The sixth mass extinction

Painting of the Dodo by Roelandt Savery executed in ca. 1626 and held at the NHMUK, London.

Mass extinctions had shaped the global diversity of our planet several times during the geological ages. The fossil record indicates that more than 95% of all species that ever lived are now extinct. During times of normal background extinction, the taxa that suffer extinction most frequently are characterized by small geographic ranges and low population abundance. Occasionally extinction events reach a global scale, with many species of all ecological types dying out in a near geological instant. In a conservative palaeontological sense, a mass extinction occurs when extinction rates accelerate relative to origination rates such that over 75% of species disappear within a geologically short interval (typically less than 2 million years).

Over the past 500 years, humans have triggered a wave of extinction, threat, and local population declines that may be comparable with the five previous mass extinctions of Earth’s history. 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. 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.

The percentage of species of land mammals from five major continents/
subcontinents in the period ∼1900–2015 (From Ceballos et al., 2017)

The most recent Living Planet Index (LPI) has estimated that wildlife abundance on the planet decreased by as much as 58% between 1970 and 2012. Several species of mammals that were relatively safe one or two decades ago are now endangered. The highest percentage of decreasing species is concentrated in tropical regions, mostly in the Neotropics and Southeast Asia. In 2016, there were only 7,000 cheetahs in existence, and less than 5,000 Borneo and Sumatran orangutans. Populations of African lion has dropped 43% since 1993, and populations of giraffes dropped from around 115,000 individuals in 1985 to around 97,000 representing what is now recognized to be four species (Giraffa giraffa, G. tippelskirchi, G. reticulata, and G. camelopardalis) in 2015.

Amphibians offer an important signal to the health of biodiversity; when they are stressed and struggling, biodiversity may be under pressure. Decreasing amphibians are prominent in Mexico, Central America, the northern Andes, Brazil, West Africa, Madagascar, India, Indonesia and Philippine. In the case of reptiles, the proportional decline concentrates almost exclusively in Madagascar; and decreasing species of birds are found over large regions of all continents. Other studies document that invertebrates and plants are suffering massive losses of populations and species. Long-term distribution data on moths and four other insect orders in the UK show that a substantial proportion of species have experienced severe range declines in the past several decades. Therefore, the acceleration of extinctions over the past decades, 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.

 

References:

Gerardo CeballosPaul R. Ehrlichand Rodolfo Dirzo; Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines, Proceedings of the National Academy of Sciences (2017) doi: 10.1073/pnas.1704949114 

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

 

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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