of people, and certainly qualifies as an “explosive” increase within Berryman’s “relatively short period of time.” The rate of growth has declined within recent decades, true, but it’s still above 1 percent, meaning that we’re adding about 70 million people yearly.
So we’re unique in the history of mammals. We’re unique in the history of vertebrates. The fossil record shows that no other species of large-bodied beast—above the size of an ant, say, or of an Antarctic krill—has ever achieved anything like such abundance as the abundance of humans on Earth right now. Our total weight amounts to about 750 billion pounds. Ants of all species add up to a greater total mass, krill do too, but not many other groups of organisms. And we are just one species of mammal, not a group. We’re big: big in body size, big in numbers, and big in collective weight. We’re so big, in fact, that the eminent biologist (and ant expert) Edward O. Wilson felt compelled to do some knowledgeable noodling on the matter. Wilson came up with this: “When Homo sapiens passed the six-billion mark we had already exceeded by perhaps as much as 100 times the biomass of any large animal species that ever existed on the land.”
Wilson meant wild animals. He omitted consideration of livestock, such as the domestic cow (Bos taurus), of which the present global population is about 1.3 billion. We are therefore only five times as numerous as our cattle (and probably less massive in total, since they’re each considerably bigger than a human). But of course they wouldn’t exist in such excess without us. A trillion pounds of cows, fattening in feedlots and grazing on landscapes that formerly supported wild herbivores, are just another form of human impact. They’re a proxy measure of our appetites, and we are hungry. We are prodigious, we are unprecedented. We are phenomenal. No other primate has ever weighed upon the planet to anything like this degree. In ecological terms, we are almost paradoxical: large-bodied and long-lived but grotesquely abundant. We are an outbreak.
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And here’s the thing about outbreaks: They end. In some cases they end after many years, in other cases they end rather soon. In some cases they end gradually, in other cases they end with a crash. In certain cases, even, they end and recur and end again, as though following a regular schedule. Populations of tent caterpillars and several other kinds of forest lepidopterans seem to rise steeply and fall sharply on a cycle of anywhere from five to eleven years. A population of tent caterpillars in British Columbia, for instance, has shown a cycle like that dating back to 1936. The crash endings are especially dramatic and for a long while they seemed mysterious. What could account for such sudden and recurrent collapses? One possible factor is infectious disease. It turns out that viruses, in particular, play that role among outbreak populations of forest insects.
Back in 1993, when the caterpillars hit my town, I got interested in this subject and did some research. It seemed peculiar to me that a critter like the forest tent caterpillar, with a very limited repertoire of behavior, a fixed set of adaptive tactics, should multiply egregiously during one or two summers and then virtually disappear by summer three. The environment hadn’t changed drastically, yet the success of one species within that environment had. Why? Variations in weather didn’t explain it. Exhaustion of food supplies didn’t explain it. I called the county extension service and pestered a fellow there with questions. “I don’t think anyone can say why you have the boom and bust,” he told me. “It just happens.”
Because that reply wasn’t satisfactory or convincing, I started reading the entomological literature. Among the experts in the field was one Judith H. Myers, a professor at the University of British Columbia, who had published several papers on tent caterpillars and an overview of insect population outbreaks. Myers offered a solution to the mystery. Although population levels are influenced by many factors, she wrote, the cyclical pattern “seems to imply a dominant force that should be easy to identify and quantify. That driving force, however, has proved surprisingly elusive.” But now ecologists had a suspect, she reported. Myers described something called nuclear polyhedrosis viruses, known collectively as NPVs, which “may be the long-sought driving force of population cycles in forest Lepidoptera.” Field studies had revealed that NPVs achieve their own outbreaks within outbreaking populations of forest lepidopterans, killing off the insects