Would biodiversity recover if humans disappeared?
An article in the October 12, 2006, issue of New Scientist magazine, titled Imagine Earth Without People, asked what would happen if all humans disappeared tomorrow. As the article is more than 3,300 words long, the main conclusions are summarized below (followed by links to related articles and books, such as The World Without Us by Alan Weisman). The New Scientist article begins:
"Humans are undoubtedly the most dominant species the Earth has ever known. In just a few thousand years we have swallowed up more than a third of the planet's land for our cities, farmland and pastures. By some estimates, we now commandeer 40 per cent of all its productivity. And we're leaving quite a mess behind: ploughed-up prairies, razed forests, drained aquifers, nuclear waste, chemical pollution, invasive species, mass extinctions and now the looming spectre of climate change. If they could, the other species we share Earth with would surely vote us off the planet." (New Scientist)
The article continues with the proposition "Now just suppose they got their wish." Would the footprint of humanity ever fade away? The scenario is that all the people on Earth are spirited away in an instant, perhaps to a re-education camp in a far-off galaxy. To gather reliable predictions for the article, New Scientist writer Bob Holmes interviewed a range experts in relevant subject areas. Their specialties cover man-made structures, pollution, climate and atmospheric changes, and biodiversity.
Ronald Chesser, an environmental biologist at Texas Tech University has worked extensively in the exclusion zone around Chernobyl since the nuclear disaster which occurred 20 years ago. He says native fauna have begun to take over. Wild boar are 10 to 15 times as common within the Chernobyl exclusion zone as outside it, and big predators are making a spectacular comeback. "I've never seen a wolf in the Ukraine outside the exclusion zone. I've seen many of them inside," says Chesser.
Not surprisingly, without humans, areas still rich in native species will recover faster than severely altered ecosystems. Natural forests are predicted to recover within 200 years, but where native trees have been replaced by single-species plantations it will take several generations of tree growth to return the forests to their natural state (i.e. several centuries).
David Wilcove, a conservation biologist at Princeton University, says some ecosystems may never return to the way they were before humans interfered, because they have become locked into a new "stable state." In Hawaii, for example, introduced grasses now generate frequent wildfires that prevent native forests from re-establishing themselves. On the other hand, highly domesticated species such as cattle, dogs and wheat would probably evolve back towards hardier, less specialised forms through random breeding. However, species in the direst straits may already have passed a critical threshold below which they lack the genetic diversity or the number of potential breeding pairs that are needed for recovery.
About half of all endangered species are in difficulty because of predation or competition from invasive introduced species. Some introduced species may decline once man-made environments vanish, but others such as rabbits in Australia and cheat grass in the American west, will probably be around for the long haul and continue to edge out imperilled native species. On balance, David Wilcove expects the number of species that benefit to significantly exceed the number that suffer.
Genetically modified crops are engineered to be resistant to pesticides, but at a metabolic cost to the plant, so in the absence of spraying they will be at a disadvantage and will probably die out. And many GM crops are seedless, forcing farmers to buy new seeds each year from the producer.
With humans gone, fish stocks could take anywhere from a few years to a few decades to recover. So few cod and other large predatory fish remain that they can no longer keep populations of smaller fish such as gurnards in check. Instead, the smaller fish turn the tables and outcompete or eat tiny juvenile cod, thus keeping their former predators in check. Eventually, in the absence of fishing, enough large predators will reach maturity to restore the normal balance.
Timescale of environmental change:
| Within 1 year |
|---|
| Light-pollution: "Pretty quickly — 24, maybe 48 hours — you'd start to see blackouts because of the lack of fuel added to power stations," says Gordon Masterton, president of the UK Institute of Civil Engineers. Renewable sources such as wind turbines and solar panels will keep a few automatic lights burning, but the lack of maintenance to the distribution grid will scuttle these in weeks or months. Nuclear reactors will not fare so well. As cooling water evaporates or leaks away, reactor cores are likely to catch fire or melt down, releasing large amounts of radiation. |
| Air pollution: Oxides of nitrogen and sulphur and ground-level ozone will wash out of the atmosphere in a matter of a few weeks, although excess nitrates in groundwater persist for much longer. Other pollutants, such as chlorofluorocarbons, dioxins and the pesticide DDT, may take a few decades to break down. |
| Within 100 years |
| Methane: The chemical lifetime of methane in the atmosphere is only about 10 years. The wild card is that there are massive reserves of methane in the form of methane hydrates on the sea floor and frozen into permafrost. "We may stop emitting methane ourselves, but we may already have triggered climate change to the point where methane may be released through other processes that we have no control over," says Pieter Tans, an atmospheric scientist at the United States' National Oceanic and Atmospheric Administration (NOAA). |
| Within 250 years |
| Man-made structures: Though modern buildings are typically engineered to last 60 years, bridges 120 years and dams 250, these lifespans assume that essential maintenance will be carried out on a regular basis, otherwise things go downhill quickly. But even though buildings will crumble, their ruins — especially those made of stone or concrete — are likely to last thousands of years. |
| Beyond 1000 years |
| Carbon dioxide: "There will be CO2 left in the atmosphere, continuing to influence the climate, more than 1000 years after humans stop emitting it," says atmospheric chemist Susan Solomon (NOAA). Eventually calcium ions released from sea-bottom sediments will allow the sea to mop up the remaining excess over the next 20,000 years or so. |
| Nuclear waste: Containers for nuclear waste are designed to survive thousands of years of neglect. Beyond that time radioactivity will have dropped a thousandfold, according to Rodney Ewing, a geologist at the University of Michigan who specializes in the management of radioactive waste. That doesn't change the fact that damaged nuclear power plants will already have contaminated the environment with radioactivity. |
What would alien visitors find in 100,000 years?
Visitors from another planet arriving on Earth in 100,000 years time would find no obvious signs that an advanced civilisation ever lived here. If they used sophisticated tools to dig down below the surface the fossil record would show a mass extinction centered on the present day. Ocean sediments would show a brief timespan with massive deposits of heavy metals such as mercury, a relic of our fleeting industrial society. There would also be a concentration of radioactive isotopes left behind by nuclear reactor meltdowns. They might also turn up concentrations of skeletons at burial sites for a large bipedal ape, indicating that these animals were able to organize an intelligent civilisation. The article concludes:
"Within a few million years, erosion and possibly another ice age or two will have obliterated most of even these faint traces. If another intelligent species ever evolves on the Earth — and that is by no means certain, given how long life flourished before we came along — it may well have no inkling that we were ever here save for a few peculiar fossils and ossified relics. The humbling — and perversely comforting — reality is that the Earth will forget us remarkably quickly."
The World Without Us
Journalist Alan Weisman has written a book on the same theme: The World Without Us (Amazon.com). An article he wrote for Discover Magazine, Earth Without People, formed the basis for the book. He presents evidence that debris from plastics accumulating in our oceans will be the longest lasting pollutant apart from radioactive waste. An abridged excerpt from the book was published in Orion Magazine: Polymers Are Forever (~5,700 words).
Related reading:
The New Scientist article didn't actually say how long it would take to biodiversity to become as varied as it was before the human era. A research study by paleontologists at the University of California, Berkeley, indicated it would take around 10 million years. The same was true for minor extinction events, not just the five mass extinctions in the fossil record.
An article by an eminent biologist, Edward O. Wilson, was published in Cosmos Magazine in September 2005: Is Humanity Suicidal?
Jared Diamond, Professor of Geography at the University of California, Los Angeles, has written three bestselling books about the history and future of human societies. In "Collapse: How Societies Choose to Fail or Survive" he provides detailed accounts of earlier societies that collapsed as a result of damage they inflicted on natural resources which were essential for their survival.
