Download Nov. 23, 2004 Extinction – the death of species Types of extinction

Nov. 23, 2004
Extinction – the death of species
Types of extinction:
Pseudoextinction
Terminal extinction
Magnitudes of extinction
Background extinction
Mass extinction
-the Big Five
-the Cretaceous-Tertiary
Effects of extinction
Last night in the museum's hall
The fossils gathered for a ball
There were no drums or saxophones,
But just the clatter of their bones,
A rolling, rattling, carefree circus
Of mammoth polkas and mazurkas.
Pterodactyls and brontosauruses
Sang ghostly prehistoric choruses.
Amid the mastodonic wassail
I caught the eye of one small fossil.
Cheer up, sad world, he said, and winkedIt's kind of fun to be extinct.
--Ogden Nash
Extinction is the rule, not the exception
Ninety-nine percent of all species that ever lived are now extinct, or, as the paleontologist
Dave Raup puts it, to a first approximation, all species are extinct.
The point here is that extinction is a very important evolutionary process, and, as a matter
of fact, an evolutionary process that is best studied by reference to the fossil record. This is
because we have, in fact no information on "natural" extinctions today. All of the extinctions
currently going on are extinctions in which humans have played (or are playing) a key role.
Consider first, three types of extinction
1. Local extinction, or extirpation. The geographically restricted demise of one of more
populations of a species, while other populations survive elsewhere. Most species are patchily
distributed, so wiping out one population may not mean wiping out all. Example: the horse
became extinct in North America about 10,000 years ago but survived in Eurasia. Horses were
“re-introduced” to North America when the Spaniards arrived.
2. Pseudoextinction, or phyletic extinction: where one species evolves into another species.
While the ancestor can be said to have gone extinct, the evolutionary lineage does, of course,
continue. There is no loss of species.
3. True extinction, or terminal extinction: where a species lineage becomes extinct. Here, there
is a loss of species
The question to ask at this point is: what proportion of extinctions seen in the fossil record are
pseudoextinctions and what proportion are true extinctions? Estimates suggest that about 20% of
the extinctions are of the pseudoextinction type and the rest, 80%, are true extinctions, but it's an
open question.
Extinction magnitude
How are extinctions measured?
What we'd like to be able to do is simply count up the number of species that make their last
appearance in some time interval. That would give us an extinction rate: the number of species
per million years, for example.
In sad fact, we have named and described only a very small sample of the number of species
actually in the fossil record. As a consequence, most paleontologists work at a higher taxonomic
level, usually the family level (species, genus, family). So, in practice, extinctions are commonly
measured as the number of families that make their last appearance in some time interval, so the
measure of extinction is number (or %) of families becoming extinct per million years.
The best record that we have is the marine record. Fossils there are relatively abundant and the
time control is pretty good. The record of terrestrial vertebrates is not as good, and that of
terrestrial plants even worse.
What the figure of extinction rate below suggests is two somewhat different magnitudes.
Background extinction and mass extinction. In fact, there is no sharp difference between
background extinctions and mass extinction, one does grade into another, but the distinction is
useful and is commonly made.
1. Background extinction
Note that the extinction rate here never falls to zero; there is always some extinction going on.
But most of the time it's going on at a relatively low rate, say about 5 families per million years.
This may be the sort of extinction that happens thanks to relatively small scale environmental
changes, disease, predation, psudoextinction, competition and the like.
Consider an interesting feature of this graph, focusing now just on the background
extinctions. There is a regular trend toward lower levels of background extinction through
geologic time.
Possible interpretations:
a. Species are becoming better adapted through geologic time. Evolution is building a
better organism.
b. The earth's environment is becoming more benign. Temperatures may not fluctuate as
much, volcanoes may not go off so often, glaciations may be less frequent. NO evidence for any
of this.
c. Change in the species/family ratio. Recall that what is plotted here is the rate of
extinction of families. What needs to be done to cause the extinction of a family is, of course,
the extinction of all the species in the family. Families with lots of species, like the Muridae, a
family of rodents, are difficult to make extinct - just `cause there's lots of them. How many
species would need to go extinct to cause the extinction of the family Hominidae? Just one.
So, if there's been a change in the number of species per family through geologic time,
such that Cambrian families had relatively few species and Tertiary families had, on average, lots
of species, family extinction rates would be high in the Cambrian and relatively low in the
Tertiary.
There is some evidence for an increase in the number of species per family during
geologic time.
2. Mass extinction. Notice how there are five groups of points on the graph that stand wellabove the background level of extinction.
These are the mass extinctions: extinctions in which extinction rates exceed background
rates by a factor of two or so (double background), on the average of about 10 families/million
years.
These extinctions affected a broad range or organisms (i.e., not confined to one
taxonomic group or another or one ecological group or another).
These extinctions may have been of relatively short duration. That is, they may have
been catastrophic, with intervals of time anywhere from a million years down to one bad
weekend. This is still a controversial topic. Were mass extinctions sudden or gradual?
These five groups of points in the marine record have come to be known as the "Big Five". A
quick look before I look at one of the big five in detail.
1. Late Ordovician 19.3 families/million years. 12 percent diversity drop (if recovery is
quick, there won't be much drop, so the drop is not a good measure of extinction
intensity).
Affected: trilobites, brachiopods, echinoderms.
Cause: associated in time with a glaciation and with a sea level drop.
2. Late Devonian. Approx 10 families/my, 14% diversity drop.
Affected: brachiopods, tabulate and rugose corals, sponges, early fish, conodonts.
Cause: Associated with black shales - low oxygen conditions? Some evidence for an
impact but it’s controversial.
3. end-Permian. Approx 15 families/my. 52% reduction in family diversity. Maybe as
many as 92% of all marine species.
Affected: brachiopods, corals, echinoderms, trilobites, cephalopods, sponges, fish....
almost every group affected.
Cause: Associated with lowered sea level and cooling climate but record in vicinity of
boundary very poor. Some evidence of an impact. Active Siberian volcanoes too.
Controversial.
4. Late Triassic 11 families/my, 12 % diversity drop.
Affected: bivalves, cephalopods, conodonts, fish
Cause: Possible impact record in Canada
5. end-Cretaceous 16 families/my, 11% drop in diversity.
Affected: dinosaurs, ammonites, marine microplankton, marine bivalves. (plants not
badly affected).
Cause: impact of a comet or asteroid. Still controversial to some.
The big picture below: diversity through time, evolutionary faunas, mass extinctions and some
possible causes for each.
The end-Cretaceous, or K/T extinctions:
The extinction of the dinosaurs on land, and the ammonites and other invertebrates in the
oceans about 65 million years ago has always been a topic of fascination and speculation.
Will Cuppy, in his book “How to become extinct” says: "The age of reptiles ended
because it had gone on long enough and it was all a mistake in the first place". Things were
more complicated than that.
The hard-core controversy kicked into gear about twenty years ago when Luis and Walter
Alvarez, a father-son team, analyzed some of the clay collected from the boundary between
Cretaceous and Paleogene (Tertiary) marine rocks in northern Italy, near a small town called
Gubbio.
What they found is illustrated below: very low levels of the element Iridium up to the
boundary, a high concentration in the boundary clay, then a quick decline to background levels
in Tertiary limestones.
Elements like Ir are not common in the rocks of the earth's crust: they are common,
however, in meteorites (samples of planetary interiors).
So, from the evidence of the unusual elements in the boundary clay they suggested what
has come to be called the Impact hypothesis: namely - that 65 million years ago, the earth was
struck by an large asteroid (10 km in diameter).
Evidence:
High Ir levels in boundary sections around the world
Rapid extinction in marine and terrestrial sections
Soot in boundary clays (fires)
Shocked quartz in boundary clays (especially in N. America) - impact feature
Glass spherules - from melted rock
Large 65 Ma crater in Mexico’s Yucatan Peninsula, 200 km diameter
Sedimentary deposits from large tidal waves “tempestites” in Mexico and Texas
Scenario
The impact pulverizes the asteroid and a good deal of the crust where the asteroid struck.
The pulverized debris was injected into the atmosphere, where it blocked sunlight (total darkness
for 50 days in one scenario) and rapidly cooled the climate.
The darkness shut off photosynthesis, killing the short-lived oceanic phytoplankton, cutting off
the food supply of herbivorous dinosaurs.
That, plus the cooling, should have been enough to cause the extinctions.
Acid rain from nitrous oxides created from passage of asteroid?
Giant tidal waves,
Global wildfires?
Arguments against:
The impact hypothesis, while it has lots of support and supporters, is not without its
weaknesses and its opponents.
Counter arguments include:
-The idea that explosive volcanism could have done the trick, by injecting lots of dust
into the upper atmosphere, etc, etc. And some volcanic eruptions are relatively enriched with
Iridium. The Deccan Traps of India, for example, are a series of volcanic deposits approximately
65 million years old. [But greater age range than extinctions, no shocked minerals, spherules, or
soot, though].
-The contention that the extinctions of dinosaurs, ammonites and the like were gradual
affairs, stretched out over several million years. If they were gradual, a sudden impact couldn't
have caused them. [Weak evidence because poor sampling near the boundary would produce a
record that looked like a gradual extinction.]
Consequences of extinction
No matter how extinctions might be caused, they have evolutionary consequences - they have
strongly affected the history of life. Consider, for example:
1. Removing incumbents. The K/T extinctions wiped out the dinosaurs. That seemingly allowed
the diversification of the mammals. So, if it hadn't been for that volcano or asteroid, we wouldn't
be here.
Graph of diversity through time for Cretaceous and Cenozoic mammals. Mammal diversity
increased dramatically after the extinction of the dinosaurs.
2. Chance effects. The biggest extinction of all, at the end of the Permian, has been estimated to
have wiped out 93 percent of all the species then living. A mighty close call. But that's not the
point.
The point is that those remaining 7 percent are the founding members of the postPaleozoic biota. With extinction levels so high, it's a good bet that some species survived just
because of chance, not because of some superior adaptations. This suggests that chance has
played a large role in determining the composition of the earth's biota. Extinction, bad genes or
bad luck?
In fact, extinction doesn’t seem to be very selective. There’s a weak correlation between
traits such as geographic range and survival (species with large geographic ranges tend to have
longer geologic ranges) and population size and survival (species with larger populations tend to
persist longer). The dinosaurs didn’t have it coming; they were just unlucky. Were mammals
”superior” just because they survived an asteroid impact?
3. Ecological effects.
Zombies
Dodo – extinct, flightless bird, discovered 1598, extinct by 1681.
Dodo and the Mauritius – islands in the western Indian Ocean.
Calvaria tree also found in Mauritius, but no seedlings known, only old trees.
Seeds were found to germinate only after the exterior surfaces were mechanically ground away
somewhat, or were passed through the gut of a turkey.
The extinction of the dodo may lead to the extinction of the tree.
Calivaria tree is a zombie, the living dead. A delayed effect of the extinction of another species.