We’ve had lots of lively discussion on my post regarding the Mark Davis et al. comment in Nature on natives and exotics. I have been traveling and otherwise occupied and have not had a chance to comment so I feel a little like the kid that kicked the anthill and then ran away. Fortunately, Holly was gracious enough to forego her post today (I promise to return the favor, Holly!) so I can chime back in.
Obviously there are lots of layers to the debate but one of the main items in the discussion is whether there is an inherent ecological advantage in planting natives over exotics. At this point the focus always seems to shift to herbivory and the question of whether native insects will eat non-native plants. There are certainly examples each way; some insects are generalists while others are highly specific. More importantly, however, plants fill many other roles in the environment beyond serving as food for insects. Moreover, species composition is just one aspect of diversity. The ecological function of landscape is also determined by how we manage other factors such as structural diversity and age class distributions. In his book “Bringing Nature Home” Doug Tallamy shows a picture of a bland, sprawling suburban landscape ( p. 24) and notes “this highly simplified community is made up of a few species of alien ornamental plants that provide neither food nor shelter for wildlife.” OK, I’ll buy that. But would the situation change if the blue grass was changed to a native grass kept mowed to 2” and the two widely spaced shade trees were changed to natives? Doubtful. The structural complexity; that is, the number and arrangement of grasses, annuals, shrubs, and trees, is likely a bigger driver of ecosystem function than whether the plants are native or exotic.
In his thoughtful comments on the blog post Vincent Vizachero sums up, “I stand by my view that the general heuristic of favoring native plants over alien plants is better than the alternative of not caring about origin at all.” I can buy that as well, but with the caveat that other factors are equal. The rub, of course, is that other factors are rarely equal. And I suppose this is where the pragmatic approach discussed by Davis et al. resonates with me. In my position I do a lot of programming on trees for urban and community forests. I go through a list of criteria to consider for tree selection. Here are some of the key factors I usually discuss:
Adaptation There is no argument that there are well-documented environmental, economic and social benefits to trees in urban and suburban areas. But in order to fulfill these roles trees must be able to survive where they are planted. This means being adapted to abiotic and biotic environmental conditions which are often adverse. In this region of the country there are some native trees that fit the ‘tough trees for tough places’ bill, such as swamp white oak, bur oak, and honey locust. Many other natives, especially understory species, are much more difficult to site.
This street planting in Lansing alternated green ash and Norway maple.
Available space This seems like a no-brainer, but it’s amazing how often this gets overlooked and we end up with too much tree and too little space. Again, we have some great small native trees; Carpinus, redbud, striped maple. But these can be limited in their site adaptability.
Diversity In Michigan some communities have lost 30% of their tree cover to the emerald ash borer. Have we learned our lesson about improving species diversity? Not really. But we need to keep trying. Exotic pests are here and here to stay. Does anyone believe that global trade will decrease in the near future? Does anyone believe that there will be quantum leap in our ability to detect and intercept hitch-hiking pests? In order to continue to accrue the benefits of urban and community forests we need to continue to diversify our portfolio; this includes a mix of natives and exotics. I doubt there will ever be sufficient data to prove one way other, but it seems reasonable to me that an urban and community forest balanced among 20-25 native and exotic species will be better able to withstand the slings and arrows of weather and pests better than one made up of 8-10 natives.
11 thoughts on “Does native matter?”
Great insights and observations! I love reading this blog!
As someone who works for a local garden center…’does native matter’? Absolutely not to the consumer! He/she wants…in this order….pretty, survivability (against all odds I might add). Native, exotic…what’s that? “I just want that plant with the pretty yellow flower.” That’s a reality we’ll always have with it, even though I personally an intensely interested in your debate and understand the nuances. I am reminded of my husband, who, as a financial consultant, sometimes gets caught up in the “shoulds” of the market. “The market should not be doing this” or “Gold should not be worth this much” “Should” implies a moral imperative, which is rarely a driving force of the market!
And if I may post yet a 3rd time…ironically, as soon as I left your website I went to my email from the Georgia Urban Forest Council, who is offering a class this summer, “Planting Native Trees and Battling Invasives” Sounds like I need to go to that one!
There are lots of folks that couldn’t disagree more. I’ve been working with local city forester who has been taken to task for having exotics on their recommended tree species list. People plant around their homes and communities, not national parks, this is where the battle is.
Speaking as a volunteer Master Gardener and a reasonably informed lay person charged with educating the public and other Master Gardeners about this topic, here are some thoughts and questions that have come up over the years:
What is the difference between “Invasive” and “Naturalized”? Examples: Queen Anne’s Lace, Daucus carota is mostly described as “naturalized”, but Dame’s Rocket, Hesperis matronalis is mostly “invasive?” And an old (1930 era) field book I have describes Purple Loosestrife, Lythrum salicaria as “naturalized” and quite beautiful. Does it make a difference if it’s growing in places away from waterways and wetlands? Obviously the definition evolves over time. But by what criteria? Is it the same criteria as Supreme Court defined pornography? “Invasiveness” is in the eye of the beholder and I know it when I see it? Local standards?
Why are the orange day lilies considered “Invasive”? Sure beats some of the other stuff that grows alongside the road. And is it their orangeness that’s a problem, or any color day lilies? Why or Why not?
How do you reconcile the recommended removal of invasive ground ivy in lawns, with recommended lower herbicide usage to the beleaguered suburban homeowner, trying to do the right thing? And if the number of bumblebees around its blossoms in the early spring is indicative of something, why isn’t that something a good thing? Have you never noticed the number of all sorts of bees and insects buzzing around the invasive privets when they flower? Or thistles, for that matter. Aren’t butterflies a good thing? Have you seen the numbers that flock around Buddleia, now on the invasive watch list?
If the behavior of a particular species mirrors its native cousin, why is it bad? E.g. Clematis ternifolia vs Clematis virginiana, or Aralia japonica vs Aralia spinosa? I’ve read about the concerns of hybridization, but why is that a concern? Is the issue strictly aesthetic in value? Whose aesthetics? How about Wineberry vs the native black raspberry or blackberry, all three of which are pretty aggressive, thriving, and co-existing, and not easy to control if you don’t want them, regardless of their native/non-native status. And, as I’ve noted before, wineberries have better flavor to my taste buds.
How do we integrate into the thought process the invasive worm stuff that has been discussed in this forum in the forests of previously glacierized areas of North America? If the introduced species are native ones from southern regions that weren’t subject to glaciation, would it matter? Don’t they have the same negative impact, regardless of their nativeness? How local does “native” have to be?
Has anybody read and then taken into consideration the work of Del Tredici and his book, “Wild Urban Plants of the Northeast” that renames “invasive species” “spontaneaous species”, and even has good things to say about Ailanthus under the right circumstances. More information here and here.
So, I appreciate the discussion, and recognize that there is not a black and white answer to these questions. I also welcome the Nature article, despite expressed critiques, since it makes it clear that the answers are not as pat as some of the environmental organizations and conservation programs would have us believe. Nor is the problem an unserious one, noting that Norway Maples, ironically introduced by botanical hero John Bartram, are one example of a suburban planting escaping and impacting native forest ecology, potentially quite heavily. I also agree with Bert’s comment above, though, having served on a committee making new zoning recommendations for new developments in a local municipality where the suggested wording from conservation groups insisted on native only plantings, with full code enforcement powers. Saner heads prevailed, but only because of the potential costs of enforcement, not on the merits.
How about some sort of useful categorization or scale of badness, with well defined, peer-reviewed within the discipline, criteria?
Example: EAB – really really bad, a ’10’ on the scale; Orange Day lilies, not so much, a ‘1’.
I’m sure I’m not alone in thinking that implementation of policy fixes to the problem, have gott
en ahead of the definition, scale, and impact of the problem, which I’m guessing is one of the reasons for the Nature article’s existence in the first place.
The pictures above show a landscape covered mostly by lawn and asphalt, so I’m wondering exactly how planting natives would really be significantly more helpful than anything else; the environment is already far from what it originally was in that place.
I had an insight into this whole debate this morning on a hike I took. Along the trail there was a spot covered in non-natural, invasive ivy that had run rampant after being planted nearby many years ago. Around here, people go out in parties to hand-pull and eradicate this invasive ivy. However, I also noticed that wherever the ivy was growing, the otherwise rampant poison oak was not. I don’t know the good or bad ecological affects of the ivy’s presence, but I know I hate getting poison oak!
Bert, what I meant is that a lot of what the discussion was about was what the criterion should be for mounting an eradication effort when native ecosystems seem threatened by any given non-native species. If it was only about whether we should remove all non-native species from our landscape choice list it wouldn’t be a very complicated discussion. I’d certainly side with you without hesitation, as I believe would any non-idealogue.
Bert, what you say seems to me quite sensible and straight forward but I don’t believe that urban and suburban plantings was what the debate was about. I was under the impression that it was about the management of wilder areas. Invasive species aren’t much of an issue in the tame and largely artificial areas you discussed here.
Ray brings up some of the questions that have troubled me most, since I’ve been a gardener for myself and others. One thing I would add is that, oftentimes in Atlanta, cultivars of a “native” species are given just as much importance as the species itself. I realize that Itea virginica grows wild along our local streams, and Itea ‘Henry’s Garnet’ might set viable seed in a garden, but it was chosen for its individuality 800 miles northeast of here.
The takeaway for me is that we need to be conscious of the meaning of what we plant, being neither a dogmatic “nativist” nor an unthinking consumer of petunia flats and sod. All of this earth is now under human influence–it is all, in effect, a “garden”. And as Bill Mollison says, “we are all gardeners.”
Regarding where the ‘battle’ against ‘native’ and ‘non-native’ takes place, there are no battle lines. The debate is not relegated to only urban landscapes, but all landscapes. Even before the current rapid and extreme climate changes, the humans species has changed habitats around the globe, influencing diversity, sustainability and survivability of all ecosystems. It is by the very nature (to be redundant) of evolution that species are on the move, some moving to extinction. Many species are establishing new habitats elsewhere. Some are filling in niches that others leave vacant. This has been going on for millions of years, on all landscapes around the planet. In the midst of this rapid change, we are concerned about changing habitats and ‘marching’ species. Perhaps we should be more concerned about human impact on altering the habitats that precede species mobility, and extinction.
Clearly, diversity, habitats and ecosystems are changing along with everything else. Rather than bicker about what is granted ‘permission’ by us to move and establish where, we might spend that effort on learning how to adapt to these changes, and acquire the scientific knowledge upon which to base decisions on what (and where) to preserve, and allow changes occur.
The term ‘invasive’ species is undergoing change, just like everything else around us. We need to be open to accepting some change, and know when to preserve the old.