A Savanna Life in a Technological World

History of Herbivory; Mass Extinction.

History of Herbivory; Mass Extinction.

You call this a mass extinction event? Ha. I’ll show you a mass extinction event. Maybe you by now you would guess that I would say something like that, since I don’t like to agree. But I also want to say that actually the mass extinction event that everyone knows about, the “KP” Extinction that killed the dinosaurs, was not that big of a deal and there are a lot of other mass extinctions that were worse. But I can’t do that. Upon a little more research I am forced to admit that the KP extinction is still my model for what a real mass extinction event looks like.

Sixty-six million years ago, a rock about the size of the city of San Francisco hit the earth near the Yucatan Peninsula. That created a wave so big that it was 300 feet high meteor impacted the shallow seas off of what is now the Yucatan Peninsula. This created a tidal wave three hundred feet high, and this tsunami wrapped around the entire world. In fact the crater in the ocean where this thing hit was dry for ten minutes after the impact. That’s how much water was displaced. But this event created much more significant waves. In many ways, they were still bouncing around for tens of millions of years. In many ways, they are still bouncing around today. But before we cover what was probably the most significant hour in the last 500 million years, I want to make sure to talk about my week, which was unusually boring even for me. In other words, the Almanac Section.

Though it was sort of apocalyptic in a Biblical Flood sort of way. With the big king tides last weekend, the water finally went over the banks of beaver slough. This apparently gives me an idea of what high water should be like in a normal year. The nine head of cattle I have left are high and dry, but they are on an island of sorts, so I’m glad it didn’t rain anymore. With nothing to do out in the pasture, I drove to town four times trying to get the same part. I am trying to get my new tractor mounted post driver in operation and I have had to hunt for a hydraulic fitting. In addition, I bought my 36” chainsaw this week and before I got done notching my first tree, I threw the chain and it got messed up. So. Back to town again today. Of course, this is what building savanna ecosystems looks like. Stay tuned.
So back to a somewhat more eventful week in earth’s history. When the Chicxulub meteor hit,* it also sent a wave of superheated dust particles that sparked a global wildfire. That dust, and the smoke from all the fires, started a nuclear winter which seems to have completely halted photosynthesis for some significant period of time. This happened everywhere on the globe. All over the world, the fossil record shows a ‘fern spike.’ Imagine a graph that tracks the fossil pollen of a given area. All the sudden after the impact, all the other plants drop way down and ferns are the dominant vegetation. Immediately after the impact, some places bounced right back. In North America however, close to the impact crater, this fern spike lasted for 200,000 years or so. I’ve been thinking about this and the only way this can really make sense to me is that tree species went extinct on the continent. This is shocking. Think about all the things that have happened to trees in North America over the last few hundred years. Save for the Florida Torreya (the exception that proves the rule) nowhere are there any tree species even mildly close to extinction. Everywhere they bounce right back given half a chance.

So that’s what happened to the plants. What happened to the herbivores? Well, they went extinct. All of them. No where on the entire globe did a single terrestrial vertebrate herbivore survive.

I guess the best explanation for this is that all that plant production ceased for long enough all herbivores died. Animals that needed a steady supply of fresh meat, also dead. However, plant production started back up again in time to save at least a few small animals, those that pulled through by eating carrion, detritous, or insects.

Even though we might not yet be anywhere near the mass extinction category, we have had a very disproportionate effect on large animals. If we are just looking at large animals I would agree that in the Holocene we are way out on the tail of a bell distribution, approaching the point where the K/P extinction event is a good analog.

This is a great natural experiment to test this assertion about herbivory; vertebrate herbivory is one of the foundational drivers of vegetation structure around the globe and through time. Just as we learned a lot about herbivory by looking at the world before herbivory, we can learn a lot about herbivory by looking at this hiatus in herbivory.

First, after that fern spike we see a big expansion in ‘rainforest’. It was only in the mid-cretaceous that we start to see signs of ‘rainforests’ as we know them today, very diverse, vegetative structures with multiple canopy levels. Rainforests really take off immediately after the extinction of the dinosaurs. One of the reasons for this is that it was very warm, a pattern that started before the impact and lasted long after. But it was as warm at times during the Mesozoic, the age of the dinosaurs, yet not rain forests. I wonder if some of the increase in rainforest during the early Tertiary is also a result of ‘herbivore release,’ in other words, a change in vegetative structure in the wake of the dinosaur extinctions. Remember, rainforests are not necessarily ‘better,’ (ie more diverse botanically) than herbivore maintained savannas, (follow this link for a study I’ve cited in the past, https://onlinelibrary.wiley.com/doi/full/10.1111/j.1654-1103.2012.01400.x).

This hiatus also shows that once lost, it can be pretty difficult to get large animals back. Large animals can become small pretty easily. Small animals can become medium sized pretty easily, but it takes a long time a lot to go from a small animal to a large animal. Specifically, it was about 30 million years from the dinosaur extinction to the re-emergence of mega-mammals in the late Eocene. Since the early Tertiary is probably the best analog we have to the Holocene as far as large animals go, there is an important lesson there. Save the large animals you have cause once you lose them they are extremely hard to get back.

But the third and final lesson from the immediate aftermath of the extinction of the dinosaurs is that large animals did come back. Large herbivorous animals are a very persistent theme through the history of life on earth. There is something about that system that seems to work very well and has survived and dominated the majority of the history of terrestrial ecosystems.

Next week we will move on to the home stretch of this history of herbivory, where we finally get to the rise of grasslands in the Miocene and we cover the last 30 million years of earth’s history as a rounding error. Oh yeah, and that is when I get to the point and tell you what the thesis of this series is, why we have been going on this quest to unearth the mysteries hidden in the history of herbivory.

*Warning: True Nature-Geeks Only Beyond This Point! In this post I am going to roll with the assumption that it was this meteor that was the big event in the K/P extinction, at least as far as terrestrial vertebrate herbivores are concerned. There is the theory that hyper-volcanism in India was already killing off the dinosaurs. I guess I have a hard time buying that. The way I read it, if you had the Deccan Traps without the meteor, worse case scenario you have a Permian/Triassic extinction, a huge event for marine organisms, but more of a transition than an extinction for terrestrial vertebrate herbivores. The argument is that Dinosaur diversity was already decreasing before the impact. Maybe but the last five or ten million years of the Mesozoic seems to have been very dino-verse. (That’s my technical term for ‘diverse in dinosaurs.’ I will be presenting it at the American Geophysical Union this year.) I heard Darren Nash put it this way; the Miocene was the height of mammal diversity and we have been losing mammal species for the last ten million years. But look around. Are we in the process of losing all mammal species? What would be the mechanism for that? Are mammals in the process of being replaced by some other group?

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