On Superiority

I’m writing this at my house in central Connecticut. Twenty thousand years ago, this spot was buried under a mile of ice.

About thirteen thousand years ago, after the ice thinned and retreated, plants swept over the bare land. They came from the southern United States, and they established the same kinds of forests and swamps that had grown in Connecticut in earlier periods between the Ice Ages.

But today, many of the most abundant plants around my house today come from distant continents–plants like honeysuckle. And Japanese barberry. And ground ivy. And the knotweed. And on and on: you can read a list of invasive plants in Connecticut here.

It’s a story repeated around the world. As ships cross the oceans and planes soar through the air, they deliver species to places that would probably never get to on their own. And sometimes they thrive amazingly, beating back the native species.

This week in the New York Times, I write about new research on why invasive species thrive. Some scientists argue that native ecosystems have to be weakened for aliens to take hold. Some favor the idea that alien species gain an advantage by leaving their parasites and predators behind. But a pair of ecologists, Jason Fridley and Dov Sax, argue that something else may be going on. It may simply be that the invasive species are superior.

Using the word “superior” is a risky thing to do, because it can trigger lots of troublesome and irrelevant associations…


But the word is correct–Fridley and Sax use it repeatedly in their paper–so I’ll stand by it even if it means I have to clarify things.

Fridley and Sax have drawn on some remarks by Darwin to develop what they call the Evolutionary Imbalance Hypothesis. They argue that species in different parts of the world have adapted to similar physical conditions. East Asia is a lot like Connecticut, for example, in terms of its climate. The species in both places have evolved, as natural selection improved their ability to survive, grow, and reproduce. It’s conceivable that a species in one place might get better at all that than a species somewhere else. It would be, in other words, superior. That doesn’t mean that this species would be some kind of Platonic ideal, or that it was a kind of Aryan paragon. It simply produced more offspring under identical conditions than another species.

Fridley and Sax hypothesized that some places might act as nurseries for superior species. These would be stable regions where evolution could play out longer than other places. As I describe in my column, they found evidence that these places–like East Asia–do in fact exist, and they send lots of invasive species to other regions. Like Connecticut. With ice sheets sweeping down from time to time, Connecticut and the rest of the northeastern United States may have been home to an ecosystem of–sorry to say–inferior species.

As I noted in my column, this hypothesis has some gloomy implications for those who want to preserve native species. Healing a native ecosystem may not be enough. Introducing an alien species’s parasites to its new range may not be enough. That’s because fighting invasive species may, in effect, be fighting against millions of years of evolution.

And this leads to a prickly question. One of the things that make ecosystems worth saving is the services they provide us with. If a superior species swoops in, it may do a better job at some of those services than native ones. An alien plant might draw more carbon dioxide out of the atmosphere and store it away in the soil, for example. Maybe we’ll be better off with an invaded ecosystem? I may not be fond of the barberry infiltrating the New England forests, but maybe our decisions about managing the wilderness have to be based on more than aesthetics.

7 thoughts on “On Superiority

  1. I read an article last night on the potential for using biotechnology to upgrade crops with a more efficient version of photosynthesis. One suggestion the author makes for preventing those plants from displacing wild plants is to also upgrade native species. I suppose we’re nearing a point where we could selectively create “superior” natives. Or alternately, find a way to downgrade invasive species, in a manner similar to the coming GM mosquitoes that preferentially produce more males (and hence reducing their reproductive fitness).

    There’s certainly potential for serious unintended consequences, with no guarantees that the genes will stay put in the species we choose.


    [CZ: Thanks for the link, Mike. I’m actually not aware of an example of a cultivated crop outcompeting native plants in the wild. We’ve domesticated these crops so that they depend very much on our care, from water to fertilizer to weeding. There are some weeds that started out as crops, but they actually invade farm fields, not natural ecosystems. Surviving in the wild requires a whole set of traits that have been selected against in crops. (If I’m missing a clear case, I’d welcome being enlightened.) If scientists create a superior photosynthesizing plant, it’s conceivable that it might overrun native plant ranges–but I’d want to know if there are any tradeoffs that are made to get that extra photosynthesis. For the moment, what seems to be the real threat to native species are the destruction of habitat, rapid climate change, and (to some extent) invasive species.]

  2. This is reminiscent of Jared Diamond’s thesis in Guns, Germs, and Steel about the advantages that Eurasian peoples had over those in the Americas based on the superior plant and animal resources available to them for agricultural domestication.

  3. Well OF COURSE we can’t focus on aesthetics alone — that’s the whole problem with gardening with or without natives (too often I think we privilege our own wants over those of other species that might help us, like pollinators that need native plants to lay their eggs on — choosing a plant because it looks pretty is only one part of why we should be choosing to bring that plant home). That being said, who’s going to spend the money to test every plant in every locale to see which is more beneficial and how and to whom? What about the evolutionary history of insects / animal species and plants? What about some bees who forage ONLY on asters in the fall, using that pollen to feed larvae over the winter? What about monarchs and milkweed? What about prairie that sequesters a ton of carbon, on par with forests (if not more some studies suggest)? Maybe exotic species can do so well because they have no competition from disease or animals or insects? Isn’t the larger issue that we elevate ourselves above all other species, and presume we are always the right thing and can do wrong? I don’t advocate conservation in the strictest sense because via climate change we’ve altered every last square inch on the planet, but I can’t say I’m a full-blown novel ecosystem guy, either. Thanks for starting the conversation.

  4. Thanks, Mr. Zimmer for spreading the word about this new research, or perhaps more accurately new evidence in support of a very old theory. You have done more to inform the public about the outdated assumptions of invasion biology than any other science writer (except Emma Marris, of course).

    I’m a life-long member of the California Academy of Science in San Francisco. I see that you will speak at the Academy on November 7th. I hope that you will have an opportunity to mention this new research and related studies.

    The native plant movement has a death grip on the public lands in the San Francisco Bay Area. Hundreds of thousands of non-native trees have been destroyed here and native plant advocates are demanding the destruction of millions more. Huge quantities of herbicides are required to prevent the trees from resprouting and to eradicate non-native vegetation. These projects are described here: http://milliontrees.me

    Anything you can do on November 7th to inform the people of San Francisco about the current status of a destructive ideology that is increasingly being discredited by empirical studies would be deeply appreciated.

    Thank you for your work.

  5. Mr. Vogt’s comment suggests that he has not understood this article. “Aesthetics” are not the subject of this article. The subject is evolution. The point of the article is that whatever your personal preference with respect to the nativity of a plant, it may not be possible to eradicate plants that have an evolutionary advantage over the plant you prefer. Plants from places with longer evolutionary histories have developed greater competitive advantages over plants that have experienced less evolutionary history. Natural selection has improved their survival mechanisms.

    There are also a couple of misconceptions in Mr. Vogt’s comment that are worth addressing. There is very little empirical evidence that supports the “predator release” hypothesis which is used by native plant advocates to predict that all introduced plants will ultimately be invasive. When careful empirical studies are conducted, equal numbers of insects are always found on native and non-native plants. Here is a report on a particularly conclusive study which was based on 30 years of data analyzed by computer to conclude that butterflies do not prefer native to non-native plants in the San Francisco Bay Area: http://milliontrees.me/2014/10/03/butterflies-of-the-bay-area-region/

    Milkweed is a good example of the common misconceptions of native plant advocates. There are 140 species of milkweed which are native to specific ranges. Anyone of those species of milkweed will serve monarchs equally well. There are also monarchs in Australia, which are presumably using milkweed that is native there. It is not the fact that milkweed is native that meets the needs of monarchs. It is the chemical properties of a genus of plant that can be native or non-native to any particular location.

  6. Millie –

    Milkweed is an introduced (and invasive) species in much of Asia and Australia. The Monarch butterfly was also an introced species there – introduced to Vanuatu and New Caledonia, they colonized Australia in 1871 and survived (and thrived) there due to the widespread use of a form of North American milkweed as an ornamental plant.

    Today, Monarch (or Wanderer in Australia) butterflies are purposely farmed for release at weddings and other events because they are showy and people pleasing. Such releases are considered more ethical because there is so much (non-native) milkweed that the butterflies thrive after release.

    Nothing ‘natural’ there, I’m afraid.

  7. Brian, Actually the North American monarch migrations are also considered anthropogenic. Richard Vane-Wright reports that the California monarch migration was first reported in 1864. He believes the eastern monarch migration to Mexico also began around the same time. His theory is that the agricultural practices of early settlers, which cleared trees, created a population explosion of the milkweed that is the host plant of monarchs. More milkweeds resulted in more monarchs and monarchs began to migrate in response to population pressure, he believes. He calls this the “Columbus Hypothesis.”

    Vane-Wright believes monarchs in Australia found their own way there as a result of population pressure. By monarch standards, it isn’t such a long way. Monarchs were observed in all these places long before the fad to release butterflies during weddings.

    The word “natural” has little meaning in the Anthropocene. Walk down the cleaning and cosmetic aisles of your market to observe how many products are touted as “natural.” The word is now primarily a marketing tool.

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