Robert
M May
Correspondence:
Robert M May robert.may@zoo.ox.ac.uk
Author
Affiliations
Department
of Zoology, University of Oxford, The Tinbergen Building, South Parks
Road, Oxford OX1 3PS, UK
BMC
Biology 2012, 10:106 doi:10.1186/1741-7007-10-106
The
electronic version of this article is the complete one and can be found online
at: http://www.biomedcentral.com/1741-7007/10/106 a which the two
main charts will be found.
Received:
13 December 2012 /Accepted: 19 December 2012/ Published: 20 December 2012
©
2012 May; licensee BioMed Central Ltd.
This
is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits
unrestricted use, distribution, and reproduction in any medium, provided the
original work is properly cited.
Extinctions
have happened ever since life began - is there anything different about
man-made extinctions?
Looked
at in the large, the history of life on Earth is one of continuous change,
driven by the interplay between evolutionary processes and the altered
environments that can result. Some of these environmental events have had
external causes (for example, the asteroidal impact that caused the most recent
of the so-called Big Five mass extinctions, which eliminated the dinosaurs),
while others have arisen from changing interactions among species (for example,
the early appearance of oxygen in the atmosphere, resulting essentially from
biogeochemical processes in primitive ecosystems). Are the recent past and
impending future extinctions, unambiguously caused by humans, different? Yes
and no. No, in the sense that the explosive growth of the animal
species Homo sapiens can be seen as just another evolutionary process
with increasingly serious ecological consequences for other species. Yes, in
the sense that - unlike earlier extinctions - the causative agent (that's us)
is aware of what is happening and could act to reverse current trends.
Unfortunately, we show few signs of doing so.
What
are the major causes of extinctions (man-made or otherwise)?
The
causes of recent, human-associated extinctions are usually listed under three
headings: over-exploitation, habitat destruction, introduced aliens. But you
could, with a bit of a stretch, brigade many past extinctions under one or more
of these headings. The above-mentioned demise of the dinosaurs, or the massive
wave of marine extinctions which mark the end of the Mesozoic, could be called
'habitat change'. The opening and closing of land bridges, as tectonic plates
moved around over the past billion years and more, introduced 'invasive
aliens', which restructured many ecosystems. More generally, over geological
time-scales, natural evolutionary processes created changes within plant and
animal populations, with new winners and new losers. In that sense, humans look
like being the main agents of the Big Sixth wave of extinctions, on whose
breaking tip we currently stand.
How
are the man-made versions distinct?
The
very big difference between past extinctions and the current human-associated
ones is we understand what is happening. And we can, in principle, choose to
modify our behavior to preserve the awe-inspiring diversity of plant and animal
life we have inherited. Even were we to do this - and we show few signs of it -
there would still, over relatively long time-scales, be changes. They would,
however, be more likely to be the pseudo-extinctions technically referred to as
'relay and replacement', as in the series of differently named species along
the continuum as Eohippus evolved into today's horse.
How much
do we know about the rate of extinction before humans started interfering?
As
in so many areas of science, we know quite a lot, and continue to learn more.
One measure of our knowledge is indicated in Table 1, by Raup, which gives
the estimated average lifetime, from origination to extinction, of a variety of
animal groups. Figure 1 complements this by showing numbers of
families (remember the taxonomic hierarchy: species, genus, family,...) of
marine animals over the sweep of geological time. The figure testifies to
increasing diversity and species richness, interrupted by episodes of mass
extinction. Overall, these data suggest average life-spans of animal species in
the fossil record to be around 1 to 10 million years, with significant
variation within and among taxonomic groups, and with the higher end of the
range being more common.
Figure
1. The history of the diversity of the marine animal families over the
past 600 million years. The solid line connects 77 data points, each showing
the total number of well-skeletonized families known from a particular
geological epoch (each of whose duration is indicated by the width along the ×
-xis). The numbered arrows indicate the five recognized episodes of 'mass
extinction'; the one labeled 3 is that which separates the Paleozoic from the
Mesozoic, and the one labeled 5 is the one that ended the dinosaurs and ushered
in our own era.
How
much do we know about the rate of extinction after humans started interfering?
Here
our knowledge should be better. Part of the problem is what a management
consultant might call the misallocation of resources. The workforce of
systematists and taxonomists is estimated to be apportioned roughly equally
among vertebrate animals, invertebrate animals and plants (with microorganisms
an order of magnitude smaller). Yet the known number of vertebrate species is
smaller than those of plant species and invertebrate species by one and two
orders of magnitude, respectively. Things get worse as we move to research
literature on conservation biology: a recent study of 2,700 papers published
over 15 years in the two top conservation research journals shows 69% on
vertebrates (four-fifths of the 69% on birds and mammals), 20% on plants, and
11% on invertebrates (one-third of the 11% on Lepidoptera). Nevertheless, if we
assume that documented extinctions among birds and mammals occur at a rate
typical of other groups, we can make an indirect assessment of the recent
acceleration in extinction rates. The IUCN Red Data Books document the extinction
of roughly one bird or mammal species each year over the past century. This is,
in effect, a group of around 1,400 species each playing a game of Russian
Roulette with one bullet in a revolver with 1,400 chambers. On this basis, each
can expect to survive around 1,000 years. In relation to the 1 to 10 million
year expectation noted above, this represents a speeding-up of extinction rates
by a factor 1,000 to 10,000. Figure 2 shows this in more detail.
Figure
2. Extinctions per thousand species per millennium. This figure, taken
from the Millennium Ecosystem Assessment, shows the estimated average lifetime
of species in particular groups of animals, at different periods. 'Distant
past' refers to average extinction rates as estimated from the fossil record.
'Recent past' is for extinction rates calculated from known extinctions of
species (the lower estimate) or known extinctions plus 'possibly extinct'
species (upper bound) over the past century or so. 'Future' extinctions are
derived from a variety of different models, all based on current trends, but
considerably uncertain (as indicated by the wider range).
Do
we even know enough about how many species there are today?
If
the Star Ship 'Enterprise' were to land on Earth, what would be the first
question the crew asked of our planet? I think it would be, how many distinct
species are there here? I think they would be shocked by our ignorance. We do
have very good knowledge of how many bird species there are. The International
Ornithological Congress says 10,448, although some would argue plus or minus
500. The mammalian total is smaller, 5,000 give or take 10%. Plant species add
up to around 300,000. There are approximately 1 million known insects, but the
true number could be several times this. Adding other smaller taxons gives a
total species count in the neighborhood of maybe 1.7 million, although
unresolved synonyms - same species identified and named separately in different
museum collections - may inflate this. Estimates of the true total, in my opinion,
are in the plausible range of 3 to 8 million distinct eukaryotic species. In
other words, we have documented only one half, maybe only one-fifth, of our
planet's biological diversity.
Why
should we be concerned about extinctions?
I
would distinguish three kinds of concern.
The
first might be called narrowly utilitarian: the plant and animal species that
are being extinguished could represent important genetic resources for
tomorrow's biotech revolution. We are burning the books before we have read
them. I think this is a weak argument, because tomorrow's advances in
understanding the molecular machinery of life will, I believe, see us (for
example) designing new drugs from the molecules up.
The
second might be called broadly utilitarian: although the services provided by
ecosystems, which are many and varied, are not taken into account in
conventional measures of gross domestic product (GDP) , they nevertheless are
very important to us (and insofar as they can be given a value, it is estimated
to be roughly of the magnitude of the more conventional global GDP). The
Millennium Ecosystem Assessment classifies these services under 24 headings,
and finds that 15 of these are being degraded, 4 are improving, and 5 are such
that we know too little to assess. Deplorable though this is, I believe we may
be smart enough to survive in a biologically impoverished world. It would,
however, be an unattractive world resembling that of the cult movie Blade
Runner.
Which
brings me to the third argument, which is that we have an ethical
responsibility not to deprive tomorrow's world of its heritage. Aldo Leopold
expressed it well, mourning the death of Martha, the last passenger pigeon: 'We
grieve because no living man will see again the onrushing phalanx of victorious
birds sweeping a path for Spring across the March skies, chasing the defeated
Winter from all the woods and prairies.... Our grandfathers, who saw the glory
of the fluttering hosts, were less well-housed, well-fed, well-clothed than we
are. The strivings by which they bettered our lot are also those which deprived
us of pigeons. Perhaps we now grieve because we are not sure, in our hearts,
that we have gained by the exchange'.
Need
it be an exchange?
That's
the question.
Where
can I go for more information?
See
references [1-5].
Books
Lawton
JH, May RM: Extinction Rates. Oxford University Press; 1995.
Raven
PH: Nature and Human Society: The Quest for a Sustainable World.
Washington DC: National Academy Press; 1977.
Millenium
Ecosystem Assessment: Ecosystems and Human Well-being: Synthesis.
Washington, DC: Island Press; 2005.
References
Raup
DM: Cohort analysis of generic survivorship.
Paleobiology 1978, 4:1-15.
Paleobiology 1978, 4:1-15.
Sepkoski
JJ: Phylogenetic and ecological patterns in the Phanerozoic history of
marine biodiversity. In Systematics, Ecology, and the Biodiversity
Crisis. Edited by Eldridge N. Columbia University Press; 1992:77-100.
Benton
MJ: Diversification and extinction in the history of life.
Science 1995, 268:52-58. PubMed Abstract | Publisher Full Text
Science 1995, 268:52-58. PubMed Abstract | Publisher Full Text
Costello
MJ, May RM, Stork NE: Can we name the Earth's species before they go
extinct?
Science 2012, in press.
Science 2012, in press.
Stork
NE: Re-assessing current extinction rates.
Biodiv Conserv 2010, 19:357-371. Publisher Full Text
Biodiv Conserv 2010, 19:357-371. Publisher Full Text
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