[MCN] The 6th Mass Extinction is already underway - mammals at risk

[Lance Olsen]

Science Advances  19 Jun 2015: Vol. 1, no. 5, e1400253
DOI: 10.1126/sciadv.1400253
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"The analysis we present here avoids using 
assumptions such as loss of species predicted 
from species-area relationships, which can 
suggest very high extinction rates, and which 
have raised the possibility that scientists are 
"alarmists" seeking to exaggerate the impact of 
humans on the biosphere (26). Here, we ascertain 
whether even the lowest estimates of the 
difference between background and contemporary 
extinction rates still justify the conclusion 
that people are precipitating a global spasm of 
biodiversity loss."
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RESEARCH ARTICLE - ENVIRONMENTAL SCIENCES
Accelerated modern human-induced species losses: 
Entering the sixth mass extinction
Gerardo Ceballos, Paul R. Ehrlich, Anthony D. 
Barnosky, Andrés García, Robert M. Pringle and 
Todd M. Palmer

Keywords
Sixth mass extinction, vertebrate extinctions, 
rates of extinction, background extinction, 
modern vertebrate losses

Abstract (full article open access at link)
<<http://advances.sciencemag.org/content/1/5/e1400253.full>>

The oft-repeated claim that Earth's biota is 
entering a sixth "mass extinction" depends on 
clearly demonstrating that current extinction 
rates are far above the "background" rates 
prevailing in the five previous mass extinctions. 
Earlier estimates of extinction rates have been 
criticized for using assumptions that might 
overestimate the severity of the extinction 
crisis. We assess, using extremely conservative 
assumptions, whether human activities are causing 
a mass extinction. First, we use a recent 
estimate of a background rate of 2 mammal 
extinctions per 10,000 species per 100 years 
(that is, 2 E/MSY), which is twice as high as 
widely used previous estimates. We then compare 
this rate with the current rate of mammal and 
vertebrate extinctions. The latter is 
conservatively low because listing a species as 
extinct requires meeting stringent criteria. Even 
under our assumptions, which would tend to 
minimize evidence of an incipient mass 
extinction, the average rate of vertebrate 
species loss over the last century is up to 114 
times higher than the background rate. Under the 
2 E/MSY background rate, the number of species 
that have gone extinct in the last century would 
have taken, depending on the vertebrate taxon, 
between 800 and 10,000 years to disappear. These 
estimates reveal an exceptionally rapid loss of 
biodiversity over the last few centuries, 
indicating that a sixth mass extinction is 
already under way. Averting a dramatic decay of 
biodiversity and the subsequent loss of ecosystem 
services is still possible through intensified 
conservation efforts, but that window of 
opportunity is rapidly closing.

INTRODUCTION
The loss of biodiversity is one of the most 
critical current environmental problems, 
threatening valuable ecosystem services and human 
well-being (1-7). A growing body of evidence 
indicates that current species extinction rates 
are higher than the pre-human background rate 
(8-15), with hundreds of anthropogenic vertebrate 
extinctions documented in prehistoric and 
historic times (16-23). For example, in the 
islands of tropical Oceania, up to 1800 bird 
species (most described in the last few decades 
from subfossil remains) are estimated to have 
gone extinct in the ~2000 years since human 
colonization (24). Written records of extinctions 
of large mammals, birds, and reptiles date back 
to the 1600s and include species such as the dodo 
(Raphus cucullatus, extinguished in the 17th 
century), Steller's sea cow (Hydrodamalis gigas, 
extinguished in the 18th century), and the 
Rodrigues giant tortoise (Cylindraspis peltastes, 
extinguished in the 19th century). More species 
extinction records date from the 19th century and 
include numerous species of mammals and birds. 
Records of extinction for reptiles, amphibians, 
freshwater fishes, and other organisms have 
mainly been documented since the beginning of the 
20th century (14, 17). Moreover, even in species 
that are not currently threatened, the 
extirpation of populations is frequent and 
widespread, with losses that far outstrip 
species-level extinctions (18, 25). 
Population-level extinction directly threatens 
ecosystem services and is the prelude to 
species-level extinction (18).

Here, we analyze the modern rates of vertebrate 
species extinction and compare them with a 
recently computed background rate for mammals 
(7). We specifically addressed the following 
questions: (i) Are modern rates of mammal and 
vertebrate extinctions higher than the highest 
empirically derived background rates? (ii) How 
have modern extinction rates in mammals and 
vertebrates changed through time? (iii) How many 
years would it have taken for species that went 
extinct in modern times to have been lost if the 
background rate had prevailed? These are 
important issues because the uncertainties about 
estimates of species loss have led skeptics to 
question the magnitude of anthropogenic 
extinctions (26) and because understanding the 
magnitude of the extinction crisis is relevant 
for conservation, maintenance of ecosystem 
services, and public policy.

Until recently, most studies of modern extinction 
rates have been based on indirect estimates 
derived, for example, on the rates of 
deforestation and on species-area relationships 
(11, 14). Problems related to estimating 
extinction since 1500 AD (that is, modern 
extinctions) have been widely discussed, and the 
literature reflects broad agreement among 
environmental scientists that biases lead to 
underestimating the number of species that have 
gone extinct in the past few centuries-the period 
during which Homo sapiens truly became a major 
force on the biosphere (1-4, 6-8, 14, 15). 
However, direct evaluation is complicated by 
uncertainties in estimating the incidence of 
extinction in historical time and by 
methodological difficulties in comparing 
contemporary extinctions with past ones.

Less discussed are assumptions underlying the 
estimation of background extinction rates. The 
lower these estimates, the more dramatic current 
extinction rates will appear by comparison. In 
nearly all comparisons of modern versus 
background extinction rates, the background rate 
has been assumed to be somewhere between 0.1 and 
1 species extinction per 10,000 species per 100 
years (equal to 0.1 to 1 species extinction per 
million species per year, a widely used metric 
known as E/MSY). Those estimates reflect the 
state of knowledge available from the fossil 
record in the 1990s (7, 9-13). In a recent 
analysis, which charted the stratigraphic ranges 
of thousands of mammal species, extinction rates 
were measured over intervals ranging from single 
years to millions of years, and the mean 
extinction rate and variance were computed for 
each span of time (7). In this way, the 
background extinction rate estimated for mammals 
was estimated at 1.8 E/MSY, here rounded upward 
conservatively to 2 E/MSY (that is, 2 extinctions 
per 100 years per 10,000 species). This is double 
the highest previous rough estimate.

Those previously estimated background rates were 
primarily derived from marine invertebrate 
fossils, which are likely to have greater species 
longevity than vertebrates (10, 15). Data 
deficiencies make it impossible to conduct 
empirical analyses (as was done for mammals) for 
non-mammal terrestrial vertebrates; therefore, we 
assume the background rates of other vertebrates 
to be similar to those of mammals. This 
supposition leads to a more conservative 
assessment of differences between current and 
past extinction rates for the vertebrates as a 
whole, compared with using the very low 
background extinction rate derived from marine 
invertebrates.

The analysis we present here avoids using 
assumptions such as loss of species predicted 
from species-area relationships, which can 
suggest very high extinction rates, and which 
have raised the possibility that scientists are 
"alarmists" seeking to exaggerate the impact of 
humans on the biosphere (26). Here, we ascertain 
whether even the lowest estimates of the 
difference between background and contemporary 
extinction rates still justify the conclusion 
that people are precipitating a global spasm of 
biodiversity loss.

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"We live in a moment of history where change is 
so speeded up that we begin to see the present 
only when it is already disappearing."

"We are not able even to think adequately about 
the behavior that is at the annihilating edge."

R. D. Laing. Introduction, The Politics of Experience. 
1967, New York. Pantheon Books, a division of Random House
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