Interwoven and Integrated: Non-native and Native Species in Life’s Web

Lock and key. Co-evolution. “Made for each other.” One of the dogmas offered by nativists is that native plants and animals, evolving together for thousands of years, form a closed interdependent ecology. According to the California Native Plant Society’s website:

Native plants, birds, butterflies, beneficial insects, and interesting critters are “made for each other.” Research shows that native wildlife prefers native plants.

According to native species advocates, exotic invasive plants disrupt this fabric; with no natural enemies in this new environment, they grow rapidly but provide very little by way of ecological benefits to any native plant or animal. They outcompete the natives, goes this argument, causing extinctions.

It’s not generally true. What we find is that native species adapt to the changing environment. In those species — like insects — where reproduction is rapid, they may actually already have evolved to use some of the new resources that introduced plants offer. [ETA Aug 2011: There’s a great story on such adaptation on the website Death of a Million Trees.] Others, like birds and mammals may have flexible behaviours so they aren’t tied to specific plants and ecological constructs.  Instead of ecosystems that can be declared Native or Non-native, they’re actually interwoven into a web of interdependencies that characterize any functioning ecosystem.

Take, for example, the Mission Blue butterfly. It’s a famously endangered subspecies of butterfly. It’s being reintroduced at Twin Peaks, with considerable effort and investment and uncertain success. Its larvae depend on a particular native host plant, lupine. But the adult butterfly needs nectar, and one of its best sources is a purple non-native thistle.

The Anise Swallowtail is a spectacular native butterfly, an eye-catching yellow that’s difficult to capture with a camera. In the 2010 butterfly count, it was the 3rd commonest butterfly in San Francisco… and all because it’s adapted to using fennel, an introduced plant as the host for its larvae.

Photo credit: 123rtf

Monarch butterflies, which famously use milkweed as their nursery plant, equally famously use (non-native) eucalyptus for their primary roosting sites in California.

Honeybees, which are non-native, have an eclectic approach to nectar-producing flowers. They use the non-native eucalyptus flowers in seasons when few others are blooming; but in spring and summer, they can be found on ceanothus and lupine and other Native flowers. By pollinating these flowers, they help propagate them. Meanwhile, native bumblebees happily sip from non-native bermuda buttercup (oxalis pes caprae), a copious nectar source. (It doesn’t actually propagate the oxalis because that plant doesn’t set seed in California.)

[ETA: 31 July 2011] In fact, according to a recent East Bay study by UC Berkeley’s Professor Gordon Frankie and his colleagues, only 72 of nearly 700 flowering plants (PDF file) they examined had measurable bee activity (at least one visit in 6 minutes on a sunny day). Of the 72, three-quarters were non-native plants, and a quarter were native. Many garden plants are bred to be unattractive to bees (bright, double-petal flowers without much nectar); but clearly, the ones that do feed pollinators sustain a substantial population not just of non-native-but-essential honeybees, but also of native bees. The researchers found 74 species of bee, of which only two were exotic.]

[ETA 25 Sept 2011: The snowy plover, a threatened shore-bird species, nests in the old Cargill salt ponds on San Francisco Bay.]


For some reason, many are convinced that birds and animals don’t live in eucalyptus forests. They do. An established and damp ecosystem like Sutro Forest is home to a large number of birds, insects, and small mammals — most of which are native.

Downy woodpeckers, hairy woodpeckers, and sapsuckers are heard there; winter wrens forage in its undergrowth and call piercingly. Juncos and towhees are relatively easy to see, as are hummingbirds. Over 40 species of birds have been seen and/or heard in this forest. They use the eucalyptus, the acacia sub-canopy, and the blackberry understory as habitat.


In a recent op-ed article “Mother Nature’s Melting Pot” in the NY Times, Professor Hugh Raffles (of New School, author of Insectopedia) reflects on integration of Native and non-native species. The tree-preservation blog, Death of a Million Trees, carried a post about this article.

And a recent article in the New Scientist discussed the great horse debate. America had horses for over a million years, and according to the article, they evolved here. Then they died out, 10 thousand years ago. So the wild horses here now, are they to be considered reintroduced native species? Or are they non-native pests? The article concludes with a reference to Mark Davis and his book, Invasion Biology… is native vs non-native really the point?

Nature, after all, doesn’t have such categories. All species compete and co-operate and evolve, whatever their origin. Evolution occurs all the time, and can show visible results quite rapidly. Charlie (who has participated in several discussions on this site) recently sent us an interesting article from an Australian site, which noted that introduced plants were adapting to their new environment by becoming similar to native plants. This leads to the obvious question: If these plants become unique to that landscape, at what point are they deserving of protection as an endemic species?

Trying to restore a landscape to a particular point in time assumes a static view of the environment that is quite unnatural in an ever-changing planet.

[ETA 24 June 2011: Para on horses edited to clarify facts and provide a link to the 17 June 2011 New Scientist article. However, this link will probably expire after some weeks.]

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