African forest elephants are among the largest animals in the world. Males can grow to be more than eight feet tall and weigh almost three tons. Until recently, forest elephants were thought to be shy cousins of African savanna elephants, but in 2010 a genetic analysis suggested the two were actually separate species. If so, forest elephants are in especially big trouble. An exhaustive study of their range found that just in the nine years it took to assemble the data, the population had dropped by more than sixty percent, mostly owing to poaching. The scientists who conducted the study warned that forest elephants could be wiped out within a decade.
Thanks to human activities—deforestation, climate change, and overharvesting, whether legal or not—species are disappearing so fast it’s often said the world is entering a sixth mass extinction. (The last one was the event—most likely an asteroid impact—that did in the dinosaurs at the end of the Cretaceous Period, 66 million years ago.) One of the things that’s different about this event is, of course, us. Another is that this time around, size matters. These two things are connected.
In previous extinctions events, it doesn’t seem to have mattered how big an animal was; puny or prodigious, species had the same odds of surviving. Outsized celebrities like Tyrannosaurus rex get the attention, but most casualties of the fifth, or end-Cretaceous extinction, were tiny. Today, by contrast, large animals are much more likely to be endangered. They’re the sort humans tend to hunt.
A recent study of land animals weighing more than 100 kilos, or 220 pounds, found that sixty percent of large herbivores are currently threatened with extinction. Twelve percent are classified as “critically endangered” or already extinct in the wild; among these are the tamaraw, a buffalo-like creature native to the Philippines, which is down to fewer than 250 individuals, and the Javan rhinoceros, down to only about sixty individuals. African elephants, which despite the genetic evidence are still considered a single species under international law, are only listed as “threatened”—but if forest elephants were granted their own legal status, they too might join the list of “critically endangered.”
The picture isn’t much better for large carnivores. All of the world’s seven species of big cats— these include tigers, lions, and jaguars—are listed as either threatened or near-threatened. A century from now, National Geographic fellow Tom Lovejoy has written, big carnivores “will probably exist only in zoos or wildlife areas so small as to be quasi zoos.”
What would a world without big animals look like? To a certain extent, unfortunately, we already know. When the first people arrived in Australia, some 50,000 years ago, they encountered a menagerie of very large beasts: oversized kangaroos, lumbering marsupials known as rhinoceros wombats, and turtles eight feet long. Within a few thousand years, all of these enormous creatures had vanished. When people arrived in North America around 15,000 years ago, they found an even more diverse megafauna that included mastodons, saber-toothed cats, and giant beavers. Within a few thousand years, those big animals too were gone.
The fact that these waves of extinction did in so many large animals—and few or no small ones—strongly suggests that they were caused by humans. In pre-human times, very big animals had no predators; from an evolutionary perspective, that’s the whole point of being oversized. But once hunters with clubs or spears showed up, the advantage of being large was quickly outweighed by the disadvantage, which is that large animals tend to have low reproductive rates.
Forest elephants, for example, don’t reach sexual maturity for two decades. Their pregnancies last twenty-two months, and they produce, on average, only one calf every five years. Presumably mammoths, mastodons, and rhinoceros wombats were similarly slow to have babies. This means that once their numbers started to decline, it wasn’t hard for our ancestors to drive them all the way down to zero.
Yet even though many large animals were lost from many parts of the world, it can be hard to identify the precise consequences of their absence. They vanished so long ago, and no one left behind a study showing what the landscape looked like in the days when the megafauna still roamed. Today, scientists are trying to anticipate what the loss of more large species will mean.
In Kenya, Douglas McCauley, an ecologist at the University of California-Santa Barbara, has created a series of “exclosures”—gated plots from which all large animals are excluded. The idea is to study the plots before they’re fenced and then come back and look at what happens once the elephants and hippos and giraffes are removed.
“It’s an experimental system where you essentially have a time machine,” McCauley said; after the plots are fenced they “look like an overhunted space of future Africa.” What McCauley and his wife, Hillary Young, also of UCSB, have found is that the exclusion of large mammals leads to an explosion in the number of smaller mammals, like rats and mice. This in turns leads to an explosion in the number of creatures living on the rats and mice, like fleas, which in turn can carry diseases like plague.
“It’s probably safe to say this is a fairly ubiquitous response,” McCauley observed.
Big animals are now disproportionately threatened in the seas too. A recent study in the journal Science found the extinction threat to marine creatures to be “strongly associated with large body size”—and in an unprecedented way. “The bad news from our study is that we can’t find an extinction event in the marine fossil record that looks like what we’re doing today,” said the lead author, Stanford University paleobiologist Jonathan Payne.
But for marine creatures, Payne noted, there’s also some (admittedly qualified) good news. Unlike on land, where many groups of large animals— mastodons and saber-toothed cats, for instance—are already gone, in the oceans only one genus has been eliminated by human hunting: the one that included the Steller’s sea cow, an enormous, dugong-like animal that lived in the North Pacific. It was extinguished in the 18th century, soon after Europeans first encountered it.
“Bringing back mastodons and saber-toothed cats is going to require technologies we don’t have today,” Payne observed. “But almost everything that we studied in the oceans is endangered, not extinct. So we still have the opportunity with the right management decisions to keep all of these species around for the long-term. We just have to have the political will.”