Shrink to Fit
Humanity appears to be ushering in a new age of minifauna—a kind of Lilliputian world full of runts and dwarves
By David Malakoff
At about the size of a dime, Kugelann’s green clock beetle would never be mistaken for a giant. But in the world of European ground beetles, Poecilus kugelanni is no runt. Indeed, some Belgian biologists recently classified the gaudy, green-winged creature as a “big” beetle.
Big, however, hasn’t proved better. Green clock populations have crashed over the past century due to habitat destruction and other threats, and the insect is now endangered in many places. And it’s not alone: dozens of Europe’s other big-beetle species are also fading away, even as many of their smaller cousins seem to be holding on.
It’s a pattern that researchers seem to be seeing everywhere. Around the planet, relatively large species are in big trouble—from lions and tigers and bears to cod, condors, and conifers. Even some heftier snails and salamanders are struggling. “Size matters,” says biologist Chris Darimont of the University of California, Santa Cruz, who notes that the assaults are coming from several angles. On one front, “a larger body size makes a species more vulnerable to all kinds of problems, from getting hunted by humans to habitat change.” One result: Nearly half of the world’s large “megafaunal” mammals—and more than half of the largest marine fish—are now considered vulnerable to imminent extinction.
Meanwhile, overfishing, overhunting, and pollution—and perhaps global warming—are fueling another downsizing trend. These pressures are causing some plants and animals to evolve with astounding rapidity, producing individuals that are on average shorter, thinner, and lighter. In other words, they are literally shrinking. It’s a “vastly underappreciated problem,” says Darimont, whose own work has shown that a wide range of hunted organisms now have body sizes that have shrunk on average by one-fifth. And human “superpredators” are causing this shrinkage to occur incredibly rapidly, sometimes in just a few decades—or up to 300 percent faster than in natural systems. (1)
Together, the trends have some observers wondering whether we’re on the verge of a new “age of the minifauna,” a kind of Lilliputian world full of runts and dwarves?
It turns out that this is a very old story with a modern twist. More than 25,000 years ago, one megafaunal species—we humans—began to spread rapidly around the globe and in the process helped to wipe out about half of all land mammals weighing more than 44 kilograms (97 pounds). “More than 101 genera perished,” Anthony Barnosky, an ecologist at University of California, Berkeley, reported in a 2008 study in Proceedings of the National Academy of Sciences (PNAS). (2) Among the victims were whole groups of mammoths, mastodons, saber-toothed tigers, giant ground sloths, and big beavers. Many vanished in just a 4,000-year span that ended about 11,000 years ago. By then, Australia had lost roughly 88 percent of its big mammal groups, South America 83 percent, and North America 72 percent. Africa did better during what is now called the Quaternary Megafauna Extinction (QME), losing about one-fifth of its big species, while Eurasia lost one-third.
Exactly what caused the QME has been the subject of long and fierce debate, but most explanations finger some combination of two ingredients: human hunters and rapid climate change. It doesn’t take a PhD to realize that big mammals roaming across vast territories were obvious, attractive targets for hungry hunters seeking the biggest bang for their buck. It takes a little math, however, to see just how quickly unconstrained “overkill” can eliminate a species, such as the elephant, that reproduces slowly (as big mammals tend to). Sometimes, it can take just a few human generations. Toss hunting pressure into an environment already changing rapidly due to events such as human-set fires and yo-yo climate shifts, and it’s no surprise that the stress “robbed global ecosystems of the biggest animals on Earth,” Barnosky told a packed lecture hall a few years ago.
By his count, just 183 large-mammal species survived the catastrophe, often in dramatically reduced numbers. And their days are numbered unless we learn from the past, he argues in his provocative and eye-opening PNAS paper. The fundamental problem, he says, is that we’re literally taking the lion’s share of Earth’s resources—and the shares needed by all other megafauna, too—for ourselves. The QME represented “a dramatic change in the way energy flowed through the global ecosystem,” he writes. Before the extinction, there was easily enough “biomass”—the fundamental source of energy created when plants convert the sun’s rays to edible tissue—to support some 350 big-mammal species. As hungry Homo sapiens spread, however, “energy began to flow toward a single megafauna species: humans.” In addition to simply eating other big animals, we grabbed vast swaths of their habitat to grow crops and raise cows, goats, and sheep. In essence, Barnosky says, “we replaced the extinct megafauna with us and in the process lost a bunch of species that are never coming back because we now have grabbed their biomass.”
Today, mammals aren’t the only group at risk of losing their larger species. In just the past few years, researchers have reported that larger fish, frogs, salamanders—and even marine snails—often face higher survival risks, as do certain groups of big birds such as vultures and cranes. Sometimes, as in the case of big tuna and the large frogs and sea snails, it happens because we like to kill and eat them. In other cases, large animals such as sharks and seabirds are forced to compete directly with us for food, including dwindling fish stocks. And last year, Belgian researchers reported that even the number of big-bodied carabid beetles in Europe—including Kugelann’s green clock—had declined noticeably since the 1950s. It’s not clear why, but in Biological Conservation they speculate that big beetles tend to have larvae that spend long periods growing in soil, making them more vulnerable to disturbance. (3)
One puzzle, however, remains: Why have just a handful of big mammals actually gone extinct in the past few thousand years, even as human and livestock populations skyrocketed? One answer is that some, such as the American bison, are actually “dead species walking”—reduced to small, unsustainable population sizes or marginal habitat fragments and unable to survive without human help. Another answer, believes Barnosky, is that we temporarily took pressure off wild ecosystems when we discovered fossil fuels and then used that energy to supercharge our ability to feed and shelter ourselves. But that era appears to be closing, he notes, as humans press in on the few remaining habitats still dominated by nonhuman megafauna. It’s no coincidence, he says, that human impacts now threaten some 90 species of large mammals, including 40 percent of those in Africa, a continent that made it through the QME largely unscathed. And with a rapidly warming climate taking hold, he says, the present is beginning to look eerily like the past: “Growing human populations and climate change? Beginning to sound familiar?”
In the modern version of the shrinking tale, however, humans have become a kind of “superpredator” quite unlike any before. Where other hunters cull the young and the defenseless or the old and the sick, we take the trophies: the biggest bucks, the meatiest fish, and the tallest trees. Ironically, however, this size-selective killing often pushes individuals within a species to grow smaller on average. “Humans have a penchant for unintentionally selecting against that which they desire most,” biologists Jeffrey Hutchings and Dylan Fraser concluded in a 2008 paper in Molecular Ecology. (4) Killing big fish, for instance, favors those that mature at smaller sizes and younger ages, enabling them to pass those traits along to more offspring and progressively downsize whole schools. “Put simply, the message to the fishing industry, resource managers, and decision makers is, ‘keep the big ones around,’” conclude Hutchings and Fraser, who work at Dalhousie University in Canada. It’s a message the University of California’s Darimont and his colleagues implicitly echo in their 2009 PNAS analysis, which found that “harvested organisms show some of the most abrupt trait changes ever observed in wild populations”—faster even than with organisms living in habitats battered by pollution or other human impacts.
Overall, more than 100 species are subject to size-selective harvesting, Phillip B. Fenberg and Kaustuv Roy of the University of California San Diego reported in a 2008 Molecular Ecology paper. (5) And that can have some unexpected and ecologically dangerous consequences. Eliminating trophy-male moose and reindeer from herds, for instance, forces females to mate with younger, smaller males—setting off a complex chain reaction that results in smaller calves that don’t survive as well as bigger babies. Killing dominant male grizzly bears and lions can cause a less dominant male to take over—and promptly kill all the young sired by his predecessor, slowing reproduction. Some fish and invertebrates change sexes as they grow, so taking just the big ones can badly skew sex ratios, imperiling reproduction. One of the more bizarre outcomes, however, involves African elephants. When hunters exclusively kill big males with big tusks, younger males can enter a life stage known as “musth” early. Musth is often marked by very aggressive behavior, including elephants attacking and killing people—and other endangered species, such as rhinoceros.
It’s not yet clear whether this kind of selection pressure is causing species-wide genetic changes that could make the shrinkage permanent. Some studies have suggested that generations of removing the biggest trees from forests, for instance, could sift out some of the genes for “bigness.” And in a simple but labor-intensive experiment, biologist David Conover—now head of the Ocean Sciences program at the U.S. National Science Foundation—showed that although it doesn’t take long to downsize a species, bouncing back can be a very long process indeed. Conover’s team raised schools of small fish called Atlantic silversides in a series of tubs at Stony Brook University in New York and then intentionally overfished them. From one tub, for instance, they repeatedly removed just the largest fish for five generations. As expected, the minnows evolved to become even smaller. Then the researchers stopped “fishing.” After five more generations, the fish were getting bigger again—but still not back to their prefishing size. “We found that the change was reversible, but estimated it was going to take at least 12 generations for the fish to recover,” Conover says. In the real world, he adds, that means “it could take many decades to see recovery from intensive harvesting.”
It may be harder, however, to reverse the potential downsizing effects of global warming. In general, scientists agree that past changes in Earth’s climate have influenced body size—and that physiological factors suggest warmer temperatures should favor smaller bodies (in part because smaller animals have a relatively greater surface area, so it is easier for them to stay cool). So far, however, there’s no strong consensus that current warming is shrinking species, although there is growing evidence that this is happening. Studies of 17 warm-blooded vertebrates, for instance, showed that nearly three-quarters had declining body mass, Joanne Isaac of James Cook University reported in a 2009 survey for Endangered Species Research. (6) And migrating birds captured at a banding station in Pennsylvania over the past 50 years have become progressively smaller, a research team reported last year in Oikos. (7) Such trends could be seen as good news, Isaac notes, since smaller species tend to have lower extinction risks. But other climate-related changes, such as changes in breeding seasons and rain patterns, could actually result “in an increase in extinction risk,” she warns.
How such changes in body size—and the loss of big species altogether—would reshuffle ecosystems isn’t always clear, but researchers say the changes could be dramatic. The disappearance of a big top-predator, for instance, can allow populations of smaller animals to explode, since they are no longer getting eaten (think mice without the cat). The loss of some big species—such as whales—can even wipe out whole “body ecosystems” that support a suite of parasites and other mutualistic organisms that can live nowhere else. Big species such as elephants can also physically “landscape” their habitats—by knocking down trees or trimming shrubs—in ways that smaller species can’t.
Big species can also play very different ecological roles than small ones do. Big mammals, for instance, tend to be wide-ranging grazers or carnivores, while small ones target seeds or insects. And because there are usually fewer big species in an ecosystem, there’s less chance that a smaller species can take on the role of a larger, lost species. Indeed, in a paper published last year in Proceedings of the Royal Society B, Susanne Fritz and her colleagues showed that maintaining an ecosystem’s “body mass variation”—by saving both big and little mammals, for instance—appears to be key to keeping its “functional” diversity intact. (8) “Ecosystems appear to be more resilient if you keep that huge variety of body sizes,” Fritz says.
That’s a lesson not lost on conservationists. Some are working hard to prevent the 25,000-year-old process of sifting relatively big creatures out of wild ecosystems from entering a disastrously new phase. One strategy is to more effectively target efforts to save big species. Last year in Ecology Letters, for example, Fritz and her colleagues suggested that big mammals living near the equator may need more immediate help than those living in cooler climes. (9) “Big is bad, but only in the tropics,” they wrote, summarizing an analysis that revealed that big-mammal species are most vulnerable in tropical regions where there are still big chunks of relatively undisturbed habitat. Ironically, that’s partly because “we’ve already caused big species to decline in temperate areas that have undergone agricultural conversion,” she says.
To prevent a repeat of that process, other preservationists are focusing on saving more than 100 “hotspots”—covering 21 percent of Earth’s surface—that still contain all the large-mammal species they had 500 years ago, according to a 2007 study in the Journal of Mammology. (10) Still others are taking sophisticated looks at the life histories and reproductive rates of some big predators to figure out just how little habitat it might take to save them. Protecting just six percent of the tiger’s remaining range, for instance, could help boost the big cat’s numbers—and some nations are already moving to defend these core areas. At least a few conservation-minded megafauna, it seems, aren’t ready to start humming, “It’s a small world, after all.” ❧
DAVID MALAKOFF is a science writer based in Alexandria, Virginia.
Darimont, C.T. et al. 2009. Human predators outpace other agents of trait change in the wild. Proceedings of the National Academy of Sciences. doi:10.1073/pnas.0809235106.
Barnosky, A.D. 2008. Megafauna biomass tradeoff as a driver of Quaternary and future extinctions. Proceedings of the National Academy of Sciences. doi:10.1073/pnas.0801918105.
Desender, K. et al. 2010. Changes in the distribution of carabid beetles in Belgium revisited: Have we halted the diversity loss? Biological Conservation. doi:10.1016/j.biocon.2010.03.039.
Hutchings, J.A. and D.J. Fraser. 2008. The nature of fisheries- and farming-induced evolution. Molecular Ecology. doi:10.1111/j.1365-294X.2007.03485.x.
Fenberg, P.B. and K. Roy. 2008. Ecological and evolutionary consequences of size-selection harvesting: How much do we know? Molecular Ecology. doi:10.1111/j.1365-294X.2007.03522.x.
Isaac, J.L. 2009. Effects of climate change on life history: Implications for extinction risk in mammals. Endangered Species Research. doi:10.3354/esr00093.
Van Buskirk, J., R.S. Mulvihill and R.C. Leberman. 2010. Declining body sizes in North American birds associated with climate change. Oikos. doi:10.1111/j.1600-0706.2009.18349.x.
Fritz, S.A. and A. Purvis. 2010. Phylogenetic diversity does not capture body size variation at risk in the world’s mammals. Proceedings of the Royal Society B: Biological Sciences. doi:10.1098/rspb.2010.0030.
Fritz, S.A., O.R.P. Bininda-Emonds and A. Purvis. 2009. Geographical variation in predictors of mammalian extinction risk: Big is bad, but only in the tropics. Ecology Letters. doi:10.1111/j.1461-0248.2009.01307.x.
Morrison, J.C. et al. 2007. Persistence of large mammal faunas as indicators of global human impacts. Journal of Mammalogy. doi:10.1644/06-MAMM-A-124R2.1.
Illustration by Philip Nagle