gone from southwest and central Asia, from Bali and Java, and from most of Southeast Asia. The entire population of tigers in the wild is estimated to be only 3,000 – 5,000 individuals, with less than half of those of viable breeding age. The chief causes of the catastrophic decline? Habitat loss and overexploitation. Tigers have been relentlessly hunted even in protected areas, and much of their former range is now home to people. In just one century, the tiger has gone from 100,000 individuals to the brink of extinction. Although many efforts are in place to save the remaining tigers from this fate, their continued existence in the wild is dire. In a world of 9 billion people, will we still be able and willing to make room for the tiger?

Like the tiger, the five living species of rhinoceroses (“rhinos”) have also been decimated over the last 100 years, likewise falling victim to the deadly combination of habitat loss and poaching. The white rhino (Ceratotherium simum) of Africa maintains a population of approximately 15,000-20,000 individuals, making it by far the most numerous species in the group. The other African species, the black rhino (Diceros bicornis), was thriving as recently as the late 1960s, with a population estimated to be approximately 70,000. However, the number of black rhinos has plummeted over the last 40-50 years, and it is now estimated that there are fewer than 2,500 individuals left in the wild.

The remaining three species of rhinos have fared even worse. The Indian rhinoceros, also known as the greater one-horned rhinoceros (Rhinoceros unicornis), occurs only in India, Pakistan, and Nepal. This species reminds one of a tank with large metal plates welded together to form a protective coat of armor. Once widespread throughout Asia and the Indian subcontinent, today only 3,000 or so remain; two-thirds of these live in a single national park (Kaziranga National Park) in India.

The two smallest species, the Sumatran (Dicerorhinus sumatrensis) and Javan (Rhinoceros sondaicus) rhinoceroses are both on the precipice of extinction. Only 275 Sumatran rhinos, and less than 50 Javan rhinos, are thought to remain in the wild. The latter may be the rarest large mammal on earth.

Although all rhinoceroses are herbivores, and therefore don’t occupy the top level of the food chain as a tiger does, they have been hunted relentlessly for their “horns.” Rhino horns are not made of bone, but of keratin, the same material that forms our fingernails and hair. Poachers kill rhinos and remove the horns, which they then sell on the black market. The horns are typically converted into a powdered “medicine” that can be consumed orally. There is no scientific evidence that this material has any actual medicinal properties, yet it can fetch tens of thousands of dollars for a single kilogram on the black market.

The tiger and rhinoceroses are examples of how interaction with expanding human populations can be devastating. In both cases, loss of habitat to human development has played a fundamental role, and in both cases, overexploitation by humans (i.e., poaching) has accelerated their population declines. The loss of tigers and rhinoceroses in the wild is due to us, and it is us who will make the choices that determine their future. If we can’t manage to co-exist with some of the most impressive mammals the planet has ever seen, then the future for biodiversity in general is a gloomy one indeed.

The Biodiversity We Know, and the Biodiversity We Don’t

To date, we have described, named and catalogued approximatley1 million species living on earth. This is not a trivial number, yet it pales in comparison to how many species we haven’t yet discovered or described. A recent study estimated that there are approximately 8.7 million species on earth. This study has received some criticism, but most scientific estimates fall within a range of 5-10 million (although it should be noted that some scientist have estimated there may be as many as 100 million species living today). A quick bit of math reveals that at best we have documented 20 percent of the species on earth, and at worst, it could be as low as 1-10 percent . In other words, we have yet to discover somewhere between 80 and 99 percent of the species on earth! How can this be?

Certainly, a large number of these undiscovered species will be the small things– insects, fungi and so on. However, we are surprisingly ignorant about the true biodiversity of even the most well known groups, such as mammals. For example, currently about 5,600 species of mammals have been documented, but it is projected that more than 7,000 actually exist. Thus, approximately 20 percent of all species of mammals alive today haven’t even been discovered! These are mammals we are talking about people, not an obscure group of microscopic organisms! On one hand, it is incredibly exciting to know that there are so many species left to be discovered. After all, the process of discovery is what most scientists really enjoy. On the other hand, these statistics are sobering; one can only imagine what amazing biodiversity we have already lost without ever knowing it existed (E. O. Wilson eloquently describes this dilemma in his book, The Diversity of Life). What we will lose in the next 50-100 years will depend on our priorities, specifically the value we place on living in a world with wild places, functioning ecosystems and a full complement of species.

The Ultimate Test

Can we succeed in achieving a balance between the inevitable increase in land devoted to human use and maintaining biodiversity? Perhaps. But the sustainable number of humans on earth is probably closer to three or 4 billion, rather than 7-12 billion. Reaching this sustainable level will be challenging and it will require dogged persistence. We must avoid dipping into our “biodiversity reserve” in order to achieve quick fixes to social and economic problems; unlike a strategic oil reserve, we can’t go back later and top off the “species tank.

The species we share the planet with today represent a heritage that extends back hundreds of millions of years. They have outlasted the dinosaurs, successfully endured the ice ages, and play vital roles in the functioning of earth’s ecosystems. They deserve respect, and they have the right to share this planet with us. Ironically, our success in preserving our fellow species will speak volumes about the nobility and character of our own.

Brian Arbogast is a member of the Ecology Global Network’s Science Advisory Counsel, and Associate Professor and Assistant Curator of Mammals, Terrestrial Conservation Biology at the University of North Carolina Wilmington. His recent work has focused on the conservation genetics of flying squirrels and other gliding mammals, the evolution of Galapagos mockingbirds and using non-invasive methods to study mammalian biodiversity in the tropical Andes of Ecuador.

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