What Killed The Mastodons?

Twenty thousand years ago, North America had a more impressive array of big animals than Africa does today. The continent was populated by mastodon, several species of mammoth, giant ground sloths, saber-toothed cats and bison twice the size of their modern counterparts. By 10,000 years ago most of these animals were gone, including the 10 species that weighed more than a ton. Many drastic changes occurred during this interval, including the arrival of Homo sapiens to the new world. Many have cited humans as the cause of this great megafaunal die-off: were H. sapiens causing mass extinctions even during the stone age?

Many changes took place at the end of the last glacial period. The transition to the Holocene interglacial was a wild affair: the climate flipped from cold to warm, then back to cold during the 1000-year chill of the Younger Dryas, before rapidly rewarming to our more familiar, more comfortable climate. There was also an increase in large fires, and the types of vegetation changed drastically. After people arrived the stone aged Clovis culture flourished for less than 1000 years. One popular theory is that the first Americans rapidly hunted the continent's megafauna (big animals) to extinction and then, with the disappearance of their major food source, suffered a population collapsed themselves.

Alternatively, some scientists have argued that an extraterrestrial object struck Earth ~13,000 years ago, triggering the Younger Dryas, starting fires, killing the megafauna, and putting an end to the Clovis culture. A new study entitled “Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America,” just published in Science, seeks to answer the question of what killed the Mastodons. Interestingly, the researchers investigated the decline of North America's largest animals by studying a tiny organism—the sores of the Sporormiella fungus. As Jacquelyn L. Gill et al. explain:

Although the North American megafaunal extinctions and the formation of novel plant communities are well-known features of the last deglaciation, the causal relationships between these phenomena are unclear. Using the dung fungus Sporormiella and other paleoecological proxies from Appleman Lake, Indiana, and several New York sites, we established that the megafaunal decline closely preceded enhanced fire regimes and the development of plant communities that have no modern analogs. The loss of keystone megaherbivores may thus have altered ecosystem structure and function by the release of palatable hardwoods from herbivory pressure and by fuel accumulation.

As explained in a perspective article in the same issue of Science by Christopher Johnson of the School of Marine and Tropical Biology, at James Cook University in Australia: “Sporormiella is a fungus that produces spores in the dung of large herbivorous vertebrates. Lots of dung means lots of spores, so Sporormiella gives an index of the biomass of large herbivores. The spores accumulate in sediments along with pollen and charcoal, allowing changes in biomass of large herbivores to be matched exactly to sediment records of vegetation and fire, which can in turn be dated and aligned with other archaeological and environmental records.” Simply put, the more spores the more dung, the more dung the more big critters.

Using dung fungus Sporormiella as an indicator of megafaunal populations to study the pattern of megafaunal decline around Appleman Lake in Indiana Gill et al. show that the decline began about 14,800 years ago (middle). The decline of the megafauna was followed by an increase of fire and development of novel plant communities—although the megafaunal extinction coincides with the presence of the Clovis people, earlier human communities may have been responsible for the initial decline (bottom). Source C. Johnson, Science.

Gill et al. analyzed sediments from a lake in Indiana looking for spores and found that megafaunal decline began ~14,800 years ago and took more than a thousand years (see the figure). Furthermore, large vegetation changes and an increase in fire came after this decline ruling out changes in plant life or fire as primary causes of the megafaunal extinction. Climate change also looks implausible because vegetation changes followed megafaunal decline—climate change would most likely have affected megafauna by changing vegetation, not the other way around. Finally, all this happened long before the proposed extraterrestrial impact so there is no smoking cosmic gun either.

Was it we evil humans that destroyed native America's megafauna menagerie? According to Gill et al., “human impacts remain plausible, but the decline predates Younger Dryas cooling and the extraterrestrial impact event proposed to have occurred 12,900 years ago.” They further state “This evidence excludes rapid-extinction hypotheses such as an extraterrestrial impact or a Paleo-Indian blitzkrieg.” If humans hunted these animals to extinction they must have been people present before the Clovis culture arose, but the existence of such people has been controversial itself. So the earliest Americans are not off the hook yet—so much for the noble savage living in harmony with nature.

There is another interesting conclusion that can be drawn from this study. Before 14,800 years ago, the environment around the site studied by Gill et al. was open savanna. Parkland dominated by grassy pastures, scattered with spruce and rare broad-leaved trees—an environment where wildfire was rare. As the big animals began to disappear, trees increased, no longer suppressed by the large herbivores. The result was a transitory spruce/broad-leaf woodland, the like of which does not exist today. The changing environment of 14,000 years ago caused an increase in major fires which helped speed the transition and kill off the remaining megafauna. Eventually, much of North America would be covered in forest.


North America during the late Pleistocene. Painting by Karen Carr.

The point here is that the megafauna didn't just live in their environment, they actively helped to maintain the conditions most conducive to their survival. As they declined in number it was as though nature turned against them, altering the environment and hastening the animals' demise. Megafaunal extinctions elsewhere would have had similar consequences during the glacial-interglacial transition, triggering significant restructuring of the world's ecosystems. Though this was a catastrophe for some species it was a blessing for others, who would take their places and prosper within the changed ecosystems.

There is one inescapable conclusion: man is not the only species that alters the environment to suit its own needs. All animals have some impact on their environment, the interactions of large herbivores with vegetation and fire can be seen at work in Africa today. So, the next time some save-the-environment-from-the-evil-humans activist type tries to tell you people are destroying the natural world ask them which world they mean, because there is no unaltered “natural” environment. The only unaltered pristine environment, indeed, the only world where climate doesn't change, would be one with no life at all.

Be safe, enjoy the interglacial and stay skeptical.

Why do you claim that fire is

Why do you claim that fire is rare in an open savannah environment or that it would become more common in a forested environment? Annual fires and low rainfall (or rainfall concentrated at certain times of year) are the deciding factors for a thriving grassland environment. Forests would only have been able to grow and spread if the rainfall increased and the fires became more intermittent.

Savanna vs Forest wildfire

The paper's authors implied that, because of the cool but arid climate, fires in the savannas of paleolithic North America were less frequent than they are today. I was basically relaying the author's statement but I suspect the key here is the intensity of the fires, more than the frequency. Wooded forestlands have a tendency to collect more high grade fuel over time so, when a fire does break out, the result can be a major conflagration.

Bear in mind, the savannas of 14,000 BC were not the same as the prairie found in the Midwest today. Because of the changing climate these savannas gave way to a mixed tree-grass forest ecosystem. Although fire is thought to have played an important role historically in the dynamics of many mixed tree-grass vegetation systems, the details are poorly understood, especially for oak savannas in central North America prior to European settlement.

Wildfire occurs naturally and plays varying roles in nearly all terrestrial ecosystems. Because different types of ecosystems produce and accumulate fuel more quickly than others, the frequency and intensity of such fires are determined by the type and the stage of development of the ecosystem in which it occurs. Most North American terrestrial ecosystems were/are dependent to some extent upon fire and many plants have evolved adaptations that protect them from wildfire.

Today, fires in the prairie usually occurred in five- to ten-year cycles, with moderate regularity. Fire in tall grass prairies acts to burn above-ground biomass, killing woody plants, allowing sunlight to reach the soil, and changing the soil pH and nutrient availability. Grassland fires can cover large areas in a short time as fire fronts are driven by prairie winds. However, because grass provides a low quality of fuel, grassland fires usually are not intense. Because of predominantly westerly winds across American prairies, trees are sometimes found on the eastern bank of streams and rivers that stop fires spread by these winds.

As for fire in modern forests, it depends on the region. For example, Southern pine forests, consisting mainly of loblolly, shortleaf, or longleaf pines are found from Texas east to Florida, and north to Maryland. Lightning ignited fires in southern pine communities are common. More frequent fires favor longleaf pines, which are more fire adapted; less frequent fires tend to favor shortleaf and loblolly pines. Frequent fires also create pine savannas when shrubs are burned away, favoring the establishment of grasses beneath the pines.

Some references: Fire Dependent Ecosystems of the United States, and Effects of Fire Frequency.

wow that is a really neat

wow that is a really neat post :D thanks for sharing it and i hope to be seeing more in the near future. keep up the good work and keep on posting.

Grey Wolves

I seem to remember that Canis lupus shows up in North America in large numbers around 15,000 years ago. I always wondered what their connection to the Mega-fauna die off was.

Hello, Doug. I wrote on the

Hello, Doug.

I wrote on the topic.

http://gunnarlittmarck.blogspot.com/2009/11/fanns-det-en-stenalders-apok...

Just months took it for Europe to be engulfed by an ice age.

http://www.newscientist.com/article/mg20427344.800-mini-ice-age-took-hol...

My thoughts are that the cold can go fast and kill life, heat slowly and creates better living conditions?

I wonder if every climate scientists would like the measurement method is said to recreate month temperature 400 million years back?

Sincerely Gunnar Littmarck

Actually glaciation is slow

According to our best records of the transitions from glacial to interglacial and back it is deglaciations that are rapid and glaciations that are slow. Not that temperatures cannot swing wildly during a deglaciation (e.g. The Younger Dryas) so, in that sense, cold can come on quickly, but warming is just as fast or faster. But a true slide into glacial conditions requires time. This makes sense when you think about the huge quantities of ice that must form by precipitation.