What s a Native Species?

A California native species, the San Joaquin kit fox | Photo: Greg Schechter, some rights reserved

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. Heres 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.

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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.

Coyote and creosote | Photo: KnitSpirit, some rights reserved

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 were 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.

California fan palms at the Coachella Valley Preserve | Photo: Tom Giebel, some rights reserved

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

LINK:	https://www.kcet.org/redefine/whats-a-native-species-thats-more-compli...
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