The Anthropocene defaunation process.

 

Richard Owen stands next to the largest of all moa, Dinornis maximus (now D. novaezealandiae). From Wikimedia Commons.

Richard Owen stands next to the largest of all moa, Dinornis maximus (now D. novaezealandiae). From Wikimedia Commons.

In 2000,  Paul Crutzen proposed use the term Anthropocene to designate the last two hundred years of human history and to mark the end of the current Holocene geological epoch. Although there is no agreement on when the Anthropocene started, it has been defined, primarily, by significant and measurable increases in anthropogenic greenhouse gas emissions from ice cores and other geologic features including synthetic organic compounds, radionuclides and ocean acidification.

Another marker for the Anthropocene is the current biodiversity crisis. The term defaunation was created to designate the declining of top predators and herbivores triggered by human activity, that results in a lack of agents that control the components of the ecosystems vegetation.

Global population declines in mammals and birds represented in numbers of individuals per 10,000 km2 for mammals and birds (From Dirzo et al., 2014)

Global population declines in mammals and birds (From Dirzo et al., 2014).

Since the industrial revolution, the wave of animal and plant extinctions that began with the late Quaternary has accelerated. Calculations suggest that the current rates of extinction are 100–1000 times above normal, or background levels. We are in the midst of  the so called “Sixth Mass Extinction”.

Although anthropogenic climate change is playing a growing role, the primary drivers of modern extinctions seem to be habitat loss, human predation, and introduced species (Briggs, 2011). The same drivers that contributed to ancient megafaunal and island extinctions.

SConsequences of defaunation (From Dirzo et al., 2014)

The consequences of defaunation (From Dirzo et al., 2014)

 

One of the most famous and well-documented extinctions come from Madagascar. Pygmy hippos, giant tortoises, and large lemurs went extinct due to human hunting or habitat disturbance.  A very interesting study by Burney et al. (2003) tracked the decline of coprophilous Sporormiella fungus spores in sediments due to reduced megafaunal densities after the human arrival on the island. Another well documented case is the Moa extinction in New Zealand. Recent radiocarbon dating and population modeling suggests that their disappearance occurred within 100 years of first human arrival. A large number of  land birds across Oceania suffered a similar fate beginning about 3500 years ago.

Some biologist predict that the sixth extinction  may result in a 50% loss of the plants and animals on our planet by AD 2100, which would cause not only the collapse of ecosystems but also the loss of food economies, and medicinal resources.

References:

Richard N. Holdaway, Morten E. Allentoft, Christopher Jacomb, Charlotte L. Oskam, Nancy R. Beavan, Michael Bunce. An extremely low-density human population exterminated New Zealand moa. Nature Communications, 2014; 5: 5436 DOI: 10.1038/ncomms6436

Rodolfo Dirzo et al., Defaunation in the Anthropocene, Science 345, 401 (2014); DOI: 10.1126/science.1251817

Braje, T.J., Erlandson, J.M., Human acceleration of animal and plant extinctions: A Late Pleistocene, Holocene, and Anthropocene continuum. Anthropocene (2013), http://dx.doi.org/10.1016/j.ancene.2013.08.003

 

 

Advertisements

The sixth mass extinction: the human impact on biodiversity

800px-Ice_age_fauna_of_northern_Spain_-_Mauricio_Antón

Woolly mammoths in a late Pleistocene landscape in northern Spain (Author: Mauricio Antón) From Wikipedia Commons

At the beginning of the nineteenth century George Cuvier, the great French anatomist and paleontologist,  suggested that periodic “revolutions”, or catastrophes had befallen the Earth and wiped out a number of species, but under the influence of Lyell’s uniformitarianism, Cuvier’s ideas were rejected as “poor science”. One century after Cuvier definition of catastrophism, Chamberlain proposed that faunal major changes through time were under the control of epeirogenic movement of the continents and ocean basins. Despite Chamberlain’s article, the modern study of mass extinction did not begin until the middle of the twentieth century with a series of papers focused on the Permian extinction. One of the most popular of that time was “Revolutions in the history of life” written by Norman Newell in 1967.

Mass extinctions had shaped the global diversity of our planet several times during the geological ages. They are major patterns in macroevolution. Andrew Knoll defines them as perturbations of the biosphere which seem instantaneous when it is observed through the geological record.

The ‘‘Big Five’’ extinction events as identified by Raup and Sepkoski (1982)

The ‘‘Big Five’’ extinction events as identified by Raup and Sepkoski (1982)

In 1982, Jack Sepkoski and David M. Raup identified five mass extinctions. The first took place at the end of the Ordovician period, about 450 million years ago.  Now, according to the current rates of extinction, we are in the midst of  the so called “Sixth Mass Extinction”.

Mass extinctions are probably due to a set of different possible causes like basaltic super-eruptions, impacts of asteroids, global climate changes, or continental drift. But now, a group of scientists like Edward O. Wilson and Niles Eldredge identified post-industrial humans as the driving force behind the current and on-going mass extinction (Braje, 2013).

The human arrival was a “key component” in the extinction of the megafauna during the late Quaternary. In North America, approximately 34 genera (72%) of large mammals went extinct between 13,000 and 10,500 years ago, including mammoths, mastodons, giant ground sloths, horses, tapirs, camels, bears, saber-tooth cats, and a variety of other animals. South America lost an even larger number and percentage, with 50 megafauna genera (83%) becoming extinct at about the same time.

 

Richard Owen stands next to the largest of all moa, Dinornis maximus (now D. novaezealandiae). From Wikimedia Commons.

Richard Owen stands next to the largest of all moa, Dinornis maximus (now D. novaezealandiae). From Wikimedia Commons.

Other extinctions on island ecosystems around the world are result from direct human hunting, anthropogenic burning and landscape clearing, and the translocation of new plants and animals. One of the most famous and well-documented of these extinctions come from Madagascar. Pygmy hippos, giant tortoises, and large lemurs went extinct due to human hunting or habitat disturbance. A very interesting study by Burney et al. (2003) tracked the decline of coprophilous Sporormiella fungus spores in sediments due to reduced megafaunal densities after the human arrival on the island.  Another well documented case is the Moa extinction in New Zealand. Recent radiocarbon dating and population modeling suggests that their disappearance occurred within 100 years of first human arrival. A large number of  landbirds across Oceania suffered a similar fate beginning about 3500 years ago.

The anthropogenic effects increasingly took precedence over natural climate change as the driving forces behind plant and animal extinctions with the advent of agriculture and the domestication of animals.

The Panamanian golden frog (Atelopus zeteki). Credit: Brian Gratwicke. From Wikimedia Commons

The Panamanian golden frog (Atelopus zeteki). Credit: Brian Gratwicke. From Wikimedia Commons

Amphibians offer an important signal to the health of biodiversity; when they are stressed and struggling, biodiversity may be under pressure.   Today, they are the world’s most endangered class of animal, while corals have had a dramatic increase in risk of extinction in recent years. Some biologist predict that the sixth extinction  may result in a 50% loss of the plants and animals on our planet by AD 2100, which would cause not only the collapse of ecosystems but also the loss of food economies, and medicinal resources.

The acceleration of extinctions over the past 50,000 years, in which humans have played an increasingly important role, has left a number of hard questions about how the Anthropocene should be defined and whether or not extinctions should contribute to this definition (Erlandson, 2013)

 

References:

T.J., Erlandson, J.M., Human acceleration of animal and plant extinctions: A Late Pleistocene, Holocene, and Anthropocene continuum. Anthropocene (2013)

A.D. Barnosky, N. Matzke, S. Tomiya, G.O.U. Wogan, B. Swartz, T.B. Quental, C. Marshall, J.L. McGuire, E.L. Lindsey, K.C. Maguire, B. Mersey, E.A. Ferrer, Has the earth’s sixth mass extinction already arrived?, Nature, 471 (2011), pp. 51–57.

Mayhew, Peter J.; Gareth B. Jenkins, Timothy G. Benton (January 7, 2008). “A long-term association between global temperature and biodiversity, origination and extinction in the fossil record”. Proceedings of the Royal Society B: Biological Sciences 275 (1630): 47–53.

D.A. Burney, L.P. Burney, L.R. Godfrey, W.L. Jungers, S.M. Goodman, H.T. Wright, A.J.T. Jull, A chronology for late prehistoric Madagascar, J. Hum. Evol., 47 (2004), pp. 25–63