effective at best and, at worst, just poisonous and futile. The caterpillars continued to chomp. When it appeared that they might move from ravaged trees to healthy ones, in search of more food, we tried to stop them by girdling the tree trunks with barriers of impassable goo. This was pointless (since, as I learned later, a tent caterpillar generally lives out its larval stage in the tree where it hatched) but reflected our desperation. I watched my next-door neighbor, Susan, muster such hopeful defenses for two giant elms in front of her house, each tree banded at waist height with a circular belt of spray-on stickum, and it seemed like a reasonable idea to me too. But the stuff failed to catch a single caterpillar.
They kept coming. They had their way. There were simply too many, and the infestation proceeded along its inexorable course. We stepped on them as they forded the sidewalks. We mooshed them wholesale in the streets. They ate, they grew, they molted their tight old skins and grew further. They marched up and down limbs, all over town, treating our trees like celery.
Eventually they finished eating. They had bulked themselves up to the limits, fulfilled their caterpillar juvenility, and now they were ready for puberty. They spun themselves up inside leaf-wrapped cocoons for a short metamorphic respite, to emerge in a few weeks as little brown moths. The crackling stopped and the treetops, what was left of them, fell silent. The caterpillars, qua caterpillars, were gone. But this vast population of pestiferous insects still lurked over our heads, almost invisible now, like a large gloomy hunch about the future.
Ecologists have a label for such an event. They call it an outbreak.
This use of the word is more general than what’s meant by an outbreak of disease. You could think of disease outbreaks as a subset. Outbreak in the broader sense applies to any vast, sudden population increase by a single species. Such outbreaks occur among certain animals but not among others. Lemmings undergo outbreaks; river otters don’t. Some kinds of grasshopper do, some kinds of mouse, some kinds of starfish, whereas other kinds of grasshopper, mouse, and starfish do not. An outbreak of woodpeckers is unlikely. An outbreak of wolverines, unlikely. The insect order Lepidoptera (moths and butterflies) contains some notable outbreakers—not just tent caterpillars of several kinds but also gypsy moths, tussock moths, larch budmoths, and others. Those are exceptions, though, to the general rule even for lepidopterans. Among all the forest-dwelling species of butterfly and moth, about 98 percent maintain relatively stable populations at low density through time; no more than 2 percent ever experience outbreaks. What makes a species of insect—or of mammal, or of microbe—capable of the outbreak phenomenon? That’s a complicated question that the experts are still trying to answer.
An entomologist named Alan A. Berryman addressed it some years ago in a paper titled “The Theory and Classification of Outbreaks.” He began with basics: “From the ecological point of view an outbreak can be defined as an explosive increase in the abundance of a particular species that occurs over a relatively short period of time.” Then, in the same bland tone, he noted: “From this perspective, the most serious outbreak on the planet earth is that of the species Homo sapiens.” Berryman was alluding, of course, to the rate and the magnitude of human population growth, especially within the last couple centuries. He knew he was being provocative.
But the numbers support him. At the time Berryman wrote, in 1987, the world’s human population stood at 5 billion. We had multiplied by a factor of about 333 since the invention of agriculture. We had increased by a factor of 14 since just after the Black Death, by a factor of 5 since the birth of Charles Darwin, and by doubling within the lifetime of Alan Berryman himself. That growth curve, on a coordinate graph, looks like the southwest face of El Capitan. Another way to comprehend it is this: From the time of our beginning as a species (about 200,000 years ago) until the year 1804, human population rose to a billion; between 1804 and 1927, it rose by another billion; we reached 3 billion in 1960; and each net addition of a billion people, since then, has taken only about thirteen years. In October 2011, we came to the 7-billion mark and flashed past like it was a “Welcome to Kansas” sign on the highway. That amounts to a lot