Ghost World - Part I. Dearly Departed

Imagine walking into a ghost town, recently deserted. It wouldn’t take you long to realize what’s missing. Cars are still parked along the streets, batteries slowly failing; sprinkler systems still spring to life on schedule, watering lawns that are steadily growing out of control; untended gardens are gradually overtaken by weeds, while insects and rodents dart in and out of homes with nothing to stop them but cats turned feral now that no one is leaving food out for them. All of these things are meant to be there, but they aren’t doing what they’re meant to do. Without people to maintain the town, it has descended into disrepair and instability.

Our world is in a state of disrepair and instability for the same reason – something important is missing. We live in a ghost world.

Take a trip back in time one million years, and you’ll find a world surprisingly similar to today’s. The continents are all in the same place, though things are a fair bit icier, and the ecosystems are mostly recognizable: birds in the sky; fish in the sea; on land, small mammals search for food while hiding from small predators; large herbivores stick together while watching out for large predators. But then there’s something else: giants. Mammoths, ground sloths, glyptodons, wooly rhinos. These aren’t just large herbivores, they’re megaherbivores, the most famous creatures of the Pleistocene Epoch, dwarfing all other animals with their exceptional size.

But here’s the thing: their size isn’t as exceptional as it seems. In fact, megaherbivores have been a feature of global terrestrial ecosystems continuously for at least the past 40 million years, from the pantodonts and uintatheres of the Eocene, to the gomphotheres and grazing rhinos of the Miocene, to the Ice Age giants of the Pleistocene. But within the last several dozen millennia, things changed. Extinction tore through Earth’s large animal populations, diminishing all sorts of “normal-sized” large animals like horses and camels, and completely erasing almost all of the megaherbivores on the planet. During the Late Pleistocene, at least 50 species of megaherbivore lived across 6 continents; today, only the elephants, rhinos, and hippos remain, a total of 9 species on 3 continents, and still today those species suffer and diminish under the weight of human activity and environmental change (the same factors that likely drove all the others to extinction).

This fantastic figure indicates how many species of  megafauna remain today on each continent,
out of the total number that were present in the Late Pleistocene. The four different maps
show megaherbivores, megacarnivores, large herbivores, and large carnivores.
From Malhi et al. 2016. 

This much, scientists have known for a while, but recently researchers have been asking the question: What happens when you suddenly (geologically speaking) remove most of the largest animals from global ecosystems that have co-evolved with those large animals for millions of years? This question was a central focus of late January’s issue of PNAS. The papers within define “megaherbivores” as land animals over 1,000kg (part of the “megafauna,” land animals over 45kg), and explore the various ways our world is haunted by the enormous ghosts of the past.

The most obvious answer is one a second grader could tell you: remove prey from an ecosystem and the predators suffer. And indeed, the Pleistocene was also home to “megacarnivores,” including many species of lions and tigers and bears (and sabertooths), more than half of which disappeared along with the megaherbivores.

The more dramatic answer is that when large herbivores are removed, the entire landscape changes. Megaherbivores often specialize in eating and trampling tough woody plants (elephants do a lot of this today). In both modern times and in the fossil record, the disappearance of large herbivores is often followed by a take-over by those plants. Much like our ghost town’s un-maintained gardens, grasslands and open woods become overgrown with dense forest full of tough woody vegetation now free from suppression. In the colder regions, the dry grassy “mammoth steppes” of the Pleistocene have disappeared with megaherbivores gone, replaced by unpalatable forest and shrubland in water-logged soils.

These “overgrown” ecosystems are a bit of a problem. For one thing, remaining herbivores find little to eat in forests full of such tough vegetation. For another, the forest and shrubland suddenly become very homogeneous – imagine a garden’s rich assortment of vegetables overgrown by one stubborn strain of weeds – and a low-diversity ecosystem has trouble adapting to new threats and changes. But most intriguingly, a region overloaded with vegetation is at higher risk for wildfires. In many parts of the prehistoric record, the extinction of megaherbivores from an ecosystem is followed by an increase in the frequency of forest fires. Smokey the Bear doesn’t prevent nearly as many fires as his prey does!

While some plants thrive too well in the absence of megafauna, others suffer greatly. One of the most fascinating results of co-evolution is that many plants cannot spread from place to place without the help of the animals that eat their fruits and poop their seeds out in far-away places. Numerous examples of modern-day plants with tough fruits, big seeds, and harsh defenses, are thought to have previously been prey to megaherbivores which would also have dispersed their seeds, perhaps over long distances. In a modern world of more modestly-sized plant-eaters, these “megafaunal fruits” may no longer be dispersing properly; like the abandoned cars in our ghost town, they’re ready for travel, but there’s no one around to drive them anymore.

"Megafaunal fruits" evolved in a world of giant herbivores. Left: the honey locust
is covered in preposterously large thorns, thought to have evolved to protect against
preposterously large herbivores. Right: the large tough fruit of osage orange isn't
eaten much by modern herbivores, and thus doesn't disperse very well in modern times.
Images from Wikimedia Commons.

Some researchers have even suggested that the loss of the giant mammals may have affected our climate. The removal of big plant-eaters can mean a lot more plants, which can mean higher rates of transpiration (moisture evaporating from plant leaves), and more CO2 being taken up and held by plants, not to mention a big reduction in animal-exhaust fumes (that is to say methane, a major greenhouse gas). An experiment in Russia has shown that soil temperatures can be lowered by over 15°C just by adding large herbivores which remove dark vegetation to expose bright snow, which reflects warm sunlight away from the soil.

For millions of years, giant herbivores have made up an important piece of worldwide ecosystems, but modern-times are megaherbivore-impoverished, and some researchers are worrying that we live in a world of incomplete and unstable ecosystems, where dense forests are full of tough inedible plants, where nutrients and seeds just aren’t dispersing as widely as perhaps they should be, and where even wildfires and local climates may be affected by the mammoth-sized gaps in the landscape. We live in a world recently abandoned by some of its most important residents, struggling to find balance in their absence – a ghost world. ­

Is it all doom and gloom for our broken ecosystems? Maybe not. For wherever there is a problem in the world, you can be sure there are cocky humans who think they can fix it (and perhaps they can). In Part II of this series, we’ll look at plans to bring back the ecosystems of the past.

References:
Malhi et al. 2016. Megafauna and ecosystem function from the Pleistocene to the Anthropocene. PNAS doi: 10.1073/pnas.1502540113 (PDF)

Bakker et al. 2016. Combining paleo-data and modern exclosure experiments to assess the impact of megafauna extinctions on woody vegetation. PNAS doi: 10.1073/pnas.1502545112 (PDF)