The Roots of Biodiversity:

1. The Roots of Biodiversity: An Investigation of Jablonski, Roy, and Valentine’s “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 Sepkoski’s 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)

8. Results • ORIGINATION: 11 Million Year Period • Tropical First Occurrences of Marine Bivalve Genera: 117 • Extratropical First Occurences of Marine Bivalve Genera: 46 • OT>OE 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)

9. 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

10. 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) • IT<IE 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)

11. 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

12. 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 <http://www.berkeley.edu/news/media/releases/2006/10/05_tropics.shtml>.