The parking spaces at Costco are especially wide, laid-out to accommodate vehicles like the immense Ford Excursion that Veronica now turns cautiously toward an open berth. She has picked a spot that is just to the left of another Ford Excursion; in fact, it appears to be precisely identical to our rental, so while we edge gradually into place, I get to watch the reflection of our own tinted windows sliding across an equivalent row of tinted windows. Sinking momentarily into the befuddlement of infinitely regressing reflections, I find myself hoping, only half-coherently, that this precise pairing of vehicles is in truth a result of my wife’s oddly powerful yen to organize. Because the alternative—that there are so many matching Excursions out here that this event is not so unlikely, after all—seems vaguely repellant.

Sidling through the narrow aisle between twin SUVs, I emerge to peer down the receding parallel rows of vehicle rear-ends, which flash blindingly in the sun. In the distance, where the lot is still vacant, the savannah of asphalt is bounded by a chain-link fence, and then the ground gives way to a downslope. The freeway is down there—I can hear it from here—and across the valley of the freeway, on the distant opposing shore, yellow machines move like insects, sculpting fresh hills out of ferrous red earth.

“Is that Hallmark?” I ask. Veronica has just told me about this on our drive over. The company well known for finding the right phrase—I love you … You’re the one and only—is now working at a more comprehensive scale, constructing planned communities. At one edge of the terra-cotta landscape, a row of freshly completed houses gleam white and pale yellow, weirdly clean. There are two different models, which alternate.

When I catch up to Veronica, who is already heading in the other direction, toward Costco, she’s reading as she walks, studying the list of a hundred or so items we are about to purchase for our field-course. This is the day we call gearing-day—24 hours in San Diego to gather everything that 15 people will need for three weeks at a remote research station on the Sea of Cortez. And if we are true to the pattern of gearing-days past, then Veronica is sure to be a force of efficiency and order, while I, for my part, will stay mostly out of the way. It is a truth quietly acknowledged between us that logistics have never been my strong suit.

“Did you check the first-aid box?” Veronica asks, without looking up from her list.

“I did,” I say, lying, because I’m pretty sure I remember what’s left from last year.

As she rattles a conjoined pair of oversized shopping carts from one of many long, telescoped ranks, she shouts to me over the clatter: “Two more!” Each of us will push a collapsed pair until we need to double our payloads, at which point we’ll expand into a train of four carts and two connective humans.

The actual aisles are U-shaped canyons carved through paletted goods, which have been stacked by machines to reach the 30-foot ceilings.

Inside the warehouse, we are instantly and exuberantly greeted by a row of televisions stacked three screens high. They are all showing the same animated movie, and their luminous images move in perfect synchrony, like a grinning line of can-can dancers. As we walk along the row, I avert my eyes and stare at our feet until we’ve made it into the low-lying archipelago of special offers that precede the actual aisles. Veronica pulls over beside something we need—large boxes of snack bars—and begins inspecting our options: 60 Powerbars; 48 Luna bars; 120 Chewy Granola bars.

“Veronica,” I say, placing my hand on the embankment of bars, “do you realize what this means?”

“No,” she says, absently, “what does it mean?” She has turned her attention back to her list.

“It means, Veronica, that the scale of commerce has shifted. What was wholesale is now household.”

She doesn’t react.

“Is that not disturbing?”

“It is,” she says, looking up from her list. “But we need four-hundred and twenty snack bars.”

“That’s disgusting.”

“Fifteen people, two bars a day, fourteen days. That’s four-hundred and twenty bars.”
As she places three boxes of Balance bars in my cart, I take one out and put it back on the embankment. “Those are Twix in disguise,” I say.

“People like Twix,” she replies, putting the box back in.

The actual aisles are U-shaped canyons carved through paletted goods, which have been stacked by machines to reach the 30-foot ceilings. As we move steadily along, our eyes pass over the canyon walls, scanning for the familiar labels of items we might need. Certain products are sold in such quantity as to seem like a visual joke—a gag on the expected proportions— and just for fun I pull one such prop off the shelf. It’s a trash-can sized tub containing 300—so the label exclaims—maraschino-red licorice twists.

“Did you need this?” I ask, holding it up with two hands.

“Yes, actually.” She takes it from me and places it in my cart.

“Come on,” I say, taking it out.

“Movie night,” she replies, pressing my wrist back into my cart.

In the middle of our next passageway sits a waist-high cube composed of orange blocks, each of which is, in turn, composed of 72 individually wrapped packages of cheddar-cracker-and-peanut-butter sandwiches. It’s a dizzying, electric-orange fractal, and it gives me an idea: As casually as I can, I pick up an orange block and place it in Veronica’s cart, saying only, “good snacks.” Then, as she stares at the new item in her cart, I wait patiently, preparing to declare—just as soon as she accepts my suggestion—that I’ve just proven her deranged consumerism, and she’d better go put back the red licorice.

“Are you insane?” she asks, and instantly it’s clear: my tactic has backfired. She takes the orange block from her cart and presses it back on me. “The wrappers will end up everywhere!” Losing patience, she says I need a mission of my own. She’ll gather the rest of the food, and I should visit the pharmacy, because I’m the one who knows what’s missing from our first-aid box.

The pharmacy … my only hope of reaching it without getting lost or, worse, waylaid by the sirensong of HDTV, is to walk transects—the systematic to-and-fro one would trace in an ecological survey. And so, starting with one edge of the warehouse, I march, my gaze fastened straight ahead. To my left, the glass cases of frozen meats pass as a beige-pink blur, and on my right, the openings to aisles are flashes in the corner of my eye—bright malls I manage to refuse. But as I walk by the bakery, with its distinctive scent of glazed donuts, a sudden grey flicker pierces the sidewall of my private tunnel.

It is a house finch: a small brownish-grey bird with a red head, which means he is male. On the smooth cement floor, he hops, pecks once, pauses to tilt his head and observe the ground, and pecks again. The movements are quick and exact, at once mechanical and vibrantly alive. He flits up to a table, where he perches on the edge of a plastic bin and fixes me with a tiny eye—a pinhead of glossy obsidian.

People are passing close to him, talking on their phones, reaching into the bin for flats of muffins, but he doesn’t seem to mind. He stays right where he is, and keeps his eye trained on me. Evidently, it is my own behavior that is suspicious, not everyone else’s. He hops a quick, irritated 180, and fixes me with his other eye: Why the interest? he seems to ask. Why the sudden interest?

Odds are you’ve seen the little brown bird with the red head, because the house finch, Carpodacus mexicanus, follows us nearly everywhere: the bird’s particular needs—plants to feed on, places to nest—happen to coincide precisely with the sort of habitat that human beings tend to generate. We convert almost every landscape—redwood forest or salt flat; peat swamp or sandy desert—into a patchwork of the weeds and grasses that provide house finches with their favorite seeds. And when it comes to nesting, the little birds seem to adore stubby ornamental trees, the overhangs of parking structures, and the ivy that grows on building walls.

Until the late 1950s, house finches were hemmed into the southwestern third of the United States, mainly by the vast swath of grasslands that covered the center of the country. While these regions offered plenty of seed, they were severely short on nesting sites. A parsimonious guess at ecological history might suggest that the finches hopscotched gradually eastward, alighting on islands of human settlement until they reached the promised land of Midwestern suburbs. But that is not, in fact, how it happened. Instead, when the invasion finally transpired, it came from the rear: a small population of Carpodacus mexicanus appeared in Long Island, New York, and from there they began a steady march westward. When they reached the Great Plains, about a decade later, they couldn’t cross from the east any more easily than they ever could from the west.

But why Long Island? How did a small but momentous flock find itself next door to New York City? The seeds of biological invasions, like the sparks that start wildfires, are often obscured by the very process they initiate: By the time an invader has advanced far enough to gain notice, it has altered the ecology of a fairly large region, and there’s no reconstructing where, exactly, the rings of expansion had their epicenter. But birds are uniquely beloved animals, monitored by a diffuse but nationwide federation of obsessively vigilant observers. Like no other organism, birds get noticed.

In this particular case, a red-headed finch was noticed in April, 1941, in Nassau County, New York, and the sighting was duly reported in the weekly ornithology column of the Nassau Daily Review-Star. The enthusiasts filing the report were well aware that their sighting represented an exception to the known range of the species, but they confidently identified the bird as Carpodacus mexicanus. The column’s editor, Dr. John J. Elliott, followed up on the intriguing report, and over the next five years, he discovered several small colonies of house finches nesting in Long Island tree nurseries. He published his findings in the May 1947 issue of the Linnaean News-Letter. As to the origin of the birds, he was dumbfounded, though he did note that the Long Island birds appeared to be especially dusky, much like the subspecies of Carpodacus mexicanus found in the vicinity of Denver, Colorado.

“Oh, that’s right,” her voice said, behind me, “you were supposed to go to the bakery, to look for birds.”

Dr. Edward Fleisher, of Brooklyn, happened to be a subscriber to the Linnaean News-Letter, and upon reading Elliott’s article, he immediately wrote him a letter. In January of 1940, he wrote, he had entered a Brooklyn pet store, where his attention was drawn to a cage labeled Hollywood Finches. To his dismay, the 20 birds in the cage were in fact specimens of Carpodacus mexicanus. As Elliott would later describe the fateful resolution made then and there, “Dr. Fleischer had previously seen Bohemian Waxwings for sale in this same store, and decided to put an end to this traffic in protected American passerines.”

Fleischer wrote to the National Audubon Society, requesting immediate action, but since the officer responsible for such matters was on vacation, the case was referred to the State Game Protector. After a brief investigation, this official informed Fleischer that the birds had come from a supplier in California, and therefore their sale was not illegal, as Carpodacus mexicanus was “neither protected in California nor native to New York State.”

Unsatisfied, Fleischer consulted the Bureau of Biological Survey, which informed him that house finches were indeed protected, as the U.S. and Mexico had recently listed them in a treaty concerning migratory birds. The Survey also dispatched Orin D. Steele, Game Management Agent, to pay Fleischer a personal visit and discuss the matter more fully. As Fleischer later described the conclusion of this episode, “A few days later, April 1, 1940, Mr. Steele sent me a letter in which he said, ‘based entirely on information furnished by you, we have been able to stop the trapping and transportation from California, and have stopped sales throughout the United States.’ ”

About this time, the Audubon Society, tardily roused by Fleischer’s call to action, sent their own team to New York area pet stores. The birdcages, they discovered, were empty of Carpodacus mexicanus. And when an undercover agent expressed an interest in acquiring house finches, he was told there had been some legal trouble, though for the right price, he might still get some—only they’d have to be sold as purple finches, which weren’t listed in a certain treaty with Mexico.

It seems quite likely, then, that the worthy efforts of Dr. Edward Fleisher, of Brooklyn, not only put a deep dent in the transcontinental shipment of protected American passerines, but also prompted a number of worried pet-store owners to release their flocks of Carpodacus mexicanus in the vicinity of Long Island, N.Y.—an ecological experiment that would have unexpectedly far-reaching and long-lasting reverberations.

As for that little inconsistency in the story—if the birds had come from California, why were they dusky, like the subspecies in Denver?—it was resolved a few years later, when a curator at the American Museum of Natural History washed one of his specimens and discovered that it was actually quite pale. Life in New York had sooted the house finches.

Eventually, Veronica found me by the muffin bin, watching a small bird forage for crumbs.

“Oh, that’s right,” her voice said, behind me, “you were supposed to go to the bakery, to look for birds.”

At first I tried to explain: Why the interest? the bird had seemed to ask, but of course it was my own question, not his—

“Pharmacy?” Veronica said, and for the next hour, I forgot all about the house finch. But later, staring blankly at propagating waves of brakelights—5 p.m. on the I-5—or examining, in my boredom, the tub of red licorice, which we had ended up wedging under the windshield, since it had proved a maddening remainder of otherwise perfect packing, I thought again of the bird and the question it had provoked: Why treat Costco as a swirl of unwanted distractions, whereas a little grey bird instantly receives rapt attention? Or to phrase it, frankly, as more of an indictment: Why can’t I enter Costco with the same sort of curiosity I would bring to a forest full of birds? Why can’t I, for instance, pick a product off the shelf and try to understand its origin and design, or the challenge of its manufacture, or the meaning of its widespread appeal?

Veronica was on her phone, resolving some complication with tomorrow’s border-crossing, so I settled into the numb rhythms of driving and began to puzzle over Costco on my own. Was there, I wondered, a real difference? Some critical distinction that made a warehouse of stuff less rewarding to investigate than a forest of birds? What occurred to me first was that the distinction might lie in ecology, for if Costco is viewed as a kind of foodweb, it exhibits drastic simplification, with all those thousands of products going straight to a single top consumer. In contrast, the forest’s ecological network is reticulate and integrated, with each organism affecting many others. So maybe that’s what makes the forest feel whole and coherent, whereas Costco gives you that fragmented, centrifugal feeling—too much stuff for you alone.

And yet, if one thinks just a bit less literally, ecological links do begin to appear, tracing their way throughout the warehouse. Certainly there is something like ecological competition (Powerbar versus Luna), as well as symbiosis (DVDs and red licorice). If one were keen about discerning cultural or symbolic relationships between otherwise disparate items—Barbie dolls and treadmills, for instance—then surely the warehouse’s ecological network would light up like a cobweb in the sun. And finally, to suggest that Costco lacks real ecology is to ignore a point of history: Early ecological thinkers, including Charles Darwin himself, found their most instructive metaphors in Adam Smith’s theories of the free market. If Darwin saw ecosystems as economies, it ought to be possible to invert the metaphor.

Ecology, then, was not going to offer the meaningful difference I was after. But every other distinction I tried—evolved versus designed, natural versus manufactured—failed just as soundly to vindicate my disposition toward the warehouse. At that point, I began to feel a bit like a detective whose investigation is steadily gathering clues that point unmistakably to himself: what made Costco less interesting than a forest was not Costco, or the forest, but me.

But what was my problem? Why couldn’t I get excited about Costco?

Ignorance, probably. If I knew a bit more—some engineering, maybe, or economics—then surely I could approach Costco with real focus and interest. I remembered, for instance, what Primo Levi, the Italian chemist and writer, could find inside an ordinary can of housepaint: a mini-drama of molecular affinities; a distillation of professional success or failure; a gift to posterity or an ugly reminder of a chemist’s incompetence. Just imagine, then, what he might have done with our bucket of synthetic licorice twists.

For five miles of traffic, I felt quite satisfied with my mea culpa: Costco could be truly engaging, if only I would follow the example of writers like Levi, who could read the manufactured world in such fascinating ways. But before I could make good my new resolution, it soured.

I remember exactly where it happened, because the street signs had something to do with it. We had just exited the highway and were now entering a vast residential development, where some friends had generously offered their home as base camp for gearing day. I was beginning to feel uneasy, wishing Veronica would get off the phone and remind me where to turn. Granted, I had driven to our friends’ house many times before, but my sense of direction is exceptionally bad, which makes this sort of neighborhood—where the houses are all identical, the blocks self-similar—my own personal purgatory. That very morning, in fact, returning from a short errand, I had unwittingly parked in the driveway of a neighbor, and blithely walked into his house.

After that incident, I had memorized our five-digit street address, but now, in my involvement with the Costco problem, I’d missed my turn, or I thought so, anyway. Veronica was immersed in her conversation, so I was craning forward over the wheel, searching helplessly for any geographic cue. Irrationally, I sped up, then swung the truck around the next corner, looking up at the green street signs.

Rock Ridge, said one.

Tamarack, said the other.

And at that moment, I disavowed my recent resolution.
 Each of us knows, from firsthand experience, that Costco is a small part of an increasingly widespread environment. A plexus of shopping malls, parking lots, and residential sprawl now extends for hundreds of miles around every American city. In many places, it actually is the city. Its consistency is both extensive and exact: the box stores and the goods on their shelves; the restaurants and the food on their plates; even the residential neighborhoods—it is all replicated, with astonishing fidelity, from one coast of the U.S. to the other. When I pulled into the wrong driveway, I could have been in Albuquerque or Schenectady.

What was my problem? Why couldn’t I get excited
about Costco?

The metaphor of cancerous tissue is loaded, but it is also precise in certain ways: In cancer, cells of a single variety replicate rapidly; they form a strangely homogenous tissue, because differentiation—the development of the distinctive properties that characterize mature, functional cells—is a relatively slow process, and cancer cells divide before they manage to differentiate. In fact, the less they differentiate, the more rapidly they tend to replicate, and soon they begin to invade and outcompete the various specific types of well-differentiated tissue.

In ecosystems and economies—just as in the body—differentiation takes time: Whether it is the biological evolution that makes the sage scrub of southern California different from that of western Australia, or the cultural development that makes San Diego taco shops different from Maine chowder shacks, the accumulation of local differences happens at a stately pace. But when that stereotyped plexus of malls and sprawl moves in, all the singular places give way to one single place.

At the corner of Rock Ridge and Tamarack, I saw a signpost of the problem. Developers often name their new neighborhoods and streets after what was there before the development. Near my hometown of Boulder, Colorado, for instance, there is a mile of replicated townhouses called Rock Creek, and I can testify, from childhood experience, that once upon a time there was indeed a lovely creek in exactly that location. I suspect that, similarly, on a certain hillside north of San Diego, there was once an actual and particular rock ridge.

Tamarack, on the other hand, does not memorialize a tree that once stood on that same hillside. Of this we can be sure, because the tamarack is a cold-weather tree, which grows around the Great Lakes and northward into Canada. It is possible, however, that the street sign in San Diego does in fact hearken back to a particular tree somewhere—in Minnesota, say—because developers have, on occasion, constructed virtually identical subdivisions in multiple locations. Perhaps, then, somewhere in the native range of the tamarack, there is a corner, marked by a pair of street signs, identical to the one in San Diego, only there, it is the word Tamarack that marks the site of a towering tree now gone, while Rock Ridge refers to a place thousands of miles away. Whether or not such a duplicate really exists, the juxtaposition of those disparate landmarks struck me as a concise testament to the erosion of geography, the end of distinctions between one place and another in the new American landscape.

So maybe Costco is just as fascinating, or wondrous, or even, in its own way, just as beautiful as any other setting in which we might immerse ourselves. At least, we could find a perspective that would make it so. But what I decided, as I turned the corner at Tamarack and Rock Ridge and found myself lost in a familiar neighborhood, was that I would not try to find that perspective. In fact, I would resist it, because we are now in the process of trading our entire library of unique volumes for so many copies of the selfsame text. And the exchange seems not just foolish, but tragic.

Twelve years after his column in the Daily Review-Star announced house finches in Nassau County, Elliott published this diagnosis: “The future prospects of the species in this region seem favorable; it would appear that this adaptable and colorful bird is a securely established resident of the Eastern United States.” To a modern ecologist, such delight at the establishment of an exotic species seems awfully innocent, because we have since come to understand that biological invaders—even colorful ones; even those that seem, at first, basically harmless—often lead to unforeseen and grave ecological consequences.

A handful of examples will illustrate how Elliott’s warm hospitality has given way to our current xenophobia. Although this small sample may shortly come to feel like a litany of modern blights and scourges, my intention is not to flagellate us until we achieve Job-like understanding of the bad things nature has to offer. My hope, rather, is that this sample will provide some evidence for more general questions about what biological invasions are, how their unstoppable march might work, and even, perhaps, what they’ve got to do with that other sort of advancing front, the one I contemplated in a suburb of San Diego.

The female brown-headed cowbird is a bit smaller than a robin, rather drab, and quiet. Whenever I see her hopping around a lawn with a flock of other birds, the image called to mind is that of a shadowy spy, her brown overcoat wrapped tight, her gaze askance and ever-watchful. What she’s looking for is a bird gathering twigs for a nest. And when she catches sight of such activity, she’ll trail the expectant parent to the new home and begin surveillance. Then, when the mark steps out, she’ll swoop in and deposit an egg of her own. If there happen to be eggs there already, she’ll make a meal of one, ensuring that when the parents return, the number of eggs in the nest won’t betray the intrusion.

This final effort at disguise seems a bit preposterous—like a bad fake moustache—because the cowbird’s egg is likely to be several times larger and a markedly different color than the rest of the clutch. Cowbirds can’t possibly lay eggs that mimic those of their victims, because their victims are so varied, ranging from miniscule gnatcatchers to hefty meadowlarks. And yet, strangely enough, the ruse usually works: the parents carry on incubating the misfit as if nothing at all were wrong, and then, when it hatches, they feed it even more than its siblings, because it is precociously large and forever hungry.

A few bird species are canny enough to foil the con. Robins, for instance, will chuck out the cowbird egg nine times out of 10. And there’s an interesting pattern here: It’s not just a random one in 10 couples who happen to be gullible; it’s couples from particular places—the places that have not yet been invaded by cowbirds. The perspicacity to spot an anomalous egg, therefore, appears to be a mental trait that has evolved in response to the danger of cowbirds. And this helps explain why cowbirds are such effective invaders: whenever they land somewhere new, the locals are defenseless against them. Consequently, quite a few little birds that nest in the U.S. are presently foster-parenting their way into oblivion.

There is something particularly grotesque about the way cowbirds invade—the deceit of it; the perversity of tiny yellow warblers struggling to feed their gargantuan brown chick—but other, less graphic stories of invasion will also prove instructive. Across the desert Southwest, the Asian salt cedar, a scraggly tree with glaucous needles, was originally planted to provide a bit of shade. But salt cedars, it turns out, have an irrepressible tendency to take over: they not only suck water with unrivaled verve, but also deposit salt in the soil, poisoning every other plant in the vicinity. The invasive trees have now conquered every waterway in the desert Southwest, and what were once diverse forests of cottonwood and willow are now dense thickets of salt cedar.

Another invader, a little weed now spreading across the American west, deploys a scorched-earth strategy of its own. Eurasian cheatgrass looks innocuous—a thin stalk bends elegantly under the weight of a loose cluster of slender, tufted seeds—but come summertime, the pretty little plume becomes highly combustible, inviting wildfire. And when the blaze rolls through, it destroys most plant life, but the cheatgrass seeds are fire-resistant: the following spring, they sprout from soil enriched with the ashes of their erstwhile competitors. In the century since cheatgrass arrived in North America, it has blanketed 100 million acres.

Just a couple more examples before we reckon with the problem in more general terms. In 1955, a colonial Ugandan administrator surreptitiously slipped a bucket of Nile perch into the waters of Lake Victoria, the largest tropical lake in the world. Scientists at the East African Fisheries Research Organization had expressly forbidden such a move, fearing that the predatory perch could have unforeseen effects on Lake Victoria’s extraordinary biodiversity. At first, it appeared the scientists had been wrong: The exotic predator established a local population, but nothing catastrophic came to pass. In the late ’70s, however, the perch began to proliferate, and over the next 10 years, they ate their way through 200 species of fish found nowhere else in the world.

Finally, our litany should not omit the invaders that are less exotic to us but no less successful in expanding their range: Across the world’s oceans, island after island has yielded to rats, mice, rabbits, house cats, goats, pigs, and other feral domestic animals. Rats and cats eat the island-nesting birds; rabbits and mice eat indigenous plants that would otherwise feed native herbivores; and goats and swine denude whole islands. This process began in the 14th century, when Europeans first reached the Azores, the Madeiras, and the Canaries, and it has continued ever since. All in all, the proliferation of European pests and domestics on previously isolated oceanic isles has annihilated dozens of species of reptiles and mammals, and at least 100 species of birds.

Now, with our mixed bag of invaders in hand, we can ask the more general question such variety seems to beg: If all these species are invasive, what really is an invasive species? What unites these powerful few and equips them to drive native inhabitants over the cliff of extinction? To answer this question would be to solve the problem of invasive species—not just intellectually, but practically, as well, because if we knew in advance which species might turn malignant, we could nip their incipient invasions in the bud.

In certain respects, however, this general question—what makes a species invasive?—defies a general answer. In different groups of organisms, different attributes are associated with a penchant for invasion. Among insects, for example, invasive species tend to be small, while among vertebrates, they tend to be big. Among pine trees, invasives tend to have small seeds, while in another group of plants, the invaders have big seeds. Such trends, confined to certain groups of organisms, are not without meaning, and in some cases they make a lot of sense: invasive birds often lay multiple clutches in a single season; invasive trees are likely to reproduce by sending out runners that sprout lots of new individuals. But even within such delimited groups, some invasives don’t fit the expected description, and many species that do fit the description never turn invasive. The trends, therefore, don’t tell us with sufficient specificity whom we need to monitor.

The fundamental difficulty is that invasions depend on the details of natural history. Cowbirds are invasive in part because other birds happen to be dimwitted about the color of their own eggs. Salt-cedars are invasive in part because no herbivores in the desert Southwest will eat them. And there are a thousand other peculiarities of natural history which, were they a bit different, would drastically alter the outcome of cowbird or salt-cedar invasion. In short, invasions are essentially accidents of fit between an organism and its new ecosystem. Consequently, the only way we can know for sure whether an organism will thrive in a new habitat is to see the experiment run its course—and by then it’s too late.

Do the different sorts of invasion—the human versus the non-human, the cultural versus the biological—really have anything to do with one another?

And yet, before we despair of moving beyond our bag of anecdotes, we should remember that there is another process—in fact, a well-understood one—in which outcomes depend on the details of natural history, and thus lie beyond the reach of our predictive abilities. We cannot foretell which novel mutation will help, rather than hurt, the individual who happens to possess it, because every mutation’s effects will depend not only on the attributes of its possessor, but also on the nature of other organisms in the neighborhood. (Imagine, for instance, a mutation that makes a songbird feel murderous toward anomalous eggs; in the presence of cowbirds, the mutation is certainly beneficial, but without cowbirds, it’s useless at best, and might even cause some accidental ejections.) Perhaps it is not surprising, then, that invasions and evolution by natural selection are intelligible in much the same fashion: They are comprehensible in retrospect, but very difficult to predict. On a case-by-case basis, they make a detailed sort of sense, but they are hard to speak of in terms of generalizations.

Charles Darwin, who cracked the mystery of evolution, was very good at finding general rules amid the riot of idiosyncrasies, so we might ask how he thought about biological invasions. In characteristic fashion, he approached the question historically, with a view to origins. That is, he didn’t even try to find the attributes common to all invasives, but instead rearranged the question a bit. He asked not what makes a species invasive, but instead what makes an invasive species. In other words, what place or process produces these organisms that overrun all others?

The most effective competitors, Darwin figured, must be crafted in the most inventive and exacting workshops of evolution. But where are those? Natural selection, he reasoned, has two crucial limitations. First, it can work only on existing variation. Or to use a more modern phrase, selection can favor mutations, but it cannot generate them. And therefore, invasives must come from very large populations: the more individuals there are to mutate, the more new mutations will arise, and the more raw material natural selection will have to work with.

The second crucial limitation of natural selection is that it can only adapt a population to existing circumstances; it cannot work with an eye to the future. And so, Darwin figured, in the course of their own evolution, invasives must have faced many other organisms: The more challenges a certain species has confronted, the more solutions natural selection will have discovered.

Finally, Darwin thought, large populations contending with great biodiversity can only occur where there is a vast continuous tract of habitat to support all the interacting organisms. So while small, isolated islands have produced many exotic new species, the really tough critters—essentially, the ones made to win—must have been crafted and tested in the setting of large continents. “In accordance with this view,” Darwin concluded, “we can, perhaps, understand some facts … for instance, the fact of the productions of the smaller continent of Australia now yielding before those of the larger Europaeo-Asiatic area. Thus also, it is that continental productions have everywhere become so largely naturalised on islands.”

It’s hard not to hear in Darwin’s conclusion an ominous reference to European colonialism. But do the different sorts of invasion—the human versus the non-human, the cultural versus the biological—really have anything to do with one another?

We can, I think, find certain parallels. The plexus of malls and residential sprawl, for instance, expands by a kind of positive feedback: the more it dominates a particular place, the less distinctive that place becomes; and the less distinctive the place becomes, the more easily large corporations—the multi-national chains and home builders—can market their single, stereotyped way of life. On reflection, this does sound a lot like cheatgrass and salt-cedar, which gradually alter the environment to make it suitable for nothing but more of themselves.

Also, in biological and cultural arenas alike, the process that generates local differences is dilatory relative to the pace of invasion, and this discrepancy helps to explain why the local variety so often loses: the same stately process that creates diversity is also the primary—and fatally slow—means by which local residents can devise adaptations to the invader. Kirtland’s warblers will almost surely go extinct before they evolve the robin’s defensive attitude toward the cowbird’s eggs. Although cultural evolution is a lot faster than the biological kind, it is but a crawl in comparison with racing invaders.

And finally, there seems to be an economic analog of Darwin’s theory about the creation of invaders: the fiercest economic competitors, the invaders-to-be, seem to emerge from the largest economy, the hotbed of competition, and to spread out across the globe, displacing the productions of smaller economic systems.

So yes, there are parallels, and they may even be illuminating. But we should not forget something else we learned from Darwin just a moment ago, when we tried in vain to define what exactly makes organisms invasive: In the face of endless variety, it may be more productive to tell a story of what happened than to probe the essential nature of what is. Instead of trying to pry beneath the distinct forms of invasion, we could ask about their interplay through history.

Here’s a hypothesis: Over the last half-millennium or so, the different sorts of invader have changed places on the colonial frontier. In the first wave of global invasions, various species other than humans led the way across the islands and continents of the world. These species were effectively an advance guard that drove back the natives and set up camp in the newly claimed territories. Meanwhile, human invaders marched mainly in the rearguard, securing and settling the conquered lands. By contrast, we are now in the second wave of global invasions: Widespread human and cultural expansions are now leading the way for a variety of species that follow in their wake.

Evidence concerning the first wave comes mainly from environmental historians. When Europeans first sailed for the mid-Atlantic isles, and then for the West Indies, North and South America, and finally, the South Pacific and Australia, the sailors carried with them what historian Alfred Crosby called a portmanteau biota—essentially, a partial ecology of Eurasian organisms, including plants, animals, and microbes. To reach the new lands, these species required human help, but once the ships hit the beachhead, the biological invaders marched out on their own, advancing well ahead of the humans who had brought them. Therefore, if we were to imagine the Europeans battling their way into the interior, where they could then establish outposts of European life—with crops, domestic animals, and even annoying pests—we would be quite mistaken.

The leaders of the biological invasion varied from one place to another: In the West Indies, pigs proliferated, while on the Pampa, it was horses and cattle; in New England, white clover and even peach trees invaded like weeds, while in Peru, it was turnips and mint. Generally speaking, however, the biological invasions followed a pattern. When Europeans landed, they unloaded a goodly part of Old McDonald’s farm: goats, sheep, pigs, cattle, horses, and fowl. Sometimes, animals were released intentionally, but more often, a few simply got away. Similarly, crops might be sown, but this wasn’t necessary to initiate botanical invasions, because the livestock’s feces and muddy feet carried plenty of seeds. If the sailors made contact with Native Americans, pathogens—most notably, smallpox—were transmitted.

The Europeans then departed, or perhaps tarried by the coast. But the other organisms were less patient. With their first Native American hosts, pathogens traveled back to settlements, where they wiped out half the humans, sometimes more. Livestock populations increased exponentially, and Eurasian botanicals overgrew their native competitors. So when the Europeans finally returned, or elected to extend their frontier, they walked into a landscape that had already been subdued and familiarized. Their pests and pestilence had laid waste to their enemies; their crops and livestock had founded their farms.

Is it ludicrous to mourn the loss of the local, the distinctive, the diverse, when the march of victors seems all but inevitable?

Certain Native American tribes recognized as much, for they viewed biological invaders as grim harbingers of the humans following behind. St. Jean de Crevecoeur wrote that when the Indians saw honeybees, which they knew to be a European import, “news of this event, passing from mouth to mouth, spreads sadness and consternation in all minds.” And news, of course, was not the only thing passing from mouth to mouth. Smallpox was spreading like a rumor.

Sometimes, the non-human invasion moved so far ahead of the human one that the link between them was lost on European observers. Bartolomé de Las Casas, a friar who arrived in the West Indies just 10 years after Columbus, asserted that a variety of Euro- pean plants were already in the islands when the Spanish arrived. He was wrong, of course. Those plants had come on Spanish ships. But their spread across the landscape had been so swift that Las Casas, arriving a decade later, could only presume they’d been there all along. A similar confusion attached the name Kentucky bluegrass to a species that in fact came from England.

For me, what is poignant about such confusion is that it pushes back, by hundreds of years, the date of onset of our culture’s acute amnesia about landscape. We imagine the wildest place we know—a place from childhood, perhaps; or from an expedition we took; or even from a nature show on TV—and we take that place to be Wilderness, Nature, a landscape undiminished. But again and again, we are mistaken. This is how Thoreau put it:

I take infinite pains to know all the phenomena of the spring, for instance, thinking that I have here the entire poem, and then, to my chagrin, I hear that it is but an imperfect copy that I possess and have read, that my ancestors had torn out many of the first leaves and grandest passages …

And now it appears that even Thoreau’s ancestors—and their ancestors before them—never read the whole poem, because the American landscape was Europeanized even before Europeans laid eyes on it. In the first wave of invasions, biological invaders— microbes, plants, and animals—transformed ecosystems all over the world, and European humans, coming along behind, moved into turnkey continents.

But the order has shifted: In the second wave of global invasions, the human rearguard has moved to the front, and a motley troop of species follows behind, inserting themselves into newly transformed landscapes. Evidence for this half of the hypothesis comes mainly from studies of invasive species, so here we can turn to the varied sample we gathered up earlier.

The brown-headed cowbird, sneaky bane of naïve warblers, did not in fact launch its expansion across North America until European settlers began to carve forests into a patchwork of fields. The cowbird prospers in this landscape, and it followed the advancing front of settlement into every corner of the continent now losing its native avifauna. Salt-cedar, too, is thriving on ground that humans have primed for its arrival. The damming of virtually every major waterway in the western U.S. has had important ecological effects: underground watertables have dropped; soil salinity has risen; and seasonal floods no longer occur. All of this, together, has greatly facilitated the expansion of salt-cedars.

But what about Nile perch? What could humans possibly do to facilitate an invasion transpiring entirely underwater? Between 1955, when the perch was secretly released, and the late ’70s, when the species began to explode, cutting of forests and fertilization of crops around Lake Victoria greatly accelerated a process that had been underway since the ’20s: The lake was filling up with biotic muck. The diverse flock of native fish, having evolved in a clear lake, struggled under the new conditions. Deprived of visual cues for species-discrimination, species started interbreeding rather haphazardly, eroding certain characteristics that had formerly distinguished them. And the veils of algae may also have tipped the balance in favor of Nile perch, which is well-adapted to dusky conditions. The evidence suggests that, even underwater, humans can usher along biological invasion.

Still, we should not make our sweeping story too tidy. There is, of course, no stark line in history, before which the invading ranks held exclusively to one arrangement, and since which their order has been wholly reversed. Certain species have played both roles—frontline as well as rearguard—and thus blend one stage of invasion into the next. Take rats, for instance. They certainly advanced ahead of the Europeans, and even helped subdue New World peoples by eating through food stores and spreading bubonic plague. And yet, rats also bring up the rear, breeding with special abandon in Europeanized landscapes.

Another important complication in the tale of two stages is that certain species have lately been ushered not only by humans and their newfound habits of land-use, but also by other non-human invaders. Cheatgrass, the pyromaniac weed, is an example. Humans help cheatgrass, because they often start the fires that reduce native competitors to fertilizer. But cattle are implicated, too, since overgrazed rangeland is often colonized by cheatgrass. And if we are no longer thinking of cattle as extensions of European settlers, but rather as biological invaders in their own right—they did, after all, march ahead of their humans—then cheatgrass has been led into its new territory not only by human activity, but also by another invasive species.

Invasion upon invasion brings us, in a sense, full circle. As house finches advanced across the splendidly suburban landscapes of the Eastern United States, from Long Island tree nurseries to farms in the Midwest, they encountered something they hadn’t seen much in their native deserts: warehouses packed with poultry. This must be where a very unlucky finch first contracted a very lucky mutant of Mycoplasma gallisepticum, an eye infection that had long pestered chickens, but had never made the leap to wild songbirds. The disease hit the house finches hard: whereas chickens often recovered, finches usually died. And the infection spread through the finch population even faster than the finches had spread across the U.S. The first case of Mycoplasma in a house finch was diagnosed in Maryland, in January of 1994. By 1997, the disease had spread through the entire eastern range of the house finch, and had killed 225 million birds.

The Great Plains, you’ll recall, had always seemed an impassable barrier for house finches. But Mycoplasma gallisepticum soon revealed that suburban expansion had spanned the gap. In April of 2002, the disease appeared in Missoula, Missouri. To quote the paper that announced this finding: “As native house finches are highly susceptible to this novel pathogen, western populations may now be at risk of high mortality, similar to that experienced by non-native eastern populations.” It is also likely that the infection will now jump from house finches to other native songbirds, including many species that are already endangered.

Earlier, we followed Darwin in setting aside questions of essence, what makes for invasiveness, in favor of an account of history. But Darwin knew, I think, from his own work on the origin of life and diversity, that when history is traced deep enough in time it may, in the end, reach beneath questions of essence. Come back, then, to this question: When European plants, animals, and microbes led their human companions across the globe, why were those species so wildly successful?

For the microbes, the answer is straightforward. A virulent pathogen can persist in a population only by finding new susceptible hosts as frequently as its old hosts die or recover. As a rule, this can take place only in populations that are sufficiently large. (Think of it this way: a population of a million produces babies, which are new susceptible hosts, a thousand times faster than a population of a thousand.) Australia and the oceanic islands simply weren’t populous enough to sponsor the evolution and maintenance of their own nasty microbes. And while there were indeed populations of the requisite size in the Americas, they were descended from much smaller groups that had crossed the Bering Strait within the last 20,000 years. All over the world, then, Eurasian emigrants had enjoyed millennia-long respites from disease, but this same blessing left them immunologically naïve, and therefore utterly vulnerable when European microbes finally arrived.

But what about the plants and animals? Why did European species dominate native ones from the Azores to the Americas? On the oceanic isles, Darwin’s idea about area applies: In comparison with the Azores, the West Indies, or even New Zealand, Eurasia is vast, and this territorial advantage translated into the evolution of better-honed competitors. But what about the other continents? Eurasia does comprise the broadest span of terrestrial habitat on earth, but still, the sweeping success of Eurasian invaders seems entirely out of proportion with the territorial advantage that Eurasia has over the other continents. There must, it seems, be more to the story.

The archaeological record in North America shows that Homo sapiens arrived and spread across much of the continent between 10 and 15 thousand years ago. It also shows that, around the same time, more than 30 genera of large mammals went extinct: woolly mammoths and saber-tooth tigers; scimitar lions and short-faced bears; giant ground sloths and New World camels—all of them, gone from the fossil record just about when garbage piles from hunter-gatherers start showing up.

The methods used to date 10,000-year-old archaeological sites have a margin of error of about 1,000 years. This wiggle-room probably explains why humans seem to appear in Patagonia a bit before they do in North America, even though they had to travel farther from the Bering Strait land bridge to get there. And in this very same window of apparent simultaneity, 50 genera of large mammals disappear from South America’s fossil record.

Humans arrived in Australia earlier than the Americas—about 50,000 years ago—and you’re probably now anticipating what comes next: At about the same time—the margin of error that far back is more like 10,000 years—21 genera of large Australian mammals laid down their last fossils.

At first, the suggestion seems preposterous: Bands of Paleolithic humans drove lions, tigers, mammoths, camels, and bears into extinction? Yet there seems no real alternative. For a while, researchers considered climate change. And indeed it seems likely that the end of the last ice age, which came about 11,000 years ago, conspired in some extinctions. But only conspired, for there were previous shifts of similar magnitude, and they did not have the same cataclysmic effects. What’s more, the climatic changes in South America were mild by comparison, but that continent lost even more mammals. And there is no indication of exceptional climate transition in the regions and times of most of the Australian extinctions.

To overcome our incredulity—cavemen wiped out lions?—we can at least remind ourselves of a few helpful details. First, besides their stone spears and axes, human beings wielded fire. They could have moved large numbers of animals by starting wildfires, which would have had the additional impact of depriving herbivores of forage. Second, the humans did not have to hunt the predators into oblivion; destruction of their only prey, the large herbivores, would have done the job. And third, animals that have evolved in isolation from humans may be especially vulnerable to them. This last point is often raised to explain why the mammals of Africa and Eurasia largely escaped mass extinction: They had lived around hominids continuously, allowing them to evolve the fear and instinctual defenses they needed to contend with a uniquely intelligent predator.

Skip forward again now, to European livestock arriving on the shores of new continents. The landscapes they entered were idyllically free of predators and competitors: no scimitar lions or short-faced bears to hunt the European species; and no camels or ground sloths to vie for the tastiest herbage. No wonder, then, that the pigs, goats, cattle, and so forth expanded exponentially. Meanwhile, the European weeds that travelled with the livestock had the advantage of being well-adapted to the animals they accompanied. “Kentucky” bluegrass, for instance, was adapted to the grazing of English sheep and cattle. By contrast, the native competitors of bluegrass had evolved in the absence of large grazers for at least 10,000 years, and were therefore less prepared for their impacts.

These facts have important implications for our account of invasion. Looking at the last half-millennium, the tale of two waves holds true: First, biological invaders led European humans across the globe; later, as roles shifted, human and cultural expansions led biological species. But if we look much deeper into the past, we see that the global wave we called the first was in truth made possible by one that had already transpired 10,000 years before. In a sense, the story becomes simpler: It was humans out front all along.

I confess that when I began this essay—and even when I sketched the waves of invasion—I envisioned pre-Columbian America as a final redoubt of pristine nature. I imagined the Adirondack glade plump with deer and turkey, ennobled by the presence of grizzlies and wolves, and I thought, now that—that was really nature. But in this fantasy of wilderness, I was submerged in our age-old amnesia concerning the natural world, our cycle of reduction followed by forgetting. For those places were anything but pristine. Their first leaves and grandest passages had been torn out long ago, because it was not sometime in the last few hundred years that humans became the prime movers in biological invasion. It was thousands of years ago, when we exterminated the fauna of three continents, and thus paved the way, millennia in advance, for the massive expansion of Europe.

Two hours down the peninsula, where the highway edges steep hillsides of chamise and agave, San Diego and Tijuana seem already far away. To our right, over the Pacific, a thick layer of clouds is just now tearing into tatters, striating the ocean below with bright silver and dark blue. Veronica veers that way now, off the highway and toward the ocean, leading our caravan of three SUVs bumping along a pot-holed road atop a jutting headland, at the tip of which perches an abandoned touristic complex. The place looks like a decaying Moorish palace, complete with ogive windows, onion-shaped domes, and minarettes. And while Alhambra may seem strangely out of place here on the cliffs of Baja California, fantasies of the exotic East have swirled around this peninsula for 500 years. In fact, the very word “California” was born in a 15th-century Spanish novel, as a fantastic eastern isle:

Know that to the right hand of the Indies was an island called California, very near to the region of the Terrestrial Paradise, which was populated by black women, without there being any men among them, and almost like the Amazons was their style of living.

When Hernán Cortés first sailed for Baja California, having just blazed through Mexico under the escort of a variety of European species, he believed he was sailing for an island, a place disconnected from the world he had conquered. He thought he might find oysters bearing pearls of a sort never seen by Europeans, and perhaps even women, black women, who lived there without men. He was, in other words, pursuing his own preposterous fantasy of the untamed and untainted wild.

Twenty miles further down the highway, in the port town of Ensenada, we pass a shopping mall that was not here last summer. There is a box store called Smart & Final, another called Bed Bath & Beyond, and, a bit further on, a Costco. To provide some shade for a pavilion out front, a resourceful landscaper has planted a row of Asian salt-cedar.

Is it quixotic, in the end, to resist this process that has been underway for millenia, this advance of human invaders and their ecological collaborators? Is it ludicrous to mourn the loss of the local, the distinctive, the diverse, when the march of victors seems all but inevitable? Is such a posture of bereavement perhaps even fraudulent? After all, I myself partake of the fruits of victory. And, what is more, fantasies of a remote, undiscovered, and exotic land—visions of the New World not entirely unlike those I have caught myself entertaining—played no small part in driving the march of Hernán Cortés himself. Shouldn’t such hints of hypocrisy give one pause?

Bartolomé de Las Casas, that Spanish friar who arrived in the Indies and reported wild proliferation of European species, went on to become a passionate defender of New World peoples, decrying the conquistadors’ extravagant violence, their destruction of civilizations, and their scorching of the earth. It is safe to say Las Casas viewed the New World with far more interest and openness than did his nemesis, Cortés. And yet, Las Casas, too, harbored fantasies of a pristine and unspoiled land: He conceived of New World natives as Innocents from Hesiod’s Golden Age, or as close kin to naked Adam and Eve. He was also guilty of certain hypocrisies: He had owned an estate and native slaves in Hispaniola. And ultimately, of course, his campaign was waged in vain: It is to the Sea of Cortez that we are now driving. But in spite of all this—the illusions, the hypocrisy, even the inevitable futility—we head for that sea hoping never to resemble its namesake.

Aaron Hirsh is chair of the Vermilion Sea Institute and the author of Telling our Way to the Sea. His essays have appeared in literary journals, The New York Times, and The Best American Science Writing. He lives in Boulder, Colorado.

This article originally ran as a print-only Prelude in our Fall 2013 Quarterly.