More Mangrove madness for May, I guess! Anyway, my previous post about some of the nifty mangroves I'd seen down in the Everglades got me thinkin': what's the evolutionary history of mangroves? Turns out, that's kind of a tricky topic, and one that ends up incorporating a lot of interesting taxonomic, ecological, botanical, and paleobotanical perspectives; as a for-instance, it requires that you make some clear definitions regarding what you mean by "mangrove". In one sense, you can talk about the taxonomic group of fruiting plants that we call "Mangroves" (which includes a lot of ~22 families). HOWEVER, in another sense, we can talk about the ECOLOGICAL niche of "mangroves", i.e., coastal swamps that develop as a result of biologically-mediated factors. A future post will explore those topics in more detail.
THIS post, however, is going to focus on some slick work (in my opinion) that has been done linking mangroves (including genetics AND fossils) with the geological factors that influenced their evolutionary history.
Workers with interests in mangroves have long recognized an interesting pattern in modern mangrove (taxonomically-defined) biogeography, namely that Mangrove species richness declines dramatically from a peak in the West Pacific to a minimum in the Caribbean/West Atlantic Zone. The figure below is from Ellison et al. (1999) that shows this pattern diagrammatically.
There are two explanations that have been put forth to explain this pattern. Originally, mangrove biogeography was explained in the context of an Indonesia/West Pacific "Center of Origin" distribution (Duke, 1995). In otherwords, mangrove taxa originated in one spot, and then subsequently disperesed globally. This is easily the oldest explanation for this observed pattern of diversity, and goes back into the literature for ~100 years.
After the plate tectonic revolution, however, another explanation presented itself: namely, that mangrove taxa evolved around the Tethys seaway in the late Cretaceous, and regional diversity is the result of diversification and evolution in the context of plate tectonics and the movement of continents (Ellison et al., 1999). This has been termed the "vicariance hypothesis", and is the big focus of the paper by Ellison et al. (1999).
Ellison et al. (1999) explore several lines of evidence as a means of testing the validity of one or the other hypotheses, and frankly, I think they're rather clever: FIRST, they take a look at the paleobiogeography of fossil mangroves (pollen or macrofossils) and compare that to modern biogeographic patterns. Ellison et al. (1999) compile a substantial list of fossil mangrove occurrence, and demonstrate that, actually, the fossil record is pretty damn good! The figure below is from Ellison et al. (1999; their figure 2), and shows generalized maps of mangrove occurrence through time.
Importantly, on the basis of their fossil mangrove occurrence database, Ellison et al. (1999) report that, statistically, there isn't any significant correlation between mangrove species-area occurrences and distance from the Indonesian/West Pacific region throughout the Tertiary. What is demonstrated, however, is that mangrove fossil occurrence shows a relativly wide-spread distribution of different mangrove taxa along the ancient Tethyan shoreline. Ellison et al. (1999) recognize that this conclusion provides testable hypotheses for relating other strongly correlated ecological aspects of mangrove environments.
This conclusion leads to their SECOND approach to interrogating mangroves....SNAILS. Turns out, mangroves have some pretty conservative associations with different gastropod taxa; in otherwords, the specific ecology and environmental stresses associated with Mangroves has also resulted in snail populations that almost always co-occur. Cooly, Ellison et al (1999) compile a fossil-occurrence database of gastropods, and compare that distribution to their fossil mangrove database! Neat, huh? Their results are shown below, in their Figure 3:
They proceed to explore ecological "nestedness" of gastropods and mangroves through time, and do some pretty nifty statistical manipulations of the data to test whether mangroves, their gastropods, and the species-area relationships can be associated with a "center-of-origin". Ellison et al. (1999) conclude that, on the basis of the fossil distributions of both mangroves, gastropods, and the patterns of their species-area relationships, that a probable Tethyan origin, and then subsequent diversification patterns are related to tectonic reconfiguration of shorelines and continents. Neat, huh!
It's a pretty neat use of both modern biogeographic patterns as well as ancient data, and it makes me wonder what other possible insights could be mined from both this and other settings? For instance, is there anything we could learn about large-scale paleo-oceanographic circulation patterns from these paleobotanical collections? Can we see other ecosystems being strongly influenced by tectonics in this way? Can we take the fossil record of mangroves, tie it to associated mangrove-dominated depositional environments, and see any statistical patterns in stratigraphic occurrence, mangrove biogeography, and tethyan tectonics? Or how expanding out from the "taxonomic" mangroves into the "ecologic" mangroves, which have a (potentially) older fossil record? Are there any statistical patterns to be elucidated from those occurrences? Who knows!?
Regardless, mangroves are obviously pretty amazing plants, but also seem to have a geological story to tell us as well! Maybe it's time us rock-o-centric types start really thinking about some of the critter-o-centric fields, and how one can inform the other in new and exciting ways.
WORKS CITED:
Duke, N.C., 1995, Genetic diversity, distributional barriers and rafting continents - more thoughts on the evolution of mangroves: Hydrobiologia, v. 295, p. 167-181.
Ellison, A.M., Farnsworth, E.J., and Merkt, R.E., 1999, Origins of mangrove ecosystems and the mangrove biodiversity anomaly: Global Ecology and Biogeography, v. 8, p. 95-115.
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