In Jonathan Swift's "Gulliver's Travels," Lilliput was a fictional island nation inhabited by people "somewhat under six inches high" – the Lilliputians. Modern paleontologists have adopted the term, "the Lilliput effect," to mean something slightly different. Organisms that survive mass extinctions tend to be much smaller than those that came before.

A variety of presentations on the Lilliput effect at the recent annual meeting of the Geological Society of America (GSA) in Philadelphia explored the many reasons organisms can shrink. They include the knockout punches of volcanic activity and an asteroid strike thought to have killed off the dinosaurs 65 million years ago, and the peculiar evolutionary pressures exerted by islands. (Many of the papers from the GSA meeting won't be published until 2007.)

Of intense interest to all is how humans, often said to be causing Earth's "sixth mass extinction," will affect evolution on the planet. As with naturally occurring global catastrophes of the past, human activity has suddenly changed the definition of evolutionary fitness. Some evidence indicates that the human "footprint" is leading not only to the selection of smaller species over larger ones and generalists over specialists, but also the dwarfing of individuals within a species. The ecological disturbance implied by such miniaturization has implications not only for the species themselves, which may disappear, but for humankind's well-being.

Conventional wisdom says that as lineages move through time, they tend to get bigger, not smaller. Small protohorses, for example, evolved in North America, crossed the Bering Strait when it was dry land and evolved into the larger modern horse in central Asia, even as they died out in the Americas.

This "bigger is better" process is known as Cope's rule, named after 19th-century paleontologist Edward Drinker Cope. This rule is based on what would seem to be the obvious advantages of largeness: the ability to fight off predators and take advantage of a wider variety of resources, larger and potentially more numerous offspring, and better metabolic efficiency. But heftiness is advantageous only when the going is easy.

After a catastrophe, the opposite tendency – the Lilliput effect – prevails. "It's not that the things that survived became small," says Peter Harries, professor of geology at the University of South Florida. "It's that the small survived." Smaller species often have shorter reproductive cycles, enabling them to quickly recover from population losses. If they suffer a rapid decline, like the one implied by the huge asteroid impact that extinguished an estimated 50 percent of all species, they recover faster. They also need less food, which is probably in short supply after a cataclysm, than big animals do.

As with many "rules," exceptions to the Lilliput effect abound. While the overall size of land animals shrank after the dinosaurs died off, some mollusks stayed as large as ever, says David Jablonski, a professor of geophysical sciences at the University of Chicago: "It happens except for when it doesn't happen."

When times are tough, many organisms compensate by reaching sexual maturity faster than their progenitors did, says Gerta Keller, professor of geology and paleontology at Princeton University. But with limited resources, accelerated reproduction comes at a cost: Reaching sexual maturity in half the time, the new generation may be half the size, too.

"It seems to be a universal way of adapting," Professor Keller says, "getting smaller and reproducing faster." By having more offspring in a shorter period of time, organisms accelerate evolution and improve their lineage's survival chances.

But overly specialized species like the California condor – or, by extension, the dinosaurs – cannot "Lilliputify" fast enough. They become extinct, even as generalists such as rats and some of our mammalian ancestors (the "disaster opportunists") thrive.

A dramatic shift in body size also occurs when species end up on islands, both literal and figurative. The "island rule" says that when isolated from founding populations, small species tend to get large, and large species, small.

As recently as 11,000 years ago, dwarf elephants with only 2 percent the mass of their mainland cousins wandered the Mediterranean islands. Pygmy mammoths lived on the Channel Islands off the California coast until about the same time.

At the other extreme, the extinct dodo bird of Mauritius was descended from a much smaller member of the pigeon family, while Komodo dragons (which still live on Indonesian islands and can grow to 10 feet) may be descended from a small monitor lizard.

"Size reduction comes along quickly," says Larry Agenbroad, director of the Mammoth Site in Hot Springs, S.D. – maybe in as few as three generations. As islands generally have less food than mainland areas, pressure toward smaller body size is acute.

A 2003 study published in Conservation Ecology found that human activity in Denmark over the past 175 years was having a related effect on local fauna. Habitat fragmentation had caused 25 species, including mice, hares, and foxes, to change in size. Larger species were getting smaller, and smaller species, larger.

Tellingly, in all places except Africa, and to a lesser extent, South Asia, the arrival of Paleolithic hunters coincides with the vanishing of large animals, especially in the Americas. Although for different reasons (namely, humans' hunting prowess and their appetite for meat), some argue that the advent of modern humans turned largeness into a liability, as had previous catastrophes.

Others think that the removal of large terrestrial species – especially predators – has affected human health by allowing disease-carrying animals to multiply. Since humans hunted the eastern grey wolf out of existence, the incidence of disease carried by deer- and mouse-borne ticks has risen across eastern forests.

But nowhere is human-exerted pressure toward Lilliputianism more apparent than in the seas. Highly efficient modern fishing practices have largely removed large fish such as sharks, bluefin tuna, and swordfish. "We have skimmed off the cream of all the large predators," says Les Kaufman, professor of biology at the Boston University Marine Program.

Even within some fish species like cod, the average individual size has diminished. Bigger individuals get caught in nets, while smaller ones pass through, selecting for ever-smaller fish that mature at ever-younger ages. "That's why there are so many little fish in the world today," says Professor Kaufman.

While the disappearance of large species, the shrinking of others, and the proliferation of disaster opportunists worries scientists, what this means for the ecological web to which humankind still belongs worries them more.

"Our survival will depend to a large extent on how many other species we can keep alive," Keller says.

How wolves help aspens grow

Scientists have long noted that the removal of large, "keystone" species from an ecosystem can have unexpected and often bizarre effects.

For example, wolves' removal by human hand from Rocky Mountain ecosystems led to fewer aspen groves. Absent their traditional predator, deer no longer feared to tread in heavily wooded areas. They gorged themselves on the tasty trees.

Wolves' recent reintroduction in Yellowstone has reversed the trend. Seeking to avoid ambush, deer once again cluster in open spaces, and aspen groves have rebounded.

Dubbing it "the ecology of fear," Les Kaufman, a professor of biology at the Boston University Marine Program, says this relationship holds true across ecosystems.

For similar reasons, the removal by overfishing of large marine predators like sharks has contributed to the demise of coral reefs worldwide.

Without the top predators, the population of mid-sized predators skyrocketed. In what is called a "trophic cascade," the populations of the mid-sized predators' prey – small grazing species – plummeted. Without the grazing species, the seaweeds these small organisms ate soon overgrew the seafloor, crowding out other species and making it much harder for already stressed coral reefs to regenerate.

In other words, "there is a relationship between making shark fin soup and the death of the world's reefs," Professor Kaufman says.