North American Megafauna

(Smilodon image from here.)

This is what used to be called Columbus Day. It is now called Indigenous Peoples Day. 

People came to the "New World" at least 15,000 years ago. There is some evidence that they may have made it here as much as 20,000 years ago. Regardless, the ecology here was very different from where they had left. I am referring to the megafauna that were here for quite some time and seemingly disappeared about 11,000 years ago.

These were the animals living here when the Native Americans arrived.

When Columbus sailed into the Caribbean there were thriving populations of human beings already here. Some of the largest cities in the world were in the "New World"-- cities supported by agriculture without the aid of cattle or horses. Smarter people in that they had to be ingenious in different ways without domesticated large animals.

One wonders how North America might have turned out if humans had realized they could domesticate the horses and camels that were already here, long before they "old world" humans figured it out. SF writers: go for it.

The animals usually spoken of in this context are, besides the horses and camels, were mammoths, mastodons, smilodons, giant sloths, glyptodons (an armadillo on steroids), short faced bears, an American cheetah, a giant beaver, and the dire wolf. Many were much larger than equivalents at the same time in the "Old World." Of course, humans had fifty thousand years to take care of them. 

My two favorites are smilodons and mastodons. 

Smilodons were the saber toothed cats you saw in Ice Age. They ranged from medium cats coming in at 55-100 kg (Smilodon gracilis) up to 280 kg (S. fatalis). To give some comparison, modern lions top out at about 225kg. (BTW: there was a North American lion, Panthera atrox, that weighed as much as 420kg.) 

These were extremely successful predators until they weren't. They hunted things like bison or camels-- one wonders if they left the really large herbivores to the American lion. No one knows if they hunted in packs or singly. But they arose 2.5 million years ago and came to an abrupt end along with all the others.

While smilodons were in the cat family, they were unrelated to tigers. They were no more a saber toothed tiger than a similarly named Tasmanian tiger. It would be interesting to know which was the apex predator: P. atrox of Smilodon. Most of the literature I read suggested Smilodon was an apex predator. However, it's hard for me to believe that when P. atrox is on the scene and twice as big. As far as I can tell, atrox had not abandoned its predator ways. Of course, without a precise knowledge of animal ranges, it's hard to tell. Smilodons might have been apex predators in locations not frequented by P. atrox.

Lions and Smilodons appear to have different strategies for killing. Smilodons sacrificed bite force for precision with their sabers. Where lions and their relatives developed significant jaw bones and muscles. This is one possible way they co-existed: different prey selection. However, one study suggested the dire wolf, Smilodon and atrox hunted the same prey, suggesting all three were in competition. I find this interesting. Unless there were a huge diversity and quantity of prey available to them, this could not have been permanent. 

Let's move on to mastodons, the mammoth's less popular, scrappy little brother.

Mammoths get all the press with their curvy tusks and long hair. They're taller so they get all the attention.

Mastodons are smaller, flatter and have long, flat tusks. Mastodons ate rough fair: woody small trees and bushes. Mammoths were grazers and liked grass. Mastodons were everywhere, all over North America, Russia down into China and down into Viet Nam. Mammoths had a comparatively narrow range: a band in North America, northern Russia and China. Mammoths ranged further north than mastodons. Mastodons ranged much further south.

(Although, I think this opinion might be revised. There was a tremendous mammoth site found in mid-Mexico recently. Or maybe they just managed to get far enough south to reach the end of their range. Also, I'm including the range of all the mastodon species here, including some where the genus attribution is still under discussion.)

While mastodons have been depicted as hairy as their mammoth cousins, there's no evidence for this. Some studies have suggested they were more like elephants-- which might account for their relatively southern range. They looked more like elephants than mammoths but were only distantly related to either one. 

One study of mastodon mitochondrial genomes suggests significant dispersion along with the glacial shifts. Different groups would migrate into new areas as the glaciers retreated and then get pushed into new locales as the glaciers returned. This pushed different groups together, isolated one group from another and mixed things up. All of this appears to be shown in the variation in mitochondrial DNA.

People have a tendency to come across a given place and presume that it's always looked like that. The state we initially encounter is what we think of as the natural state. Subsequent changes are compared against this natural state. When the Europeans came to Atlantic coast and found these huge forests, they thought that state was primal and without human intervention. Given that human beings had been there at least thirteen thousand years by that point, nothing could be further from the truth. Of course,  the vast majority of Native Americans had been obliterated by European diseases and couldn't argue the point. (I strongly recommend reading 1491.) 

The ecology of North America had been isolated from human beings for millions of years. The megafauna I mentioned above was integrated into that ecology. And disappeared virtually overnight. 

Mastodons , like most of its elephantine relatives, were drivers of that ecology. (Another such driver was the beaver. I suggest reading Eager, an ecological analysis of the role of the beaver. But I'm not going to discuss that right now.) They, along with mammoths, had played that role for over two million years when glaciers came and went. Then, they were gone. Smilodons, gone. Giant sloths, gone. But the ecology they drove didn't disappear with them. It just stumbled along with great holes punched in it.

Herbivores-- especially giant herbivores-- exert a strong downward control on vegetation. (See here.) They limit the spread of trees and buses by eating them-- only a subset survive to become large. They can increase grasslands by knocking down or eating grass competitors. Elephants have been known to dig out waterholes when they extract minerals. The elephants go on their way but the waterholes remain. The vegetation responds. Some take advantage of the animals by using them for seed dispersal or pollination. Or just by using the enormous amounts of dung-- a mammoth might eat 300kg of food a day. That high quality fertilizer had to go somewhere. 

Apex predators have a similar influence on the size of their prey. Predation is expensive. A predator can't end up expending more energy getting food than the energy that food will supply. So, large predators imply large prey. Not always-- baleen whales can be considered predators for the herring and krill they consume. But usually, on land, a large predator tends to hunt single relatively large prey animals rather than consuming many, many small animals. Again, not always-- this is a tendency, not a law. There has been some evidence that in the north country of Canada of wolves consuming large amounts of rodents. But those same wolves are also hunting deer and elk. 

When predators get big, some of their prey get bigger to escape them. Very large predators escape them altogether and at that point the predators descend on their sick and young, leaving much of the population intact. Since, then, the predators go after smaller herbivores, they are, in effect, reducing the competition to large herbivores. Smilodon went after horses and camels-- mid-range, grass consuming herbivores. This left the field open to mammoths.

An interesting side effect of increase in size is this large increase in biomass. The consequence of that is ecological control. By "control" I do not mean the mastodons are sitting around figuring out their next move, I mean the population is interacting with the ecology towards a new equilibrium.

This study shows that dispersal of phosphorous (an essential mineral for life) was radically different in the megafauna age compared to now" "...we estimate that the extinctionof the Amazonian megafauna decreased the lateral flux ofthe limiting nutrient phosphorus by more than 98%, withsimilar, though less extreme, decreases in all continentsoutside of Africa." This resulted in a long term decrease in phosphorus all through the Amazon that is still ongoing.

The ecological implications of the megafauna extinction to parasites, micro-predators, medium predators and mega-predators are interesting. (See here.) The giant vampire bat (Desmodus draculae and D. stocki) were not able to switch prey and went extinct. Smilodons, the dire wolf and  P. atrox went extinct-- which meant medium prey such as bison and elk had less predation until smaller predators stepped up. 

The question always comes up: why did the megafauna go extinct?

One study suggests the shift towards megafauna was a response to ecological instability. This instability came to an end around 11k years ago and the result was that there came a selection against large megafauna in North America. Recently, as these systems equilibrated, the world evolved into "stripes"-- areas  that had a reliable temperature and rainfall. These areas supported a different selection criteria for animal size. They suggest the extinction didn't happen suddenly but over the last 100k years as the ice age ceased and the climate stabilized. Thus, the "stripe" that allowed megafauna became sections of India and Africa and the temperate megafauna downsized to bison and cattle sized animals. 

Humans might have played a role. The "overkill hypothesis" suggests the Clovis hunters with their superior technology were able to bring these large animals down. It's certainly true that mammoths and mastodons were hunted by human beings. While their great size proved an impediment to the larger predators, it did not seem to deter people. That said, it's unlikely that there were enough people in North America to directly hunt these animals to extinction.

However, it's also true that all of the megafauna species were under some stress. One idea was that humans hunted enough of the larger animals to shift the ecology and that shift served the final blow. 

My own feeling is that humans pretty much devastate the landscape wherever they go. I suspect this is what they did to Neanderthals. Not so much kill them but leave the land barren enough to push them over the edge. There were no Neanderthals in North America, so we made do with the megafauna.