A few years ago, I had the thrill of meeting Lonesome George, the last known Pinta Island giant tortoise. Goats were introduced to his island home in the Galapagos in 1958. They obliterated all of the vegetation on the island, and by the time he was discovered in 1972, all of the other tortoises of Lonesome George’s subspecies — those that hadn’t already been eaten by whalers and Galapagos settlers — had starved to death. For his safety, George was moved to the Charles Darwin Research Station on Santa Cruz Island in the Galapagos.  Forty years later, on June 24, 2012, Lonesome George was found dead in his pen.

Lonesome George

Lonesome George, the last known individual of his subspecies of Galápagos tortoise.

Goats browse a very wide variety of plants, preferring the tips of woody shrubs and trees. Feral goats — domestic goats that have become established as a population in the wild — put pressure on plant populations, and can quickly change the ecosystem.  They can quickly transform a lush tropical island forest into a barren, rocky wasteland.  For the wasteland, this can mean “locally extinct”.  But for an island species, with no members beyond that island,  a wasteland can mean “extinct.”

The IUCN (International Union for Conservation of Nature) Red List of Threatened Species is the most comprehensive, objective, global approach for evaluating the conservation status of plants and animals.  As part of the process of assessing conservation status, the IUCN has developed a Unified Classification of Direct Threats1, a meticulously organized hierarchy of factors that can endanger a species.  At the broadest level of classification, there are 11 Direct Threats:

  1. Residential & Commercial Development (Threats from human settlements or other non-agricultural land uses with a substantial footprint)
  2. Agriculture & Aquaculture (Threats from farming and ranching as a result of agricultural expansion and intensification, including silviculture, mariculture, and aquaculture)
  3. Energy Production & Mining (Threats from production of non-biological resources)
  4. Transportation & Service Corridors (Threats from long narrow transport corridors and the vehicles that use them including associated wildlife mortality)
  5. Biological Resource Use (Threats from consumptive use of “wild” biological resources including both deliberate and unintentional harvesting effects; also persecution or control of specific species)
  6. Human Intrusions & Disturbance (Threats from human activities that alter, destroy and disturb habitats and species associated with non-consumptive uses of biological resources)
  7. Natural System Modifications (Threats from actions that convert or degrade habitat in service of “managing” natural or semi-natural systems, often to improve human welfare, including fire and fire control, dams and water use)
  8. Invasive & Other Problematic Species, Genes & Diseases (Threats from non-native and native plants, animals, pathogens/microbes, or genetic materials that have or are predicted to have harmful effects on biodiversity following their introduction, spread and/or increase in abundance)
  9. Pollution (Threats from introduction of exotic and/or excess materials or energy from point and nonpoint sources)
  10. Geological Events (Threats from catastrophic geological events — volcanoes, earthquakes/Tsunamis, avalanches/landslides)
  11. Climate Change & Severe Weather (Threats from long-term climatic changes which may be linked to global warming and other severe climatic/weather events that are outside of the natural range of variation, or potentially can wipe out a vulnerable species or habitat)

We can’t take credit for Geological Events that threaten the survival of a species, but the other ten threats are of our doing.  Lonesome George’s feral goats belonged to threat 8 — ‘Invasive & Other Problematic Species, Genes & Diseases’.  Feral dogs, cats, pigs, and goats help destroy the birds of Hawaii, which dropped from about 125 species when humans arrived to about 30 today.

For each of the ten human-caused threats, our intuition is that the threat is growing geometrically, as the human population increases.  And the population continues to increase!

Human Population Growth

Human Population Growth

With a growth in threats, we see a growth in extinctions.


Species Extinctions Since 1800

Species Extinctions Since 1800

The latest update of the Red List shows that 17,291 species out of the 47,677 assessed species are threatened with extinction. Threatened: 21% of all mammals, 30% of all  amphibians, 12% of all  birds, 28% of reptiles, 37% of freshwater fishes, 70% of plants, and 35% of invertebrates.2

Only an impact from a meteorite could produce a higher extinction rate than humans now produce.  Edward Wilson estimates we are losing 27,000 species per year3. Niles Eldridge estimates 30,000/year4.  David Tilman estimates 50,000/year5. Paul Ehrlich has estimated 60,000-130,000/year.  Malcolm McCallum concludes that the current amphibian extinction rate may range from 25,039–45,474 times the “background extinction rate6 7

The arrival of humans in Australia, Eurasia, New Zealand, and the Americas seems to correspond to a time of extinction for many species of birds, mammals, and reptiles. For instance, humans reached Australia between 41,000 and 46,000 years ago.8 Beginning about 46,000 years ago, all megafauna weighing more than about 220 pounds went extinct. These turtles, birds, and mammals likely had no fear of humans, and were easy prey for humans.9 With the arrival of the first Europeans in Australia, the pace of extinction increased. In the first 200 years of settlement, three-quarters of the surviving rainforest was cleared, and half of what remains suffers from significant human impact.10

Hunting is a common cause of extinctions.  The Dodo was first mentioned by Dutch sailors in 1598, who had killed all of them by 1693.11 The Passenger Pigeon was one of the most abundant birds in the world in the 19th century. But less than 100 years later, it was extinct. In one town in Michigan alone, hunters killed 50,000 birds a day for nearly five months12.

Part of the problem of determining the human factor in early extinctions is fixing an exact date for human arrival, and the fact that because continents are such big places, quite a few humans would be needed to cause the extinction of any pervasive species. So a few Vikings arriving in Labrador cause no problem for any species. In some isolated areas, it is possible to better connect the time of our arrival with the timing of extinction. In Vanuatu, in the Southwest Pacific, one study suggests that a horned turtle went extinct within 200 years of human arrival. The remains of the turtles that the scientists discovered were mainly leg bones, indicating off-site butchery of the turtles.13

About 10,000 years ago, the lion was the most widespread land mammal after humans. Since then, its numbers have dropped, its range shrunk. The African lion once ranged through Europe, too, but became regionally extinct there in the days of Julius Caeser.14 Its decline is increasingly rapid, with a 30%-50% drop in the last 20 years.15 Today, the main threats to lions are indiscriminate killing (primarily as a  retaliatory or pre-emptive killing to protect life and livestock) and prey base depletion. In addition, habitat loss and conversion has led to a number of populations becoming small and isolated16.

Rhino Poaching Soars Along With Demand for Horns

A black rhino carcass with the horn hacked off.

In Africa and Asia, rhino poaching has increased more than 2,000 percent in the last three years. Using helicopters, night-vision scopes, silenced weapons, and drugged darts, the poachers are rapidly pushing several species of rhino toward extinction. And as the supply of rhinos dwindles, the price paid for their horns soars, putting even more pressure on the survivors.

Where fauna evolved alongside humans, in Africa and Southeast Asia, there have not been extinctions comparable to those elsewhere, when humans suddenly arrived on the scene. Given time, a species can develop fear of humans, wariness, and defenses against them. But that is co-evolution. With invasion, there is no time for evolutionary response.

Bogus Theories of Extinction

Can we blame climate? Climate was changing as humans spread around the world, and this climate change may have had some impact on wildlife populations. But some island megafauna survived for several thousand years after the disappearance of their continental cousins. Ground sloths survived on the Antilles long after North and South American ground sloths were extinct. Those in the Antilles went extinct when the islands were colonized by humans. Similarly, woolly mammoths died out on remote Wrangel Island 7,000 years after their mainland extinction. Steller’s sea cows also persisted off the isolated and uninhabited Commander Islands for thousands of years after they vanished from continental shores of the north Pacific.17  Before man, climate change was so slow that species were able to adapt. Today, if can blame climate change for extinction, we ourselves are still to blame.

Can we blame disease?  We now believe that an introduced disease can cause extinction in snails, amphibians, mammals18, and probably all species if the right conditions exist.  European diseases helped the Conquistadors defeat the Aztecs and Incas, and smallpox blankets helped the American settlers defeat the Native Americans.  Today, a chytrid fungus, whose spread is facilitated by climate change, has already eliminated 67% of the 110 species of harlequin frogs.19  But it seems unlikely that  disease could be the cause of extinction of hundreds of genera of megafauna just as humans arrived.

Can we blame a meteorite? No, a single meteorite would crush life immediately and regionally. The extinctions that occurred as humans dispersed took place over the period of that dispersal — an overall period of several thousand years, and an area of the entire world.

There are many difficulties in precisely measuring our murderous success. For starters, most species on earth have not yet even been discovered, so we are not likely to notice their absence.  For a rare species, an absence of reports does not prove that the species is gone.  But Lonesome George is gone.

Lonesome George was a poster child for species threatened by humans.  Now he is gone.

End Notes

Show 19 footnotes

  1. IUCN-CMP Unified Classification of Direct Threats. Version 3.1, 2011. link.
  2. IUCN. Extinction Crisis Continues Apace. link
  3. Wilson, Edward O. The Diversity of Life, The Belknap Press of Harvard University Press, Cambridge, Mass.1992.
  4. Eldredge, Niles, Life in the Balance. Humanity and the Biodiversity Crisis. Princeton University Press, Princeton 1998
  5. Dan Olson. Species extinction rate speeding up. Minnesota Public Radio link
  6.  “This is also known as the ‘normal extinction rate’, the background extinction rate is the standard rate of extinction in earth’s geological and biological history before humans became a primary contributor to extinctions. This is primarily the pre-human extinction rates during periods in between major extinction events.” Background Extinction Rate. link
  7. Malcolm L. McCallum. “Amphibian Decline or Extinction? Current Declines Dwarf Background Extinction Rate” Journal of Herpetology, Vol. 41, No. 3, pp. 483–491, 2007  link
  8. Bowler, James M. et al 2003, New ages for human occupation and climatic change at Lake Mungo, Australia
  9. Flannery, Tim (2002), “The Future Eaters: An Ecological History of the Australasian Lands and People” (Grove Press) link
  10. Humans and Rainforests. The Royal Botanic Gardens and Domain Trust link
  11. Dodo
  12. Passenger Pigeon
  13. Arthur W. White, Trevor H. Worthy, Stuart Hawkins, Stuart Bedford, and Matthew Spriggs. Megafaunal meiolaniid horned turtles survived until early human settlement in Vanuatu, Southwest Pacific. Proceedings of the National Academy of Sciences of the USA. link
  14. Panthera leo
  15. Lion. link
  16.  IUCN. Panthera leo. link
  17. Quaternary extinction event. link
  18. Brandon Klein. Disease Can Cause Extinction of Mammals. Wired Science. Nov 5, 2008. link
  19.  J. Alan Pounds, Martín R. Bustamante, Luis A. Coloma, Jamie A. Consuegra, Michael P. L. Fogden, Pru N. Foster, Enrique La Marca, Karen L. Masters, Andrés Merino-Viteri, Robert Puschendorf, Santiago R. Ron, G. Arturo Sánchez-Azofeifa Christopher J. Still and Bruce E. Young. Widespread amphibian extinctions from epidemic disease driven by global warming. Nature 439, 161-167 (12 January 2006) | doi:10.1038/nature04246; Received 2 June 2005; Accepted 21 September 2005 link