The Roots of Biodiversity:

1
The Roots of Biodiversity An Investigation of
Jablonski, Roy, and Valentines Out of the
Tropics Evolutionary Dynamics of the Latitudinal
Diversity Gradient Elizabeth Carlton December
7, 2006
2
The Issue

• The Latitudinal Diversity Gradient
• Increase in the number of species from the poles
  to the tropics
• Over 10 times more species in the tropics than
  the arctic
• Scientists are still trying to figure out what
  causes this gradient
• Hillebrand Study (2004)
• Performed meta-analysis searching abstracts with
  search terms latitudinal gradient, latitude
  AND diversity, and latitude AND species
  richness
• Analyzed 232 studies each contained different
  variables relating to latitudinal gradient (ex.
  Thermoregulation, habitat type, longitude)
• Found general trend that latitudinal diversity
  gradient occurs in almost all groups of
  organisms, however, the strength of the gradients
  differ based upon the variable examined

3
Why do we find this gradient?
The Issue

• Two Hypotheses
• -The tropics are a cradle of biodiversity
• -The tropics are a museum of biodiversity
• The Formula for Diversity
• Diversity
• Origination Rates - Extinction Rates Changes in
  Geographic Distributions (O) (E) (I for
  Immigration)

4
The Cradle Hypotheses
High biodiversity is attributed to high
speciation rates in tropical regions
Origination Rates higher in tropics than in
extratropical regions Extinction Rates and
Immigration Rates are equal in tropics and
extratropics
5
The Museum Hypotheses
Tropics gradually accumulate species over time
but maintain more biodiversity because of low
extinction rates
Extinction Rates lower in tropics than in
extratropical regions Origination Rates and
Immigration Rates are equal in tropics and
extratropics
6
Methods

• The Organism
• Marine Bivalves
• (Prior to this study, Jablonski determined
  evolutionary relationships of bivalves)
• Why are these organisms particularly good to use
  for this study?
• Strong LDG for both species and genera- can do
  large scale paleontological analysis
• Fossil record of this organism is rich and
  densely sampled
• Bivalves are found in the ocean at all latitudes

7
Methods

• The Evolutionary Tool of Choice
• THE FOSSIL RECORD
• Used a revised Sepkoskis Compendium to identify
  all living genera of bivalves that originated
  within the past 11 million years
• Used museum collections and primary literature to
  determine where each taxon first occurred
  (tropics or extratropics)
• Used these same resources (Compendium, museums,
  and primary literature) to estimate tropical and
  extratropical extinctions
• Examined species in late Miocene, Pliocene, and
  Pleistocene ages up to the present?11-million
  year time interval
• Less sampling has been done in tropics as
  compared to extratropics (may have effect on
  results)

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Results

• ORIGINATION 11 Million Year Period
• Tropical First Occurrences of Marine Bivalve
  Genera 117
• Extratropical First Occurences of Marine Bivalve
  Genera 46
• OTgtOE

Number of Taxa first occurring in Tropical vs.
Extratropical Regions
Pleistocene (11,550 to 1.8 million ybp)
Pliocene (1.8 to 5.3 million ybp)
Late Miocene (5.3 to 11 million ybp)
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Results

• EXTINCTION 11 Million Year Period
• Number of Exclusively Tropical Genera
  Extinctions 30
• Number of Extratropical and Cosmopolitan (widely
  distributed over the globe) Genera Extinctions
  107
• ET?EE
• Most inconclusive of the three variables because
  of undersampling in the tropical regions

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Results

• IMMIGRATION Tropics are bounded between 25oN
  and 25oS latitude
• ?75 of taxa that first occur in tropics occur
  in extratropics today
• Only 2 of the ?75 taxa have completely left
  the tropics
• Found that average age of living bivalve genera
  in tropics is lower than the average age of
  extratropical genera (tropics harbor young and
  old taxa- extratropics harbor older taxa)
• ITltIE

Modern Poleward Limits of Taxa With Tropical
Origins
Pleistocene (11,550 to 1.8 million ybp)
Pliocene (1.8 to 5.3 million ybp)
Late Miocene (5.3 to 11 million ybp)
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The Out of the Tropics Model
The Tropics are a Cradle and a Museum
Origination Rates higher in Tropics than in
Extratropics Extinction Rates lower in Tropics
than in Extratropics
GENERA ARE ORIGINATING IN TROPICS AND IMMIGRATING
TO EXTRATROPICAL REGIONS
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What Does This Mean For Society?

• 75 of modern day genera of bivalves originated
  in the tropics and spread outward into
  extratropical regions
• Tropics are the center of biodiversity for the
  earth
• Destruction of tropics means destruction of
  biodiversity in higher latitudes
• By destroying our tropical habitats, we are
  subsequently destroying our own habitats
• Conservation of biodiversity in the tropics is a
  global issue

13
References

• Hillebrand, Helmut. "On the Generality of the
  Latitudinal Diversity Gradient." The American
  Naturalist 163 (2004) 192-211.
• Jablonski, David, Kaustuv Roy, and James W.
  Valentine. "Out of the Tropics Evolutionary
  Dynamics of the Latitudinal Diversity Gradient."
  Science 314 (6 October 2006) 102-106.
• Sanders, Roger. "Tropics are Source of Much of
  World's Biodiversity." UC Berkeley News. 05 Oct.
  2006. Media Relations. 4 Dec. 2006
  lthttp//www.berkeley.edu/news/media/releases/2006/
  10/05_tropics.shtmlgt.