MODELING THE RELATIONSHIP BETWEEN BIOGEOGRAPHY AND DIVERSITY: AN EXAMPLE FROM THE AZOOXANTHELLATE CORALS

Little is known about relationship between biogeography (in particular endemism) and diversity. The central question of this research is how has apparent endemism impacted the diversity curve of the Neogene azooxanthellate corals? Azooxanthellate corals in the Caribbean had relatively low diversity over the last 24 millions years. with turnover between 4 and 1 Ma during the closure of the Central American Isthmus. In addition, they experienced accelerated speciation in the last million years during Northern Hemisphere glaciation. However, these diversity patterns are heavily influenced by apparent endemism (and sampling intensity).

In the Caribbean, there is a significant difference (F-test, p < 0.01) in the number of endemic species (species restricted to a region) between the Neogene and Recent azooxanthellate corals (75 % versus 12.4 %). The low percentage of endemic species for the Recent azooxanthellate corals suggests that many of these species should have long stratigraphic ranges. However, only 8 of the 129 Recent azooxanthellate coral species are represented in the fossil record. In addition, the frequency distribution of biogeographic ranges for the Recent azooxanthellate corals is polymodal, while ranges of Neogene azooxanthellate corals have a right skewed distribution with an upward concavity ("the hollow curve") indicating significantly more endemic species than widespread species.

In order to understand the relationship between biogeographic ranges and diversity, species-by-regions presence/absence matrices were simulated. Also, in order to create diversity curves for this data, each region was assigned a geologic age. The species-by-regions matrices were degraded by randomly removing some of the species occurrences from the matrices. Lastly, diversity curves and the corresponding frequency distributions of biogeographic ranges were plotted. Results indicate that as the distributions shift from monomodal to the hollow curve, the overall diversity drops with the ends of the diversity curve more greatly affected. Many view the hollow curve as a true pattern in the fossil record, but it is difficult to separate it from sampling intensity. Overall, the increase in the diversity of the azooxanthellate corals in the last million years may be caused by decreased endemism.