Paper 39: Sepkoski 1988
Alpha, Beta, or Gamma: Where Does All the Diversity Go?
Blog Author: Lam Le
Blurb Author: Peter Wagner
-Associate Professor in Earth and Atmospheric Sciences and Biological Sciences
-Research interest:
“I focus on both phylogenetic methods and sampling theory, in order to construct phylogeny-based tests of macroevolutionary hypotheses using fossil data either about or accounting for the complexities of anatomical evolution and the inconsistent nature of fossil sampling rates over time and across geography. To date, I have applied these approaches largely to early Palezoic molluscs (particularly gastropods), but in recent years I have engaged in studies of numberous other taxonomic groups.”
Paper Author: James H. Brown
-University of Chicago paleontologist
-Best known for global compendia of marine animal families and genera, data sets that continue to motivate a tremendous amount of paleobiological research. He identified 3 great Evolutionary Faunas in the marine animal fossil record. Each of his Evolutionary Faunas, the Cambrian, Paleozoic, and Modern Faunas, is composed of Linnean classes of animals that have covarying diversity patterns, characteristic rates of turnover, and broadly similar ecologies. Most importantly, they sequentially replaced one another as dominant groups during the Phanerozoic.
Summary/Main Points:
-Background:
Global taxonomic diversity can be divided into 3 components: alpha diversity (richness within particular habitat), beta diversity (differentiation among adjacent environments and regions), and gamma diversity (differentiation among different faunal provinces). Both alpha and gamma diversity have been discussed in relation to the evolution of global marine diversity. Alpha diversity increased episodically followed by long intervals of constancy through the Phanerozoic. There was low diversity in the Cambrian, higher in Paleozoic, lower again in early Mesozoic, and increased diversity thereafter. There is a problem of “missing diversity” that produces a great increase in global diversity (gamma). Beta diversity could also play an important role. This paper investigates changing levels of beta diversity during Paleozoic and assess its contribution to global diversification during Ordovician radiations.
Data:
-505 fossil marine faunal assemblages of Cambrian through Permian age from published literature.
-The fossil assemblages are represented by faunal lists taken from paleoecologic and biostratigraphic studies. Lists were selected that included all dominant and subdominant macrofossil taxa sampled from well-circumscribed lithofacies.
-Microfossils were deleted from the fauna lists because of inconsistent documentation.
-Reefs, mounds, and hardgrounds were avoided in the collection of data, only level, soft-bottom assemblages were used.
-Most assemblages were derived from Laurentian portion of North America. 7% were from Europe for situations where there were insufficient North American data.
-Imposible for even sampling. Some environmental zones have a slightly denser concentration of assemblages than others.
Results:
Patterns of Alpha Diversity
-Diversity is lowest near shore, called “high stress” environmental category but also includes some variable nearshore environment. Alpha diversity remains unchanged through Paleozoic.
-Away from near shore, diversity increases to maximum in “open marine” category. The increase is subdued in the Cambrian but pronounced in later periods.
-Differences between 2 zones in “variable nearshore” categories are minor.
-Beyond “open marine”, alpha diversity decline across outer shelf and into deeper, off-shelf waters.
-Through time, all environmental zones off-shore exhibit increased alpha diversity since Cambrian, but not in Paleozoic.
- The highest average alpha diversities are in Silurian.
-Average alpha diversities for Permian are slightly lower.
-Evident in Figure 3 are very similar to those found in Bambach (1977). This pertains to the problem of “missing” diversity. The data proves far below 300 percent increase in global generic diversity.
Patterns of Beta Diversity
-Use Coefficient of Community to measure inverse of beta diversity, measured over discrete time intervals.
-Trends found between-zone beta diversity are very similar to those at alpha level: beta diversity lowest in Cambrian and higher but not continuous in Paleozoic.
-The Cambrian exhibits significantly higher mean values than all other periods. There is substantial component of the fauna that was without habitat specialization and living all environmental zones across the shelf.
-Mean values of Coefficient of Community for Paleozoic, Ordovician and Devonian exhibit decay.
-Coefficient of Community for Carbonifeous and Permian exhibit a pattern intermediate of Cambrian and mid-Paleozoic, but then increases.
Discussion:
-The increase in habitat differentiation implies an increase in ecological specialization between Cambrian and Paleozoic.
-Change in beta diversity in Ordovician radiations implies increased differentiation of ecological communities.
-Increase in alpha and beta diversity over evolutionary time scale: There is competition between species living together. New species must enter by: exploiting resources not utilized by existing members or wedging into habitat gradient at points between overlap of 2 or more species. The first increases alpha, while the second increases beta.
-Alpha and beta diversity are not the only possible sources of global richness. Gamma diversity is also important. However, gamma diversity plays a subordinate role.
-Remaining global diversity that contributes to change in alpha and beta diversity comes from “hidden” sources of beta diversity (reefs, hardground communities, bryozoan thickets, crinoid gardens).
Comments/Questions:
-This paper talks about a lot of paper that I am not familiar with. Sometimes the author mentions 3 or 4 papers within one sentence. Besides that, the paper is structured nicely. It’s a pretty interesting paper.
-On page 230, he talks about new species wedging into the habitat gradients at points between overlap of 2 or more species, and that increases beta diversity. Is there an example of a species coming in at the overlap of 2 or more species?
Overall, I agree with Lam that the paper was interesting and easy to read. I did appreciate the way the author described the datasets used in the research, taking time to explain how the data was the best representation of distribution across taxa. I also liked that there was discussion about the response seen in diversity to new habitats opening up in certain time intervals.
ReplyDeleteLet me start by sayin, I loved Pete's introduction to the paper and his comparison to the rock and roll music.
ReplyDeleteThis paper defines gamma diversity as biogeographic differentiation in diversity, however, the other paper mentions that climate should not be taken into account for diversity, but isn't a huge part of gamma diversity climate changes?
I think I'm a little confused. The original problem was that each site's diversity increased by 50% but global diversity increased by 300%. Were they assuming that since each site increased by 50%, global diversity should have increased by 50%? Since they were looking at local and global, weren't they sort of already looking at alpha and gamma diversity? Did they assume that each site had the same species increasing by 50% and were surprised that different sites had different species- thus beta diversity?
ReplyDeleteEither way- I liked having a bit of geology in this paper and never quite thought about not having the different zones as options for habitats.
I´m also a bit confused about this, but I think that the problem is that if you increase average local species richness by 50%, leaving beta and gamma the same, you would expect a 50% increase in overall diversity, so a 300% increase should include changes in the other components of diversity??
DeleteI think the what they meant was global diversity average was much higher than some local sites. And, the local diversity has to increase by 50% in order to increase the global diversity by 300%.
DeleteI also loved Pete's introduction. And just a Willow's, I am also confused by their estimates. I am not sure on how to lead the discussion for this paper, but if someone had a clearer understanding of those numbers it would be great!
ReplyDeleteAnyway, I liked how they compared their results to the previous study with the smaller sample size. I thought it was interesting how the patterns didn't vary as much even when the sample was doubled in size.
I´m curious about the statement in the blurb that "pulses of extinctions occur nonrandomly at an astronomical timescale". At least for the big five mass extinction, the time interval and causes have been different, is it non-random?
ReplyDeleteI think this paper is easy to understand, and well written. I wonder how much time it takes to occupy a new habitat and become specialists. What are the major factors for diversity staying constant? For answering Alex's question, they should consider climate change. It might not be taken into account for diversity because it is a marine environment. They might think that physical environmental change has more to do with the gamma diversity in water. I like they mention that ecological differentiation was evolving simultaneously. Even though some diversity assemblage seems constant, I think that it should be evolving and changing all the time.
ReplyDeleteI found it interesting that near-shore environments had the lowest alpha diversity. This seems a bit counterintuitive to me, thinking about extant coral reef communities in shallow ocean waters. The pattern of increasing alpha and beta diversity over time made sense to me, however.
ReplyDeleteSidenote: Valentine (1973) called Cambrian animals "grubby", ay? Haha.
I like how much of an easy read this paper was. It's worded in a way that, in my opinion, sounds quite interesting and made it extremely easy to understand. The graphs used were simple yet insightful and left little confusion if any when referring to graphs throughout the paper. Having the well explained introduction of alpha, beta, and gamma diversity was a nice touch in understanding the whole paper and even better with understanding the results and summary. Overall, I really enjoyed this paper!
ReplyDeleteAlso a big fan of Peter's rock band analogy. Haven't had much exposure in these different diversity categories so this was also an interesting read for me. Curious about the "hidden" beta environments.
ReplyDelete