Full disclosure: I was really into dinosaurs when I was a kid. And I don't mean all that Land Before Time Petrie stuff. I'm talking about taking a serious interest in paleontology. There must be something appealing to a three-foot-high and therefore powerless kid about a creature that could smush houses and authority figures alike. Our family cat was lame by comparison. But besides the zoo I've never gotten close to any animal that even approximates a dinosaur's size. "Megafauna"—as they're scientifically known—are increasingly hard to come by, and our current biggest animals are starting to shrink.
Dinosaurs were the first megafauna, and they set the bar pretty high for what has become a contested taxonomic term. Since the end of the Jurassic, large animals have consistently met the dinosaur's fate, and we've had to scale down what qualifies under our definition of megafauna. These days, any animal with a biomass larger than 44kg (97 lbs) gets to hang out in the megafauna club. This means that most humans--on average 148 lbs--are considered megafauna. In fact, we're on the upper end of the contemporary megafauna spectrum, though we still lose the size game to players like whales and elephants (the most mega in sea and on land, respectively). Most animal species today fall between 11 and 23kg in size—not even close to the megafauna cut-off.
That's no biomass match for even a medium dinosaur like 20-ft long Ceratosaurus. So how did puny humans get so near top of the size chain? Most of us are aware of (or do not dispute) the great extinction of the dinosaurs, but the greatest animal extinction ever took place much more recently. Between 50,000 and 7,000 years ago, the Quaternary Megafauna Extinction (QME) killed off two-thirds of mammals and half of all species with a biomass over 97 lbs, according to a 2007 paper published by the National Academy of Sciences. In North America, among the disappeared were the elephantine woolly mammoths and mastodons, giant ground sloths and saber-toothed tigers.
And just as a combination of human hunting and climate change killed off the megafauna of yore, it still threatens the (not so) megafauna of today. Not only are humans pushing big beasts to extinction, we're also impacting the average size of the surviving species: megafauna are getting smaller. Doomsday inclined scientists are warning that if we don't wise up to megafauna extinction patterns, we'll be left without any big fuzzy, feathered or scaly buddies.
You must be this big to ride
Scientists have gone back and forth between "big chill" or "big kill" hypotheses for the cause of the QME. Initially, the Ice Age, which blanketed half of the North American continent with glaciers, was blamed for freezing megafauna to death. But in the late '60s, geoscientists proposed the "blitzkrieg hypothesis" that laid guilt on humans migrating across the Bering Strait for North American megafauna extinction. Upon new continental arrival, we started hunting mastodons and other large game out of the picture.
The NAS article argues for a combination of the chill and kill hypotheses. Biologist Anthony Baronsky writes that changes in climate, while certainly megafauna-altering, were insufficient on their own to cause the QME. Rather it was climate change coinciding with a rise in human activity that led to mass extinction.
Over this period, as average human biomass increased, average non-human megafauna biomass decreased: as we got bigger, everyone else got smaller. It's not that the big guys up and shrank (what self-respecting woolly mammoth would want to become a woolly medium?); to put it simply, we killed off anything bigger either on purpose or through resource competition.
And even if they could outrun us, the megafauna couldn't escape the Ice Age, which knocked out species that couldn't handle the cold or invent fire like humans. Then, the first great global warming took place when the Ice Age's ice melted about 13, 000 years ago. This development proved lucky for tropically inclined hominids but unlucky for the megafauna who had by then adapted their large biomass to chillier climes (blubber proves inconvenient on 80-degree days).
For Baronksy, it was the intersection of human biomass increase with climate change that caused the QME and made it "an ecological threshold event." Afterwards, "humans became dominant in the global ecosystem."
This newfound global dominance supports the blitzkrieg hypothesis for more recent megafauna extinctions. When the rest of the continents had waved farewell to their flat-headed, pig-snouted Platygonus and long-legged Camelops, isolated Australian meagafauna were running amok. Then human colonization wiped out the wildlife—85 percent of Australia's megafauna disappeared following human contact. Most of those lost were grazers, larger-than-ostrich birds and giant marsupials (kangaroos are cute, but imagine a marsupial with room in its pouch for you!).
I swear it was this big
According to Chris Darimont, a biologist at the University of California, Santa Cruz, modern-day human predators have a palpable effect on biodiversity. Given the numbers of species extinct or endangered due to human influence (including those during the QME), this may seem obvious. But Darimont's analysis goes deeper to examine the effects human predators have not only on diversity among species, but within a particular species. By selecting the largest, most mature members of a species as our targets, we are effectively making animals smaller, affecting not only extinction rates but now evolution. In a research summary in January's Nature, Darimont and his colleagues found "predation by humans drives changes in exploited prey much faster than other evolutionary pressures do."
Fair enough, but it's the speed at which our hunting depresses populations that counts. The QME took place over tens of thousands of years, but by examining 29 species of modern mammals and fish, the researchers found "changes in human-harvested systems occurred more than 300 percent faster than in natural systems, and 50 percent faster than in systems affected by other human influences, such as pollution." They concluded, "Human predation works so quickly because it is often felt by large proportions of the adults in populations." That fisherman holding up his big catch might actually be changing the genetic possibilities of a species.
In a simple genetics lesson, targeting large phenotype animals—the biggest bass or the biggest bison—eliminates their big genes from the pool and means only the smaller members of their species can reproduce, passing their small-size inheritance on to future generations. Average biomass decreases.
Already, two biologists are arguing in this month's Science for a redefinition of the term 'megafauna' to be context-specific. In the face of further megafauna extinctions as well as human-related downsizing of big beasts, they think the current megafaunal threshold is inadequate for some ecosystems. For example, if all endangered South American fruit-eaters went extinct, the largest frugivore left would be the howler monkey. Weighing in at 9kg, it is 70 times smaller than the giant sloth, South America's reigning largest mammal. "Mega" then becomes a relative term.
All the paleobiological history points clearly to the present moment. Baronsky places our current global ecosystem at a "similar crossroads" to the QME, "because growth of human biomass in the past few decades has moved us to the point where we are beginning to co-opt resources from, further displace, and cause extinctions of species with whom we have been coexisting for 10,000 years." And don't forget that "earth's climate is warming even faster than the rates of climate change that characterized the QME." In short, trade your emus for chickens, your grizzlies for badgers. And even if we keep the big guys around, bringing them down to our biomass level loses something more than weight. Even if you're not a hardcore conservation biologist, there's something disappointing about a world where rhinos and giraffes are the size of Midway game prizes.
SIMONE LANDON B'10.5 prefers giant pandas.