invasives – The Garden Professors™

In defense of weeds?

Blog reader Shawn sent this link to me yesterday. It’s a pretty short take on a complex topic, but even so I was troubled by the perception that all nuisance weed species are our own fault.

Sure, it’s true that humans have moved plants or plant parts around with them for centuries. Sometimes it’s been deliberate, and sometimes it’s been accidental. But other animals also move plants around, especially seeds. When we draw this kind of distinction between what we do and what other animals do, philosophically we are removing ourselves from the natural world. True, we have technology and all kinds of other human inventions, but as a species we are still part of the biosphere.

Ivy’s little dispersal units – spread by birds

Philosophical issues aside, there’s another part of this blithe acceptance of weedy species that concerns me. Though plants take advantage of animals as a means of dispersal, the rate at which nonnative, weedy species are spreading and colonizing new environments is unprecedented (this is where technology comes in). Ecosystems can adapt to new species and other environmental challenges – but when the rate is accelerated, the adaptive process is impaired. Thus, some native species go extinct when the rate of change is too great.

Ivy left to its own devices in a natural area

These are basic ecological concepts – and we ignore them at our own peril.

Excerpt from Davis et al. letter to Nature on natives vs aliens

In yesterday’s post I linked to a letter in Nature by Mark Davis and a number of other ecologists on the role and native and alien plants. Unfortunately the journal requires a subscription. Copyright laws prevent me from re-printing the entire article, however, below is an excerpt from the conclusion, which I think captures most of their message.

“Most human and natural communities now consist both of long-term residents and of new arrivals, and ecosystems are emerging that never existed before. It is impractical to try to restore ecosystems to some ‘rightful’ historical state. For example, of the 30 planned plant eradication efforts undertaken in the Galapagos Islands since 1996, only 4 have been successful. We must embrace the fact of ‘novel ecosystems’ and incorporate many alien species into management plans, rather than try to achieve the often impossible goal of eradicating them or drastically reducing their abundance. Indeed, many of the species that people think of as native are actually alien. For instance, in the United States, the ring-necked pheasant, the state bird of South Dakota, is not native to the great plains of North America but was introduced from Asia as a game bird in the latter half of the nineteenth century.

“Specifically, policy and management decisions must take into account the positive effects of many invaders. During the 1990s, the US Department of Agriculture (USDA) declared several species of introduced honeysuckles to be alien (harmful), and banned their sale in more than 25 states. Ironically, from the 1960s to the 1980s, the USDA had introduced many of these same species in land reclamation projects, and to improve bird habitats. Recent data suggest that the agency’s initial instincts may have been appropriate. In Pennsylvania, more non-native honeysuckles mean more native bird species. Also the seed dispersal of native berry-producing plants is higher in places where non-native honeysuckles are most abundant (Gleditsch, J. M. & Carlo, T. J. Diversity Distrib. 17, 244-253 (2010).

“Clearly, natural-resource agencies and organizations should base their management plans on sound empirical evidence and not on unfounded claims of harm caused by non-natives. Another valuable step would be for scientists and professionals in conservation to convey to the public that many alien species are useful.

“We are not suggesting that conservationists abandon their efforts to mitigate serious problems caused by some introduced species, or that governments should stop trying to prevent potentially harmful species from entering their countries. But we urge conservationists and land managers to organize priorities around whether species are producing benefits or harm to biodiversity, human health, ecological services and economies. Nearly two centuries on from the introduction of the concept of nativeness, it is time for conservationists to focus much more on the functions of species, and much less on where they originated.”

Ecologists weigh in on native-exotic debate

Charlie Rohwer, a frequent guest contributor to the Garden Professors, brought to my attention a recent letter in Nature by Mark Davis and 18 other ecologist entitled, “Don’t judge species on their origins.” Davis is a leading authority on invasive species and author the book Invasion Biology. In their article, Davis and his co-authors make many of the same points that I’ve made here on the blog (Are natives the answer? Dec. 14, 2009; Restoration ecologists you need us – part 2. Aug. 9, 2010) regarding the native/exotic debate. The main point is that we need to develop a more pragmatic approach to managing landscape systems and this includes natives and exotic species; it’s not an either/or question anymore, not that it ever really was.

A couple of excerpts from their letter:

“It is time for scientists, land managers and policy-makers to ditch this preoccupation with the native–alien dichotomy and embrace more dynamic and pragmatic approaches to the conservation and management of species — approaches better suited to our fast-changing planet.”

The pragmatic approach that Davis and his co-authors (and I) advocate recognizes a several realities:

Exotics can fill many of the same ecological roles and niches as natives, Doug Talamy’s book notwithstanding.

Any inherent “ecological superiority” of natives over exotics has long since been negated by anthropogenic land-use change, alien pests, and climate change.

Attempts to eradicate alien species are largely warm-fuzzy exercises with little likelihood of success without liberal use of industrial-strength herbicides.

W.O.W. (aka Why oh why do nurseries sell this plant)?

Since we’re back on the alien train (spaceship?), I thought I’d bring up another of my least favorite shrubs – Scots broom – as our next installation of WOW (why oh why?).

Scots (or scotch) broom (Cytisus scoparius) is a much-reviled intruder in the western and eastern United States. Originally introduced as a sturdy ornamental, this legume quickly invaded disturbed areas and is labeled as a noxious weed in several western states. In Washington, it’s quarantined. Research dollars have been dedicated to studying best methods of eradication. So it doesn’t take a rocket scientist (or even a garden professor) to figure out that it’s probably not a wise addition to one’s landscape.

But apparently some nurseries either (1) haven’t paid attention or (2) don’t care. In a quick look at the internet, I found nurseries in many states, including Oregon and California, that sell this species. Many will argue that they are selling “less invasive” or “sterile” cultivars, which is a poor excuse in my opinion. Readers of this blog know by now that cultivars often revert to wild type, and there’s no reason to assume that broom cultivars are exempt from this ability. Furthermore, we know that plants have the ability to extend their ranges past what we think they are (hello kudzu?).

Just popping in to say hello

There are many ornamental alternatives to Scots broom that can easily be found online or in print. I’d love to hear some rational arguments from nursery owners, landscape designers, or anyone else justifying the sale of this plant.

Where to draw the line on a vine

Last week’s column on “why do nurseries sell this plant?” struck a chord with many readers as well as with Holly! So here is this week’s submission: that ubiquitous vine, English ivy.

First of all, we’ll stipulate than many ivies are sold as English ivy (Hedera helix) but may be entirely different species. Genetic research on invasive ivy populations in the Pacific Northwest identify most as H. hibernica (aka Atlantic ivy), with H. helix making up only 15% of the invading populations. Regardless of their species identity, it’s obvious that Hedera is a genus with the potential to escape gardens and invade remnant forests or other environmentally sensitive areas. It grows so vigorously that it can create monocultural mats on forest floors; it grows into trees where its sheer weight can break limbs and in some cases topple entire trees.

But what about other regions of the country – or the world, for that matter? In colder climates, Hedera spp. are much better behaved, dying back to the ground every winter and rarely able to flower and reproduce. The absence of a seed bank means the vine can be kept in check more easily. And it does tolerate tough environmental conditions where other groundcovers might not succeed.

Yet consider another invasive: kudzu (Pueraria montana var. lobata) For decades this noxious weed was thought to be too cold sensitive to expand past the American Southeast. Yet populations have been found in Maine, Oregon, and most recently in Ontario, Canada. Plants adapt!

So – is it worth the risk to buy a plant known to be invasive elsewhere, simply because it’s not a problem yet?

And why oh why do nurseries in Washington state continue to sell Hedera spp. as ornamentals?