Lions and saber-toothed tigers and cave bears, oh my!

Today is another special request, this time from Marissa, who both suggested starting this blog, and fortuitously picked my favorite flavor of mad science. Without further adieu, let’s explore the slightly impractical but totally awesome concept of Pleistocene rewilding.

Pleistocene rewilding takes the theory of conservation ecology and simply applies it to its logical conclusion. If we as a species can undertake projects to rectify the damage we did to ecosystems in the last few decades (restoring bald eagle populations damaged by overused of DDT, etc.), then why can’t we undertake projects to rectify the damage we did to ecosystems in the last few millenia? After all, evolution moves at million year timescales - just because a species went extinct 10,000 or so years ago does not mean that the role that species played in a healthy, functioning ecosystem has been restored.

                     We saved one. Why not the other?

 

Before we get any further, let’s define a few terms. The Pleistocene in Pleistocene rewilding refers to, well, the Pleistocene, an epoch of time extending from about 2.6 million years before present to 11,700 years before present. The Pleistocene was in some ways different from our own (the Eocene) - glaciers sometimes covered large portions of the Earth, and mastodons, saber-toothed tigers, and predatory birds with a 25-foot wingspan all prowled those parts of the Earth that were not buried under upwards of a mile of ice. Many animals were much larger than present day relatives - giant ground sloths, giant tortoises, and what can only be described as a giant guinea pig were all around during or shortly before the Pleistocene.

 In other ways, however, the Pleistocene wasn’t particularly different from the Eocene. Many of the birds, mollusks and flowering plants we take for granted today existed in their present form during the Pleistocene - oak trees, beech trees, grouse, magpies and the aptly named Iowa Pleistocene Snail . Anatomically modern humans were also in existence for much of the Pleistocene. And that, in a roundabout sort of way, brings us to the “rewilding” part of the idea.

It’s been a long time since mastodons and massive predatory birds had free range of the planet. There are a number of proposed reasons why the megafauna (larger animals) went extinct - the Pleistocene ended when the Earth’s climate switched from periods of intense glaciation to the current (pre-global warming) reduced state of the glaciers. As so often happens when the climate shifts, the species assemblage shifted. However, the expansion of humans and the rapid evolution of human hunting techniques very likely played a defining role in the extinction of the megafauna - humans preyed on the herbivores, and competed with the carnivores.

These days, most of the megafauna are gone. The Americas have lost mastodons, camels, native horses, elephants and giant sloths, to name a few. The dire wolves and Irish Elk are gone from Europe, and the remaining megafauna of Asia, Africa and Australia (lions, tigers, elephants) are largely teetering on the edge. While every species in an ecosystem is important, the really large animals tend to exert an effect disproportionate to the number of individuals. Larger animals eat more, and tend to strongly regulate whatever food source they rely on (be it smaller animals or vegetation). When we looked at sharks we talked a little bit about the effect on a food web of removing a dominant predator. The same effect is at work with megafauna. In addition to regulating food webs, large, dominant animals alter their surrounding environment in such a way as to provide habitat for other, frequently smaller, animals. It isn’t always easy to figure out exactly what role extinct megafauna played in their environments, but looking at extent megafauna can be helpful.

Let’s examine the case of the Bolson tortoise. These guys are hard to find, but it’s worth your while - they grow to the size of coffee tables and live in the Chihuahuan Desert of Mexico and the southwestern United States. At the end of the Pleistocene, the tortoises were evenly distributed throughout the desert. Since then, human predation has steadily reduced the tortoises to a few small patches of range. While it it is difficult to study exactly what role the tortoises play in their environment, due to the range reduction, the burrows of a closely related species provide habitat for well over 300 species, some of which are endangered. Given that the Bolson tortoise is known for its complex, extensive burrow system, it is likely that the tortoise hosts a similar array of species in its burrows. At this point in time, efforts are underway to study the burrowing behavior of the Bolson tortoise, and to re-expand the current range to something nearing historic levels. 

Put me back!

Moving back in time to an extinct, instead of near-extinct species, we have the example of extinct North American elephants and the increasingly extinct Maclura genus of plants. Elephants are known to be capable of dispersing large-seeded plants (like the Maclura), unlike most other North American mammals, and the extinction of the North American elephant was closely followed by a decline in Maclura diversity. While Maclura was once a relatively wide-spread plant family, the Osage Orange is the only surviving member of the family in North America. Loss of the only animals capable of dispersing the seeds is likely at least partly responsible for the decline of Maclura. 

 I'm so alone...

 
Horses are a more complicated example. North America used to have horses, but native horses went extinct at the end of the Pleistocene. The arrival of Europeans reintroduced horses to the continent, and feral horses now live in habitats from the barrier islands of Virginia to the arid grasslands of Nevada. In some places, the animals are treated as pests, but really, they’re filling an empty niche. The difference is that this time, there are no saber-toothed cats to keep the populations in check. An empty grazing niche was filled, but the mustang population, to name but one example, is now capable of doubling every four years, with serious accompanying threats to vegetation and soil erosion. At this point, government culls and sterilization programs are used to keep the mustang populations in check, but wouldn’t it be simpler to reintroduce some sort of predator? 

                                  And they would run even faster with a lion chasing them.

Wolf reintroduction (which I swear, I will devote an entire post to) works on a similar concept - elk and deer are part of the small assemblage of surviving North American megafauna, but overpopulation has had some serious ecological effects. The idea is that reintroducing wolves will act as a check on the population.

Pleistocene rewilding expands the example of the accidental rewilding of the North American horse. While it would be really cool to genetically engineer mastodons, that isn’t really the point. Instead, Pleistocene rewilding seeks to take animals from surviving megafauna populations and reintroduce them to habitats suffering from a lack of megafauna. So, in the case of the Maclura decline, elephants or buffalo or some other large grazing animal capable of dispersing Maclura seeds could be introduced to an area, with the aim of restoring the range of Maclura, or other large-seeded plants. Introduction of lions or tigers could keep the populations of wild horses in check. Other suggestions include introducing camels and cheetahs. Radical? Certainly. Crazy? I wouldn’t be so quick to say so. It’s difficult to tell exactly what the world looked like before humanity contributed to the mass extinction of megafauna, but it certainly is different these days. Maybe reintroducing megafauna will help existing ecosystems better deal with the shocks brought on by a changing climate. Pleistocene rewilding is a concept well worth exploring, and who knows, maybe cheetahs roaming the Great Basin really are the answer to a lot of our problems. 

 We can dream