Pay attention to the natural world for any length of time at all and you’ll come up against one of the biggest divides in the life sciences: “native” species versus “introduced” species.
It’s a basic concept in ecology. Ecosystems develop with certain kinds of organisms as members, and over time a predictable set of relationships develops among those organisms. Then some new organisms parachute in and change the whole system for a while, until relationships re-evolve and things get predictable again. That process happens all the time in nature, but over the last few centuries we’ve sped up the rate of introductions to a dizzying speed. Now we introduce species to new places every single day. Some turn out to be invasive, harming the ecosystem that was there beforehand.
That’s especially true in California, which has seen introductions of new species at a ever-increasing rate in the last two centuries. And as California already had a startling number of species before the new introductions started, that raises the stakes. We have more than our share of organisms that live nowhere else, and as we introduce new species, the risk of harm to the pre-existing species mounts.
But it’s not always so simple. Here's a look at just what we mean when we call something a “native species,” and whether there’s really a clean dividing line between those natives and “introduced” species.
In theory, it should be pretty straightforward to define the difference between a native species and a species that’s been introduced. After all, you might think, a species has either been introduced to a place or it hasn’t.
For some kinds of organisms, it is that straightforward. Dr. Jann Vendetti, an evolutionary biologist in charge of the mollusks department at the Los Angeles County Museum of Natural History, doesn’t have to spend a lot of time worrying about how to define which of the local snails she studies are native and which aren’t.
“For snails it’s pretty easy,” says Vendetti. “In Southern California, a snail species is native if it evolved here. If it evolved somewhere else, it’s been introduced.”
Southern California’s historic varied topography, with sun-baked, south-facing slopes interspersed with cooler northern slopes and forested creeks, provided not only ample spots for the ancestors of today’s snails to thrive, but near-impassable barriers between those spots as well. Snails in one creek often found it hard to move to the next watershed over; that meant no interbreeding between the two watersheds’ snails, and reproductive isolation is the canonical force that drives splits between species.
There are plenty of species other than snails that qualify as native by Jann Vendetti’s rule of thumb. Lots of them are plants. We live in a state in which it seems every coastal county has its own cluster of species of Ceanothus, each of which evolved in situ. Monterey pines, now one of the most widely planted trees in the world, were restricted a couple hundred years ago to the Monterey Peninsula, Cambria and the northern Santa Cruz County coast, with outlier populations on Guadalupe and Cedros islands off the coast of Baja. Each population is distinct, and each is clearly evolving in place -- or was until we started planting their progeny all over.
When you start talking about species that are a little better at migrating than snails, things can get more complicated. Next time you’re in the Mojave Desert and glimpse a coyote shading itself under a creosote bush, consider this: you’re seeing two species considered native to California. One of them, the creosote, became a species a very long way away, in the Andes. The coyote evolved in western North America, more than likely, but it likely evolved all across western North America rather than in one specific location.
Some native species evolved here. Others got here at some point after evolving. That second group of species is the group that, on rare occasion, can push hard up against our definitions of “native” and “introduced.”
You might well ask, “introduced by whom?”
The obvious answer is “us.” But we're not the only species that introduces other species to new places. Creosote bushes came to California without human help, but likely aided by seed-eating birds and mammals. Whitebark pines originated in Asia, and were brought step by step southward along the western mountain ranges by Clark’s nutcrackers and other animals. Coyotes got themselves here however they did, which probably involved sneaking. Gray wolves are reintroducing themselves to California after a century of absence. Native species all.
Eucalyptus trees, on the other hand, near ubiquitous in coastal California, were brought here deliberately by human beings less than two centuries ago. Mustard has been here only a little longer, brought by Spanish colonizers. Pampas grass and turf grass and daffodils and almond trees and opossums and honeybees: all introduced by human beings.
Clear-cut, right? Mostly. We’re still working out some of the odd exceptions. For one thing, western science is just starting to overcome an embarrassing tendency to consider Native peoples in California, and elsewhere, as not fully human. Native introductions that happened before western science was on the scene to record them seem not to quite count as “human introductions” in some eyes.
Southern California offers two remarkable examples. An increasing scientific consensus holds that the federally Endangered Island fox, found only on six of the eight Channel Islands off the coast of Southern California, was introduced to at least the three southernmost Channel Islands by Native people. To my knowledge, no one has suggested stripping the title of “Native species” from foxes on San Nicolas, San Clemente, and Santa Catalina islands just because they were probably introduced by humans.
And then there’s the California fan palm, Washingtonia filifera, which at least one researcher has suggested was carried into the present-day state of California from oases on the Baja peninsula by Native people, who used the trees for food, fiber, and building materials. The idea that fan palms were introduced to California by people is by no means universally accepted, but it’s certainly plausible, and might force a reevaluation of the tree’s inclusion on native plant lists.
Not all species introduced by Native people are considered native species, by the way. Native people in Cuba, New York and Manitoba were growing maize when Columbus arrived, but no one claims that maize — which was first bred in central Mexico — is native to any of those places. It may be that California’s reputation as a place where indigenous people never developed agriculture — an inaccurate description — promotes the assumption that people weren’t moving plants around before the Spaniards got here.
Native introductions aren’t the only place where the boundary between native species and others gets a little fuzzy. I mentioned the gray wolf above as an example of a species once native to California that has reintroduced itself without help — in fact, despite active opposition — from humans. We recently covered the possibility of reintroducing the grizzly to California. California had a native subspecies of grizzly bear, but that native subspecies was killed off in 1924. If we imported grizzlies from British Columbia or somewhere similar, would those grizzlies be California natives, or introductions, or some of each?
Wolves and grizzlies have been absent from California for only a short time, and the wild ecosystems that remain in the state probably haven’t changed too radically since they've gone other than being fragmented and having much more available parking. Horses, on the other hand, went extinct in North America somewhere between 11,000 and 13,000 years ago. Reintroduced to the North American wild by accident in the 1500s, the horse is a source of intense controversy as regards its proper place on the native-introduced continuum. The horses that escaped from Mexico and spread across the continent are generally considered to be the same species as the horses that died out here during the end-Pleistocene. The landscape and the ecosystems on it have changed radically in the interim. Are horses native to North America, or did their native status expire sometime after the Ice Age ended?
Here’s another puzzler: Many neotropical migratory birds travel through California as they move between their wintering grounds in Mexico, Central America, and points south, and their summer territories in Canada and Alaska. Some of them maintain a year-round presence in California. Some don’t. Migrating Arctic terns pass the California coast twice each year on their way from the far north to Antarctica. They’re in California waters reliably as clockwork, though they aren’t here for long and they rarely venture inland. Are they California native birds?
If the Arctic tern is too easy a “no,” then what about the similar common tern, which visits well inland in California, is seen in abundance during the fall migration, but which never breeds in California and is entirely absent from the state during the winter?
Speaking of birds, what about the cattle egret, an African bird that made it to the Americas with no apparent direct help from human beings? Cattle egrets have been aided post-arrival by humans’ conversion of much of the Western Hemisphere into agricultural lands, but they made it to South America apparently without human help in 1877, and arrived in the U.S. by 1941. They’re now year-round residents of California, with concentrations in the Central Valley’s remaining wetlands and the Salton Sea. It would seem the cattle egret’s arrival in California might be an example of a species introducing itself into new territory the way it used to happen before humans started mixing things up. Is the cattle egret now a California native?
Lastly, there’s the little issue of the differences between ecological and political boundaries. Those historic hypothetical people who might have moved fan palms from Baja California into California weren’t importing plants into a new world, just into new canyons in the same desert homeland. The international border came much later. Those fan palms are now considered native to California, but they’re really native only to a fraction of a percent of California’s landscape. Plant them in Humboldt County or around Lava Beds National Monument and they’d be an introduced species, as out of place as redwood trees in the Mojave or kelp in Mono Lake.
None of this should be taken to mean that the concept of native species isn’t a valid one. We regularly disrupt ecosystems by bringing new species into them, and in order to understand that disruption we need a concept to describe what was in that ecosystem before we disrupted it. The concept of native species is useful and valid, and easy to define probably 99 percent of the time. But it’s a human concept, and nature always finds ways to muddy our concepts’ nice clean boundaries.
In recent decades we’ve changed the rate at which organisms moved into North America by orders of magnitude. The era of introductions started with sailing ships and handfuls of mustard seed cast out into the Coast Ranges by wandering Franciscans. Now, hundreds of planes land each day in California from far-off places, each one carrying hundreds of people who might well be carrying invasive plants, animals, or pathogenic microorganisms. And those planes almost don’t matter, given the massive number of shipping containers full of random uninspected things arriving every day at the state’s seaports, and the ballast water those ships carry from place to place, and the myriad other ways in which we move whole ecosystems around the world.
In Part Two of this series, we’ll look at some of the impacts, bad and not as bad, of those introduced species.
Banner photo: California poppy, by Dagmar, some rights reserved.