Transcript Biodiversity

Biodiversity
How has the diversity of multicellular life
changed over time?
• Multicellular life started from only one kind
and is now many kinds, so, duh. Yes, the
number of different kinds of organism must
have increased over time.
• BUT has it been a steady increase? Is diversity
still increasing now? Or has the earth reached
some kind of limit on the number of kinds of
creatures that can exist?
Look at the numbers
• How many species are there now?
– About 1.8 million species of plants and animals
described
– Is that all there is? How could you know how many
more there might be?
– Estimates range from 5 million to 20+ million
• How many fossil species are there?
– About 300,000 have been named
– Hard to figure out from this how many species have
lived
Why is it hard to figure out past
diversity?
• Level of tabulation:
– Species problem – are we accurately recognizing
species? Some groups probably overestimated,
others underestimated.
– Orders and above are pretty far from natural
groups and are contentious
– Family level is reasonable compromise – close to
natural classification but easier to recognize than
species
Why is it hard to figure out past
diversity?
• Missing groups: unfossilizable taxa
– Perhaps at some times there were many softbodied organisms contributing to diversity
– The insect problem – unfossilizable but most of
the described species
Remember this?
Two thirds of all
described
multicellular life
are insects
Why is it hard to figure out past
diversity?
• Discovered fossil problem – sampling depends
on:
– Exposure of rocks of that age
– Volume of rocks of that age
• Silurian-Devonian transgression means there’s a lot of fossils
of that age, Carboniferous regression means there’s few of
that age
– Interest of paleontologists – different ages have
attracted different amounts of attention from
paleontologists
• Ease of access, economics, coolness factor
Why is it hard to figure out past
diversity?
• “Pull of the Recent”: tops of fossil ranges are
usually underestimates EXCEPT for extant
species.
• So living families count more (e.g. are
represented in more time periods) than
extinct ones.
So what can we do with the data we
have?
• Look at family or genus level data for skeletonized
marine invertebrates
• Eliminates the problem of identifying species
• Helps standardize identification (researchers may
be splitters or lumpers when it comes to species,
but families are universally recognized)
• Eliminates the problem of selective preservation
– we’re only comparing critters with hard parts
across time
• Eliminate the insect bulge
What kinds of patterns could we see?
• Equilibrium: there is a limit on the number of
species (families) that can exist, determined
by physical or ecological factors
• Expansion: either there is no limit on the
number of kinds of organisms, or we haven’t
reached that limit yet
Possible ways diversity could increase:
• Exponential: what you would expect if speciation
and extinction rates stay constant.
• Linear: implies that speciation rates decline, or
extinction rates increase
• S-shaped curve: diversity rises quickly at first,
then levels out as the Earth approaches some
limit on how many species there could be
First pass at the data –Sepkowski, 1990
What are the
patterns?
1. Two S-shaped curves: in the
Cambrian, then the
Ordovician
2. Rising diversity through the
Ordovician, then Paleozoic
plateau marked by a few
extinctions.
3. Big extinction event at end of
Permian lowers diversity.
4. Mesozoic to Cenozoic:
increasing diversity (linear?),
with downward blip at
Cretaceous-Paleogene
extinction.
So…
Expansion or equilibrium?
Constant speciation rates (exponential)
OR
Decreasing speciation rates or increasing
extinction rates
OR
Speciation rates damped by outside
forces? (S-shaped curve)
More detailed analysis: We can recognize three great faunas
in the history of life:
Cambrian
Paleozoic
Modern
The three faunas
• Cambrian: Trilobites, lingulate brachiopods,
archeocyathids, primitive echinoderms
• Paleozoic: rhynchonelliform brachiopods,
stony bryozoans, stromotoporoids,
cephalopods, crinoids, blastoids, graptolites
• Modern: bivalves, gastropods, echinoids,
crustaceans, vertebrates
The three faunas ecologically
• Cambrian: benthic organisms living only a few
cm above & below sea floor
• Paleozoic: sessile (attached) benthic
organisms, some rising above sea floor up to a
meter or so, slightly deeper burrows
• Modern: more mobile benthic fauna, deeper
burrowers, swimmers, abundant planktonics
Notice that the faunas exist outside of the time period they are named for, but are
most important during the time period of their name.
But maybe the way we are looking at
the data is not valid…
• Sepkowski’s database is by time period (first
and last)
• But the time periods are not the same length
• His analysis did not statistically account for
other biases: rock volume, abundance,
intensity of sampling.
A new approach: Alroy and others,
2001
• Divide the Phanerozoic into equally sized time
periods
• Sample the existing collections to statistically
simulate equally sized collections
• Perform rarefaction to evaluate whether the
number of groups represented is an accurate
picture of the total number of groups that
would have been alive
Rarefaction
• A method of statistically determining the
maximum expected number of different kinds
of things in a collection by repeated sampling.
• If lets you estimate how many more different
kinds of things you should have found that
you didn’t
Rarefaction activity
1. Create a data table: # of items, # of kinds
2. Reach in your bag of gumballs and pick one
without looking.
3. On your data sheet, record the number of items
you sampled (1) and the number of kinds of
things you found (1). Duh.
4. Put the gumball back.
5. Now sample 2 gumballs. How many different
kinds do you have? Record your data. Put the
gumballs back.
6. Continue the process. Each time you will sample
one more gumball then the last time.
Now let’s graph the data.
So what did Alroy
find with this new
method of analysis?
1. The Paleozoic plateau is gone
2. Instead there are peaks of
diversity in Devonian and
Permian
3. The Cenozoic increase is there,
but not as dramatic, and seems
to be plateau in Neogene
Another way to explore this question…
1. Rarefaction curves separated by latitude for Ordovician show similar numbers of genera in
both tropics and nontropics, but in Neogene there’s a big difference between the two.
2. There are probably still more Neogene tropical species to be found. – the curve has not yet
flattened out.
3. There are many more Neogene genera overall than in Ordovician.
So what to conclude?
• The increase in Cenozoic diversity seems to be
real but less dramatic than previously thought.
• No good evidence one way or another on
whether there’s a limit to possible diversity
• Diversity levels may be linked to plate
configurations or climate:
– Decrease in Paleozoic may track Pangaea formation
– Cenozoic increase may track increasing latitudinal
difference in climate.