Thursday, June 4, 2009

How Big (Dead) Mammals Respond(ed) to Global Warming: Paleontology and Our Climate Crisis

ResearchBlogging.orgAfter all of the commotion over "Ida," I'm happy to point out a new, thought-provoking paper in PLoS ONE that perhaps has more relevance to modern humans than any old primate of debated affinity. This new contribution ties two rather cool issues together: charismatic megafauna and global warming. And what might they have to do with each other?

Within the scientific community, our current cycle of climate change ("global warming") is pretty well-supported by numerous lines of evidence. In light of this change, many biologists, conservationists, and policy-makers want to know exactly how this change will affect living things (humans and wildlife alike). Will animals adapt to the new conditions? Will they die out? Will different animals cope in different ways? Models, simulations, and short-term studies are all useful, but only provide one small piece of the puzzle. Short-term studies (by short-term, I mean on the scale of months, years or decades) provide useful ground-truthing for the models, but in matters of conservation and policy, they may come too late for imperiled ecosystems. It's like having your house burn down around you before you can see the smoke.

An oft-overlooked source of data comes from the fossil record. Earth's climate has changed numerous times over the millenia, and by studying previous warming or cooling episodes we may be able to understand our own times. This is where a new study, led by Larisa R. G. DeSantis in collaboration with Robert Feranec and Bruce MacFadden, comes in.

Ice Age Antics
Beginning around 2.58 million years ago, in the late Pliocene, our planet has been in an Ice Age. This ice age is characterized by cooling periods (glacial periods, in which the ice sheets advance) and warming periods (interglacial periods, in which the ice sheets retreat). For the last 10,000 years or so, we've been in an interglacial period (and our present climate change is above and beyond this). As a neat natural experiment, DeSantis and colleagues decided to look at how large animals reacted (in terms of diet, etc.) to the switch from a cool period to a warm period.

Grind Up Fossils in the Name of Science

The team focused on two sites from Florida: one from a glacial period, between 2.0 and 1.6 million years old, and another from an interglacial period, between 1.6 and 1.3 million years old. Using a little drill, the researchers sampled tooth enamel from a variety of Ice Age organisms, including horses, deer, tapirs, elephants, and other herbivorous critters that roamed Florida during that interval. And why grind up fossil specimens? It turns out that you can run the enamel powder through a spectrometer that measures the proportions of various isotopes of carbon and oxygen.

And what do these isotopes tell us? Simply put, you are what you eat. Different plants use different pathways of carbon fixation (C3 and C4 were investigated here). Animals eating lots of C4 plants (primarily "warm season grasses") have one isotopic signature for carbon, and animals eating C3 plants ("cool season grasses," trees, and shrubs) have another. Furthermore, oxygen isotopes are different for arid environments and relatively wet environments. So, by looking at oxygen and carbon isotopes in concert, you can get an idea of the relative aridity of the area as well as the diet of a given animal.

And now the modern tie-in: According to DeSantis and colleagues, many studies and models have concluded that under environmental change, animals tend to try to be pretty consistent in what they eat. In other words, if you start out at a grass-eater, you will try and stay a grass eater. So, mammals don't really do much in response to warming (or cooling). Of course, this has pretty important implications for conservation: once the grass disappears in the face of a changing climate (whatever the cause), our grass-eaters are toast.

The Results
Interestingly, it turns out that different mammals had different stories over the course of the glacial cycle. Based on the isotopic data, the types of plants changed over time, with C3 plants dominating the cooler cycle (as would be expected) and C4 plants predominating in the warm interval. And, many of the same animals are found in both the "warm" and the "cool" study sites. Although some apparently maintained similar diets (e.g., tapirs), most other animals (e.g., deer and horses) showed very different isotopic signatures over time. They were eating different foods. . .thus, these animals were quite adaptable!

What It All Means
A striking implication of the study is that some animals may not be as vulnerable to climate change as previously thought. These Ice Age species changed their ecological niches in the face of climate change. So, if large modern animals can adapt their diets relatively easily, they may be able to escape extinction too. The bottom line still is that previous assumptions of do-or-die dietary stability for large mammals are not valid in all cases. Here we have yet another cool example of how paleontology can provide important information for "real-world" problems!

Parting Thoughts
The paper, posted at PLoS ONE, covers much more than the little bit I've highlighted here. There are some interesting tidbits on changes in rainfall and ecological partitioning, among other things. It's a quick and very accessible read (weighing in at 7 PDF pages, including figures and references), and even this non-geochemist followed the text pretty easily. So, go check it out! As always, you can rate the paper or make comments at the PLoS ONE website.

The Citation
DeSantis, L., Feranec, R., & MacFadden, B. (2009). Effects of global warming on ancient mammalian communities and their environments PLoS ONE, 4 (6) DOI: 10.1371/journal.pone.0005750

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