The Native Plants Debate

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Should we be paying closer attention to where our plants come from? Not in terms of where the plants were purchased, but rather where they evolved? It wasn’t until I read Douglas Tallamy‘s book Bringing Nature Home (Timber Press 2012) that I was persuaded to choose native plant species whenever possible.1 But it turns out that not everyone agrees. Setting aside the troubling native/alien binary, some have argued that supporting native plants — particularly those that are struggling or disappearing — is wrongheaded. I’ve seen this argument play out frequently in permaculture circles or in conversations about reintroducing natives and restoring ecosystems. If native species are failing, the argument goes, why try to prop them up — particularly when doing so frequently requires the use of toxic herbicides to clear out the invaders? Shouldn’t we embrace the successful spreaders instead?

It’s a complicated issue to be sure. I briefly consider both sides of this debate here.

A monarch perched on Liatris aspera, a wildflower endemic to the Midwestern and Eastern US (though not New England).

A monarch perched on Liatris aspera, a wildflower endemic to the Midwestern and Eastern US (though not New England).

Why Plant Native Species?

The entomologist Doug Tallamy is probably the most visible advocate for the native plants movement. His essential argument is this: native plants sustain a greater and more diverse insect population than do introduced species. This is because native plant and insect species evolved together in their ecosystem. This shared history between plant and insect is important because most herbivorous insects can require centuries to adapt to eating the tissues of a particular plant (due to the chemical constituents in the tissue that discourage herbivory). This means that whenever a certain plant’s population dwindles in an ecosystem, the insects whose larvae depend on that plant for food will also decline (think milkweed and monarch butterflies). So every time we choose something like an autumn olive (Elaeagnus umbellata, from Japan) over a native species like downy serviceberry (Amelanchier arborea), we deprive insects like red-spotted purple butterfly a potential host plant. (Tallamy summarizes his argument in a brief NY Times piece here.) Even worse, while many introduced species have a relatively minimum impact on our ecosystems, coexisting peacefully with native plants (like the broccoli in your garden, which isn’t taking over your entire yard), some non-natives, like the autumn olive, “escape” cultivation and spread rapidly in the wild, crowding out native plant communities and becoming “invasive.”

Tallamy’s book paints a grim picture of a future where a decline in insects causes similar declines in the animal species that eat them (e.g. birds and lizards), and so on up the food chain. His work has inspired other calls to action among gardeners and landscapers, including Benjamin Vogt’s recent book, A New Garden Ethic: Cultivating Defiant Compassion for an Uncertain Future (New Society Publishers 2017). Here in Maine we have a nonprofit organization, the Wild Seed Project, that was founded to encourage the planting of native species in order to maintain biodiversity.

A wood anenome (Anemone quinquefolia) blooms above emerging Canada mayflower (Maianthemum canadense) leaves.

A wood anenome (Anemone quinquefolia) blooms above emerging Canada mayflower (Maianthemum canadense) leaves.

Who's Advocating for Invasive Species?

This rise in native-plants advocacy, however, is not without its critics. In fact, recent years have seen a surge of books insisting that we should embrace hardy species of any provenance in this era of rapid climate change. Consider titles like Fred Pearce’s The New Wild: Why Invasive Species Will Be Nature’s Salvation (Beacon Press, 2015)2 or David Theodoropolous’s Invasion Biology: Critique of a Pseudoscience (Avvar Books, 2003). There are also ecologist Ken Thompson’s Where Do Camels Belong? Why Invasive Species Aren’t All Bad (Greystone Books, 2014) and Tao Orion’s Beyond the War on Invasive Species (Chelsea Green 2015).

What I find interesting as a farmer is that a number of these attempts to rehabilitate invasive species are written (or embraced by3) permaculturists. This makes some sense. Permaculturists tend to be pragmatists who adopt the most productive plants for their needs and bioregions — some of which, such as the famously productive hardy kiwi (Actinidia arguta), are potentially invasive. Consider one of the movement’s founders, David Holmgren, who decries the “nativist orthodoxy” that dominates ecologists and national ecological policies. He elaborates in his Foreword to Orion’s book:

The criticism of permaculture by the environmental orthodoxy was [...] because of the perceived audacity of using ecological arguments to justify the use of a wider range of species not indigenous to a site or even a bioregion. [...] Most permaculture teachers and designers accepted the findings of invasion ecology at face value and sought to minimize risks of unintended naturalization. For me, this pragmatic accommodation drove permaculture away from the principle of working with rather than against nature. (x)

For Holmgren, the prevailing notion that “all species naturalizations4 at all scales represent ecological degradation, and should be avoided” is “pernicious” and neither holistic or ecological (“Weeds”). He frequently frames his arguments as opposing an established, industry-funded orthodoxy in ecological research. 5

Though Holmgren fires the first volleys at “nativism” in his Foreword, Orion’s book itself is more nuanced. I’d like to devote a little more space to it here, as it points out how complex these arguments about nativeness and invasiveness can become.

Orion, an organic farmer and permaculturist who took charge of an ecological restoration project in Oregon, interrogates the distinction between native and invasive plants throughout her book. And though she admits to removing invasive species from her own property, she generally uses the book to expose flaws in our thinking about invasiveness, both as a label and as an ecological threat. According to Orion, simply determining what makes a particular species “native” poses serious difficulties, since North American habitats are so profoundly disturbed (having been actively managed by the indigenous peoples who lived here before European colonization6).

One of Orion’s central messages is that our invasive species management practices do more ecological harm than good. She describes the dangers of herbicide application and reveals how chemical manufacturers inform regional ecological policies. There’s big money in species management. Orion also discusses a number of cases where invasive plants (often the subjects of multi-million dollar eradication campaigns by the government and conservation groups) fill ecological niches left vacant due to human interference. In fact, she argues, a number of these invasive species, like the salt cedar along the Colorado River, play important roles that native species no longer can as a result of anthropogenic environmental changes such as increased soil salinity.

Orion’s arguments — particularly her exposure of massive herbicide use — strike me as rational, even if her examples of maligned invasive flora are more anecdotal than systemic. (I’d love to see more of a comprehensive literature review of current ecological research on the question.) I wish Orion had found opportunity to engage with Tallamy’s work, which she never cites. His arguments about insect biomass and host plants seem to offer potentially damning rebuttals to some of her claims. But she does cite an older article (“The California Urban Butterfly Fauna is Dependent on Alien Plants”) to demonstrate that alien plants can successfully serve as hosts to native insect populations. The authors of that article, Graves and Shapiro, studied California’s butterflies and concluded that, although at least three butterfly species had laid eggs on alien plants toxic to their larvae, in most other cases introduced flora is largely beneficial to the insects; in fact, some of the butterfly species rely entirely on introduced plants for nectar and oviposition (i.e. egg-laying). The Graves and Shapiro article often surfaces as a critique of Tallamy’s logic. I’ve seen other permaculturists cite this same study to similar ends (Shapiro at one point sounds almost like Holmgren, referring to “native-plant ideologues” (35)). Unfortunately, Shapiro doesn’t appear to discuss Tallamy, and vice-versa; I’d be curious how the biologist and entomologist would settle their seemingly contrary claims about non-native host plants.

In at least a couple passages I had hoped that Orion would delve more deeply into the complexities she raises. Take her defense of the invasive candleberry in Hawaii:

In one Hawaiian study, the invasive nitrogen-fixing candleberry contributed 18 kilograms per hectar (kg/ha) of nitrogen per year in an area where nitrogen fixation by native plants amounted to 0.2 kg/ha per year... Enriched soils have been shown to attract large populations of invasive earthworms (yes, even worms are considered invasive in certain places). Worm biomass was shown to be up to eight times higher under the invasive candleberry compared to the native myrtle. (121)

Here Orion’s parenthetical confirmation that yes, some people consider even worms invasive, glosses over a situation that’s actually quite serious. Invasive earthworms in North America, in fact, are engineering new ecosystems. This is particularly true of northern temperate forests, where invasive worms consume the O (or organic) horizon of leaf litter atop the forest floor. This activity alters soil structures and microbial communities in such a way that many plant species that evolved under the old ecological system can no longer survive due to the worms’ interference7. (Japanese Barberry (Berberis thunbergii) is having something of a similar effect in northeastern forests, increasing both worm and tick populations.)

Thus the presence and concentration of earthworms in Hawaii is not necessarily a mark in candleberry’s favor.

A monarch caterpillar feeding on a swamp milkweed (Asclepias incarnata) leaf in my old Virginia garden.

A monarch caterpillar feeding on a swamp milkweed (Asclepias incarnata) leaf in my old Virginia garden.

What's to be Done?

I raise these points not to quibble with Orion, but to point out the missed opportunities for dialog between these camps. That dialog is crucial because both sides make important points. We absolutely should ensure that there are sufficient host plants to sustain insect biomass. Anyone familiar with the monarch’s (Danaus plexippus) dependence on native milkweed (Asclepias spp.) understands what’s at stake when host populations dwindle. At the same time, our attempts to eradicate invasive species shouldn’t cause greater ecological harm than good; furthermore, we should be conscious of when an ecosystem has changed so radically that its native flora can no longer survive without human assistance and interference.

It’s clear that we’re radically changing our planet’s ecosystems. Though I don’t take this pattern of human disturbance and interference to be license to do whatsoever we please going forward (since there is no going back), I do think a combination of conservatism and flexibility is warranted.

I’ll close with words written to me by a couple ecologists whom I consulted on this question a couple years ago (I post them here anonymously). Here’s one summarizing comment:

In the last four decades the biodiversity that we have lost worldwide and even in Maine is simply stupefying. As Americans, we have a poor sense of history and don't like to admit that we did a bad job and lost the game. So we invent new rules and proclaim victory ("nature's salvation"). Like, the world has always had invasive species and extinctions, and so it's all good. It's not. The world is much less sparkling and interesting and heterogeneous than even 10 or 20 years. Tallamy's pleas would be right in line with older conservation biologists. The so-called "new conservation scientists," on the other hand, criticize their predecessors as stodgy and xenophobic. I do think it's easy to overstate the ecological benefits of native vs. introduced species

And another that emphasizes the complexity of these questions about plant provenance and ecological utility:

I love native plants, and spend a lot of time encouraging people to plant them. However, I'm not a native plant purist ...  after all, if we planted only native plants... we wouldn't have vegetable gardens. I do think that we should plant a high percentage of native plants in our landscapes (I don't really have a number in mind ... 50%? ... 75%?), because Maine is undergoing considerable land development. If we want Maine's ecosystem to continue to function, we need to make sure that we preserve natural areas and also add native plants to our landscapes that surround our development projects.

I think ecologists wouldn't all agree. Here's an example: most people would agree that purple loosestrife is problematic in our wetlands, and a lot of money has been spent trying to manage it. But I work with an entomologist who thinks purple loosestrife has its good points ... it's very good forage for some insects. That's just one small example, but it does point out that your question is very complex.... [W]e usually think of white clover as a great cover crop in production fields, but it is one of the most invasive species of plants in a broad worldview. As I said, there are no "good" and "bad" plants.

Sources Cited

  1. My use of native/invasive plant terminology follows the USDA definitions listed here. But as is the case with so many aspects of this conversation, definitions for terms such as alien, invasive, native, weed, and naturalized can be contentious. See discussions in Richardson et al, Colautti and MacIsaac, Orion 48ff

  2. For a rebuttal, see Liam Heneghan's review in the Los Angeles Review of Books. Fred Pearce also has a piece over at Salon entitled "Invasive species will save us: The new way we must think about the environment now."

  3. See for instance David Holmgren's review of Theodoropolous' book (PDF)

  4. Richardson et. al give the traditional definition of "naturalization" as the stage when a "species establishes new self-perpetuating populations, undergoes widespread dispersal and becomes incorporated within the resident flora" (94).

  5. To give you a sense for how fierce the debate can be for advocates on both extremes of this native/invasive spectrum, consider the lengthy argument kindled in the comments section to Sue Reed's article on the subject, "Permaculture's Internal Contradiction" (archive available here).

  6. But see Reichard, who asserts that the land management practices of the first peoples "mostly either maintained a matrix of native species somewhat more diverse than would naturally have occurred or created small 'gardens' of native species translocated somewhat out of their native context" (3).

  7. See Snyder et al., Bohlen et al