Extinctions and Biodiversity in the Fossil Record
Ricard V Sol´e and Mark Newman
V olume2,The Earth system:biological and ecological dimensions of global environmental
change,pp297–301
Edited by西瓜汁怎么做
Professor Harold A Mooney and Dr Jop G Canadell
呈现英语
in
Encyclopedia of Global Environmental Change
(ISBN0-471-97796-9)
Editor-in-Chief非你莫属的意思
Ted Munn
John Wiley&Sons,Ltd,Chichester,2002
Extinctions and Biodiversity in the Fossil Record
Ricard V Sol´e and Mark Newman
Santa Fe Institute,Santa Fe,NM,USA
Life has existed on Earth for more than three billion years. Until the Cambrian explosion about540million years ago, however,life was restricted mostly to single-celled microor-ganisms that were,for the most part,poorly prerved in the fossil record.From the Cambrian explosion onwards,by contrast,we have a substantial fossil record of life’s devel-opment,which shows a number of clear patterns,including a steady increa in biodiversity towards the prent,punc-tuated by a number of large extinction events that wiped out a significant fraction of the species on the planet,and in some cas,caud major reorganizations amongst the dominant groups of organisms in the ecosphere.
The history of life on the Earth begins in the oceans during the Achaean eon,somewhere around3.8billion years ago, probably with the appearancefirst of lf-reproducing RNA molecules an
d subquently of prokaryotic single-celled organisms.At the start of the Proterozoic eon,around 2.5billion years ago,the atmosphere changed from reducing to oxidizing,as a result of the depletion of stocks of elemental iron in the Earth’s crust,and oxygen-breathing life became possible.Eukaryotes,multicellular life,and xual reproduction all appeared for thefirst time during the Proterozoic,although the exact order and dates are still in dispute,since the fossil record of this period is poor.The earliestfirm evidence of multicellularity dates from about 575million years ago.
About540million years ago,for unknown reasons,an enormous increa in the diversity of multicellular animals took place.This event,known as the Cambrian explosion, produced all the major body plans of animals en today,as well as a number of others that have since become extinct. Thefirst land-dwelling plants appeared during the Sil-urian period,about430million years ago,followed shortly afterwards by thefirst land-dwelling animals,which were incts,and then thefirst land-dwelling vertebrates during the Devonian.About250million years ago,at the end of the Permian period,the largest mass extinction of all time took place,killing at least90%of all species on the Earth, and ending the eon named the Paleozoic.
The Mesozoic,colloquially known as the age of the dinosaurs,followed the Paleozoic.In addition to dinosaurs, the Mesozoic also saw thefirst appearance of mammals and offlowering plants.It ended a
bout65million years ago with the Cretaceous–Tertiary(KT)extinction event,which wiped out the dinosaurs along with about70%of all other species then alive.The interval from the KT event until the prent,known as the Cenozoic eon,saw the radiation of the mammals tofill many of the dominant land-dwelling niches and,eventually,the evolution of mankind.
This information comes from geological studies and from the substantial fossil record of extinct lifeforms.The currently known fossil record includes about one-quarter of a million species,mostly dating from the time interval between the Cambrian explosion and the prent.There are numerous bias in the fossil record,which make accurate quantitative investigations difficult,including the following.
1.Older fossils are harder tofind becau they are
typically buried in deeper rocks than more recent ones.
2.Accurate dating of fossils is difficult.Radiocarbon
dating,for example,is not uful for rocks that are hundreds of millions of years old.Radioactive isotopes other than carbon with longer half-lives are ud for most of the geologic time-scale,but resolution of dates using the isotopes can be poor.
3.Particularly rich fossil beds,or particularly zealous
investigators,may produce very complete records for some time periods or groups of organisms,while other periods or groups may be comparatively poorly rearched.
4.Prehistoric environmental disturbances can upt the
deposition process by which fossils are formed and give ri to time periods in which the fossil record is poor.
5.Marine organisms tend to be much better prerved
than land-dwelling ones,becau deposition is much more uniform and reliable in the oceans than it is on land.
Despite the bias,a number of trends are clear from the fossil record.
Figure1shows a plot of the best estimate of the number of living families of marine organisms,as a function of time since the start of the Cambrian.As the plot shows, diversity appears to increa substantially over time,and this is believed to be a real trend.On average,there have been more famil
ies,and indeed more species,alive in recent times than earlier.Over the cour of the plot, diversity appears to increa by a factor of aboutfive, although some of this increa is an artifact of the greater availability of fossils of recent species in more easily accessible rocks.
Figure2shows the extinctions,again of marine organ-isms,as a function of time over the same interval.The graph gives the number of families becoming extinct per strati-graphic stage.Stages are uneven intervals of time bad on recognizable geological and paleobiological markers,with typical length about7million years.As thefigure shows,
2
THE EARTH SYSTEM:BIOLOGICAL AND ECOLOGICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE
菠萝蜜籽能吃吗
Time before prent in millions of years
200400600
80010001200N u m b e r o f k n o w n f a m i l i e s
Figure 1Number of known marine families alive over the time interval from the Cambrian to the prent.(The data are taken from Sepkoski,1992)
500
400
300
200
100
Time before prent in millions of years
F a m i l i e s b e c o m i n g e x t i n c t p e r s t a g e
Figure 2Estimated extinction of marine animals in fami-lies per stratigraphic stage since the Cambrian.The arrows indicate the positions of the big five mass extinction events discusd in the text
there has been considerable variation in the intensity of extinction over prehistoric time.Of particular note are the five large peaks in extinction marked with arrows.The are the big five mass extinction events that marked the ends of the Ordovician,Devonian,Permian,Triassic,and Creta-ceous periods.A sixth peak in the Cambrian is also visible,but this is thought probably to be an artifact of poor fossil prervation during that period rather than a real extinction event.The basic features of the big five extinctions are as follows (e also Table 1).
The end-Ordovician event about 440million years ago appears to have occurred in two bursts,parated by about 1million years,which between them wiped out about 85%
Table 1Extinction intensities at the genus and species level for the big five mass extinctions of the Phanerozoic.Estimates of genus extinction are obtained from directed analysis of the fossil record while species loss is inferred using a statistical technique called rever rarefaction.(Figures are taken from Jabionski,1991)Genus loss (%)Species loss (%)Extinction
(obrved)
(estimated)
End Ordovician 6085Late Devonian 5783Late Permian 8295End Triassic 5380End Cretaceous
47
76
of then-living species.The event was con fined to marine species,since multicellular life had not yet colonized the land.Particularly affected were brachiopods,bivalves,echinoderms,bryozoans,and coral
s.The immediate cau of extinction appears to have been the continental drift of a signi ficant landmass into the south polar region,causing a global temperature drop,glaciation,and conquent lowering of the a level,which destroyed species habitats around the continental shelves.The a level ro again with the end of the glacial interval about a million years later and caud a cond burst of extinction.
The late-Devonian extinction around 360million years ago is complex and rather poorly understood.It is probably,in fact,compod of a number of parate events –as many as ven –spread over about 25million years,including particularly notable extinctions at the ends of the Give-tian,Frasnian,and Famennian stages.Overall,about 80%of living species died out in the late Devonian.Particularly hard hit were corals,brachiopods,bryozoans,ammonoids,and fish.The caus of the extinctions are unclear.The leading theories suggest that changes in a level and ocean anoxia,possibly triggered by global cooling or oceanic vol-canism,were most likely responsible,although the impact of an extraterrestrial body such as a comet has also been considered.
The late-Permian extinction around 250million years ago was the largest extinction event of all time,killing some 95%of marine species and about 70%of land-dwelling species.Like the end-Ordovician event,it ems to have been compod of two bursts,parated by an interval of about 1
0million years,the cond being the larger of the two.Notable extinction happened again amongst bra-chiopods,ammonoids,and corals,as well as gastropods and,unusually,incts.Despite an enormous amount of rearch on the subject,the caus of the late-Permian event are still a subject of debate.It is clear,however,that the a level ro during this period,levels of oxygen in the oceans were low,and carbon dioxide (CO 2)levels were high.There is some suggestion that a cometary impact may have been involved,or a shift in ocean circulation driven
EXTINCTIONS AND BIODIVERSITY IN THE FOSSIL RECORD3
by climate change,or CO2and sulfur relea following large-scale volcanic activity.The late-Permian event had a profound effect on the terrestrial ecosystem,which is still being felt today,a quarter of a billion years later.
A particularly notable example amongst marine faunas is that of the bivalves,a relatively minor group during the Paleozoic that took advantage of the ecological vac-uum left by the extinction to establish a solid grip on shallow-water environments,leading to their dominance over the previously very successful brachiopods and gas-tropods.
The end-Triassic extinction around210million years ago is probably the most poorly understood of th
e bigfive extinction events.It appears to have killed about80%of species then living,either in one burst or possibly in two, parated by about20million years.Major extinction is obrved particularly amongst ammonoids,bivalves,gas-tropods,and brachiopods.Leading theories of the caus of the end-Triassic event are ocean anoxia,massive volcanism, or possibly an asteroid impact.
The end-Cretaceous event,usually called the KT event, has attracted the most popular interest of any extinction becau it saw the death of tho perennial movie stars,the dinosaurs,but it was in fact the smallest by quite a wide margin of all the bigfive.The KT event appears to have been a single pul of extinction around65million years ago,which wiped out about70%of all species then living. As well as the dinosaurs,it extinguished many other land-dwelling vertebrates,especially large-bodied ones,along with large numbers of(marine)bivalves,gastropods,and foraminifera.The proximal cau of the KT event was, almost certainly,the impact of a large comet or meteor near the prent site of the town of Puerto Chicxulub on the Yucat´a n peninsula in eastern Mexico,with an associated drop in a level and possibly short-term cooling or heating, or acid rain.
The fossil record can give us valuable insight into the nature of extinction and the effects of large-scale envi-ronmental change.It also indicates that recovery from extinction is a slow process by human standards,typically taking on the order of5or10million years.Thus,it is important to fend off su
ch extinctions before they happen, rather than hoping that the ecosystem will prove robust enough to take care of itlf.
Comparison between fossil and prent-day extinction is not straightforward since,as mentioned above,the fos-sil record consists largely of marine organisms,whereas interest in contemporary extinction focus mostly on land-dwelling organisms.Also recently extinct or currently endangered species tend to be rare,whereas the fossil record primarily reflects the most abundant and numerous biotas. Still,there are a number of general patterns in the fossil extinction record that may help us in the conrvation of modern-day biodiversity.
First,we note that habitat loss,such as the destruction of shallow-water environments on the continental shelf as a result of changes in a level,appears to have been an important cau of extinction again and again.A ri in a level as a result of global warming over the next century,for example,could be devastating for reef communities.In this context,human-driven habitat loss is leading to extensive habitat ,to the generation of spatially isolated subareas.In many cas the subdivision of a large population into many weakly connected subpopulations substantially increas extinction risk.Again,the fossil record is very uful in providing a source of comparison. Habitat fragmentation enhanced the extinction of large mammals during the Pleistocene
(75%of them are extinct). The area reduction during glacial cycles led to widespread habitat fragmentation and eventually to the extinction of many species.A considerable number of current endemisms in plant species are due to the confinement to small areas during the Ice Age.
Second,most past extinction events appear to have been lective to some extent.The end-Ordovician event,which was associated with a period of global cooling and glacia-tion,particularly favored species that were well adapted to cold-water conditions,and was particularly harsh on tho who were not,for obvious reasons.The KT bound-ary event,as mentioned previously,appears to have come down especially hard on large-bodied animals.A number of explanations for this latter effect have been put forward. Large-bodied animals have smaller populations and greater area requirements,and are thus more nsitive to habitat loss or fragmentation.Moreover,their trophic requirements and low rates of population growth make them slow to recover from environmental change.It has been suggested that the loss of large herbivorous faunas could trigger major changes in biogeographic vegetation patterns that could in turn trigger further extinctions.
This last obrvation leads us to an additional question: to what extent do interactions between species affect the respon of an ecosystem to environmental stress?Ecolog-ical interdependence between species may have heightened the impact of some of the mass extinctions;some studies, for
example,have suggested that the collap of marine food chains at the end-Cretaceous contributed to the KT extinction event.The KT event had a rapid effect on most biotas and a subquent long-term effect perhaps related to a decrea in primary productivity.This ems par-ticular likely in the ca of marine biotas,which show a marked dip in the rate of accumulation of carbonates, especially calcium carbonate,following the KT boundary, indicating a decrea in productivity.The resulting decrea in food supply would then produce extinctions at higher trophic levels.Similar mechanisms may also have been at work during the human-driven end-Pleistocene extinctions of mastodons and mammoths in North America,which were
4THE EARTH SYSTEM:BIOLOGICAL AND ECOLOGICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL
CHANGE
Rabbits
Grass-I
Grass-II
Large blue butterfly
Ants-caterpilars
+−
−
−
+
Figure3An example of coextinction:the introduction of
the myxoma virus to rabbit warrens in England triggered
the extinction of the Large Blue butterfly(Maculina arion),
an already endangered,endemic species.Myxomatosis
caud a dramatic decline in rabbit populations and
a conquent surge in the abundance of a species of
grass on which the rabbits previously grazed(Grass-I).
席与时
Another previously dominant species of grass(Grass-II)
was unable to compete and declined substantially in its
abundance,giving ri to a shortage of nesting material
for certain variety of ant(Mynnica).As a result of a
symbiotic dependence between the butterfly Maculina and
this species of ant,the butterfly then became extinct
associated with widespread changes in vegetation patterns
and the disappearance of many other species.One may小兔子的特点
well ask whether the current extinction of large herbivorous
species in African ecosystems will result in similar con-
current extinctions.It is also believed that symbiotic and
parasitic species are particularly vulnerable to extinction,
as a result of their dependence upon a partner or host.
Many living species may,in fact,already be doomed to
extinction,becau of the loss of a partner in an esn-
tial symbiotic relationship(Figure3).The situation with
parasites may be even wor,since most parasites are spe-
cific to a single species of host,and extinction amongst
parasites often goes unnoticed,even though their impor-
tance in maintaining diversity has been stresd by many
studies.Since all known animals and plants have some
parasitic load,extinctions of parasites must be widespread
in evolutionary history.The eggs of what are believed to
be ectoparasitic mites have been discovered on fossilized
dinosaur remains,indicating that coevolutionary parasitism,
and hence coextinction(the coupled extinction of ecological
partners)is an ancient phenomenon.Estimating extinc-
tion rates for parasites is unfortunately difficult,since they
are poorly reprented or difficult to identify in the fossil
record,and internal parasites are basically not prerved
at all.
Overall,recent extinction rates in most plant and ani-
mal taxa are relatively low,but for some groups they
do approach the levels associated with prehistoric mass
extinction events(Table2).The obrvations have led
some rearchers to suggest that the biosphere may be
on the verge of another such event.Some claim that the
world has already entered a sixth period of mass extinc-
tion,driven primarily by the human population explosion.
Theoretical studies of long-term ecological respons to
habitat destruction suggest that steady increas in extinc-
tion rates are to be expected in the near future.Paleonto-
logical studies indicate that rare,localized,or specialized
species that have evolved to survive in particular niches are
the mostly likely to become extinct,while widespread or
adaptable species,or opportunistic colonizers are likely to
prevail.The fossil record is thus more than a cautionary
tale.
Table2Comparison between different features of past mass extinction events(as
reported from the fossil record)and prent-day,human-driven mass extinction.
Here the effects of habitat loss are widespread but different mechanisms
were at work:changes in a levels and continental breakup(fossil record)
and human-driven habitat destruction/fragmentation(prent day)respectively.
(Estimations of species loss are from Wilson,1992)
Fossil record Prent day
Time resolution¾105–106years1year
Most affected biotas Tropical biotas Coral reefs,rainforests
Selectivity on size Large-sized species Large-sized species
Loss of endemics Not well known Widespread
Effects of habitat loss Widespread Widespread
Time scale of recovery¾5–10Myr Not known
皇帝游戏
Direct effects on food webs Very important Very important
Indirect effects on food webs Not known Very important
Extinction rates75–80%¾104species year 1
EXTINCTIONS AND BIODIVERSITY IN THE FOSSIL RECORD5
As paleontologist David Jablonski has put it:
The lessons from the past are inevitably blurry at a coar scale.At the prent stage of knowledge,the fossil record is more revealing for potential long-term conquences than for immediate solutions.However,the history of life of Earth pro-vides an array of hat are sufficiently spectacular to militate against inaction.
REFERENCES
Jablonski,D(1991)Extinctions:A Paleontological Perspective, Science,253,754–757.家常山药
Sepkoski,Jr,J J(1992)A Compendium of Fossil Marine Animal Families,Milwaukee Public Muum Contributions in Biology
and Geology,83,2nd edition,Milwaukee Public Muum, Milwaukee,WI.
Wilson,E O(1992)The Diversity of Life,Harvard University Press,Cambridge,MA.
FURTHER READING
Lawton,J H and May,R M,eds(1995)Extinction Rates,Oxford University Press,Oxford.
Leakey,R and Lewin,R(1995)The Sixth Extinction,Doubleday, New York.
Mckinney,M and Drake,J,eds(1998)Biodiversity Dynamics, Columbia University Press,New York.
