Jablonski et al. 2006 – by Rebecca Kait

Jablonski et al. 2006 – by Rebecca K.

Jablonski, D., Roy, K., & Valentine, J. W. (2006). Out of the tropics: evolutionary dynamics of the latitudinal diversity gradient. Science, 314(5796), 102-106.

Paper Authors: David Jablonski, Kaustuv Roy, James W. Valentine

David Jablonski

·      Ph.D. from Yale University

·      Distinguished Service Professor at University of Chicago

“My research emphasizes the combining of data from living and fossil organisms to study the origins and the fates of lineages and adaptations.”

Kaustuv Roy

·      Ph.D. from University of Chicago

·      Professor at University of California, San Diego

“What are the physical and biotic processes that determine large scale spatial patterns of biodiversity? How do species and communities respond to climate change and other anthropogenic impacts and what are the ecological and evolutionary consequences of such responses? These and related questions form the focus of my research.”

Earl D. McCoy

·      Ph.D. from University of California, Los Angeles

·      Professor Emeritus at University of California, Berkeley

“My students and I study a broad range of ecological and biogeographical problems. Many of our projects relate in some way to conservation biology, either in theory or in practice. Most of our current research deals with conservation and restoration of severely threatened upland habitats, particularly sandhill and scrub, in Florida. Within this framework, my students have focused their projects on a variety of topics: structure of gopher tortoise populations, demography and autecology of sand skinks, restoration of Florida mouse populations on lands mined for phosphate, and comparative biology of common and rare frogs, for example. Other students have focused their projects on topics such as methods of ecological analysis and the composition of species' assemblages. My own research encompasses additional topics in the areas of disturbance ecology, particularly fire ecology; biogeographical theory; and the philosophical basis of ecology.

Virtually all of the research being conducted by my students is aimed at solving particular problems and, therefore, probably would be labeled "applied research" by many persons.”

Paper Summary:

The latitudinal diversity gradient (LDG) is a large-scale pattern that has been observed in a many taxonomic groups, with higher species diversity occurring in the lower latitudes compared to the extratropics and further away from the tropics. In this paper, Jablonski et al. 2016 evaluate the paradigm of the tropics as a cradle/museum of biodiversity and “evaluate the spatial and temporal dynamics that underlie present-day LDG” while looking at previous work from the perspective of their presented framework.

“From an evolutionary perspective, large-scale spatial patterns of biodiversity depend on three variables: origination rates (O), extinction rates (E), and changes in geographic distributions (expressed here as I, for immigration into a latitudinal bin) of taxa.”

The authors discuss these variables and the models that use them, highlighting assumptions and problems that might arise in the way that these variables are treated when trying to answer certain questions. Some examples that were mentioned:

-       Looking at net diversifications rates might be useful for answering many important questions, but in looking at the cradle/museum question, looking at the rates may be limited by the combinations of O and E – a place with higher O may have a higher E as well, and in rates this overall net value might be low and this can be misinterpreted.

-       It might be implicitly assumed that LDG derives largely from differences in in situ origination and extinction, but this might be an oversimplification. E.g. are taxa shifts being accounted for? (Taxa shift in response to variables such as climate changes, etc.)

Out of the Tropics

“We suggest that the available data are most consistent with an ‘‘out of the tropics’’ (OTT) model, in which the tropics are both a cradle and a museum, with taxa preferentially originating in the tropics and expanding over time into high latitudes without losing their initial tropical distributions.”

Using data on marine Bivalvia, the authors tested their hypothesis and found that:

-       (Extinction) Higher extinction rates in higher latitudes over the past 11 million years compared to the tropics.

“Taken at face value, the bivalve data show substantially higher extinctions at high latitudes over the past 11 million years; only 30 exclusively tropical genera go extinct as compared to 107 extratropical and cosmopolitan ones.” However, the authors suggest caution here – this result may be owed to the severe under sampling in the tropics… “but the presence of so many last occurrences at high latitudes constrains potential patterns and suggests that tropical extinction rates are unlikely to be substantially higher than extratropical ones.”

-       (Origination) More first occurrences in the tropics compared to extratropics (117 vs. 46)

“Restricting analyses to families having ≥75% of their genera known as fossils, tropical first occurrences of those bivalve taxa significantly exceed extratropical ones in each of three successive geologic time intervals leading up to the present day (late Miocene, Pliocene, and Pleistocene)” (this may also be a lower estimate: “because sampling is strongly biased in the opposite direction (so that some genera originating in the tropics will not be recorded paleontologically until they expand into the better sampled extratropical zones)”)

-       (Immigration) More than 75% of the taxa in the data occur first in the tropics, but now they also occur extratropically today. “The bivalve data indicate that genera originating in the tropics tend to extend their ranges to higher latitudes over time…”