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Native plants are not always best for native insects

Wildlife of a garden,Jennifer Owen. Image ©RHS Native plants are not always best for native insects, non-natives also have an important role. That’s the message following a thirty year study in a British garden. For thirty years award-winning ecologist Dr Jennifer Owen studied, identified – and counted! – the insects and other creatures that visited her suburban garden in the English Midlands. And detailed it all in her invaluable book Wildlife of A Garden (Available in North America, and available in Britain) published by the Royal Horticultural Society.

She grew well over 400 different plant species - garden plants and weeds, natives and non-natives - in her garden (below, click to enlarge) which measures just 741square meters (8000 square feet). And she counted 23 species of butterflies, 375 species of moths, 94 species of hoverflies, 121 species of bees and wasps, 305 species of bugs, sawflies, lacewings and related creatures; 21 species of beetles, 122 species of other insects including two ants – all in her suburban garden. And 138 other invertebrates. And 57 birds and six mammals. Wildlife of a garden,Jennifer Owen,garden,Leicester. Image ©RHS

And as well as counting with extraordinary determination and great skill at identifying this vast variety of creatures – she also studied their food plants. And what did she find?

Dr Owens found that non-native species were better as food plants for moth larvae than native species. Moth larvae used 27% of the native species in the garden as food plants, and 35% of the alien plants. And 46 species of moth fed on 40 native plants in the garden, while 75 alien plants provided food for 38 species of moth.

She also looked at the four moth species with the most varied diets and the plants they ate. One ate 78% non-native plants, one ate 83% non-native plants, one 62% and one 79% non-native plants. They definitely didn’t favor natives. Of course, we don’t know how much of each plant each moth actually ate – after all, Dr Owen needs to get a few hours sleep each night.

And finally, what were the most popular food plants for moths? Plants in the rose family come out top, with seven native species and five non-native species used by 27 species of moth. And one of those native species, Potentilla fruticosa, is so rare in Britain it might almost be non-native. Next comes the Buddleja family represented only by the Chinese Buddleja davidii and used by 19 species of moth. The daisy family, the largest family of garden plants, hosts just 13 moth species all of which feed on aliens and only two of which feed on natives. And just to be clear: these are all British native moth species.

Wildlife of a garden,Jennifer Owen,tortoiseshell,aster. Image ©RHS You get the picture. I could go on, but this is just a blog post of a few hundred words. Read the book. OK then, one more thing. Dr Owen reports that of the 15 most widely used food plants in the garden nine are non-natives and only six are native. And some people think that introduced plants have no native larvae feeding on them at all!

This really throws the “natives are best” notion out of the window. We may like to think that natives are best, but they’re just not. And can’t we trust the insects know what they like to eat – wherever the plants come from? So why don’t we plant what the larvae actually like to eat, instead of what we think they ought to like?

This is an extraordinary piece of research summarized in a very readable and well illustrated book.

In my next post here, I’ll be looking at which buddlejas are best for adult butterflies. Because now we know.

           

 

* This post was originally headlined: "Alien plants are better for insects than natives – it’s official!" But after reflecting on the comments below, I modified it and substituted a less sensationalist headline. I also modified the introduction.

* At present, for some reason, amazon.com and amazon.co.uk don't seem to realize they have the books in their own warehouses and are not listing them as being available. They are available for shipping anywhere in the world from The Royal Horticultural Society.

Comments

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Victoria

I find the alien plant puritans a little tedious. I think they're missing the point - surely the best policy is to grow things responsibly and sustainably, wherever they come from.
I wonder what would happen if someone suggested humans only ate native plants? No more grapefruit, avocado, banana, oranges, potatoes, tomatoes, chocolate, coffee, etc etc. (Well, here in the UK anyway.)

Graham Rice

Absolutely, Victoria - and what is native, anyway? In one California county they're ripping out the newly arrived yellow tree lupin because it's spread from - the county next door! Completely mad.

Jake Williams

Well, that really says something to all those nutters who believe that we should only grow what God ordained to grow in our area as the plants and insects and birds that He put there all formed part of a perfect relationship. If the moth larvae all prefer buddleias from thousand of miles way I'd say He got it wrong.

Susan Harris

Wow, you're so brave!
And btw, I love the photo of you on the Rant, accompanying your interview.
And another btw - my blog has THREE cats!

Susan Harris

And my final "btw" is that that last comment was from the gardenblogger Susan Harris, now venturing out off-topic, as you can see from the link I'm leaving here.
Lemme know if you speak in or near DC anytime soon!

Graham Rice

Brave, Susan?! Just telling it like it is. I'd love to hear of research like this from the US - whatever the result. There's too much opinion classed as fact, and not enough fact.

You like the mugshot! It'll be on my next book jacket. [And I like the shot of you up a tree at http://www.boomerturnons.com]

Actually we have three cats but only two on the blog - the other, Garbo, doesn't like having her picture taken, she just wants to be alone...

Rareplants

Grow a lot of bamboo in the garden and found hordes of ladybirds hibernating in culm sheaths on them hopefully a lot less aphids this year.

Graham Rice

So non-native bamboos can be good for hibernating native ladybirds. Of course, thanks rareplants (http://www.swinesmeadowfarmnursery.co.uk/).

And Linda Beutler (@oregonclematis) tweets to point out how quickly native American hummingbirds exploit introduced plants in North American gardens.

Bill McLure

So it seems to me that those "natives or nothing" people who insist we all plant only not very colorful local wildflowers in our gardens should be cringing in shame.

I'm going to plant more buddleias and I bet that if some tenured professors bothered to actually do some proper research, as Dr Jennifer Owens has done, instead of spouting their own opinion, then perhaps even ivy would turn out to be ecologically useful in countries where it's not native. It's certainly invaluable as a food plant in areas where it grows naturally as it flowers at a season when so few other plants are in bloom. The problem is that the invasive plant nuts rip it out before its full ecological relevance can be carefully studied.

Emma

I have a lovely set of photographs of ladybirds hibernating in monkey puzzle trees at my local arboretum - another non-native, of course.

I do worry whether that people will be put off from planting Buddleja now that Plantlife have published concerns about it becoming invasive.

Sylvan Kaufman

I hope that before recommending Buddlejas for the United States, you will consider reading this article: Tallamy et al. 2010. Can Alien Plants Support Generalist Insect Herbivores? Biological Invasions 12:2285-2292. You can find a pdf of the article online. The article finds that several generalist North American lepidoptera larvae perform extremely poorly on Buddleja and other non-native plant species.

Graham Rice

Yes, Emma, Plantlife is expressing caution about Buddleja in Britain. But first of all, let's not lump all buddeljas together. They're mainly referring to B. davidii, B. globosa, for example, is still listed in the New Atlas of the British Flora (2002) as "a rare escape" although Plantlife ranks it higher as problem. Also, several new interspecic hybrid buddlejas bred in the USA for gardens are completely sterile so cannot invade anywhere.

Buddleja davidii, by the way, was first introduced into Britain from China in the 1890s, and first found outside gardens in Wales in 1922. It's now found over most of England and Wales, sometimes in manmade habitats where nothing else at all will grow.

And Sylvan, probably all recommendations of plants to grow, or not to grow, "for the United States" are flawed. More precise advice is almost always necessary. In my garden in north east PA, B. davidii struggles to get through the winter without being killed and has never seeded. In colder zones, it never survives. So while in some areas I would certainly not recommend planting B. davidii, in others it's a valuable (and safe) garden plant.

I will look over the Tallamy paper you mention carefully. However, at first glance, it seems that the aim of the study is stated as being to determine whether the four native lepidoptera species chosen could have a role "in slowing the growth and reproduction of alien plants". It was not, it seems, in spite of the study's title, to compare feeding success on natives and non-natives. If that had been the aim, the study would have been set up in a different way.

Alice

I thought the argument for natives was not that insects liked them better but that native plants plus native insects creates a balance so that both can thrive. Maybe the moths are actually over-populating by eating the non-natives. Maybe not, but the question needs to be addressed.

kathi mestayer

If you want to read the case for natives in (habitat) gardens, it's all laid out in "Bringing Nature Home" by Doug Tallamy, Chair of the Dept of Entomology and Wildlife Ecology at the U. of Delaware. The data are overwhelming; the case is open-and-shut.

Graham Rice

Yes, Kathi, I'm aware of Bringing Nature Home although I don't have it to hand at the moment as unfortunately I left my copy in England - the hazards of the Transatlantic life!

Graham Rice

A correspondent to a native plant mailing list, commenting on my above blog post, said: "Oh, this is fabulous. Let's all go out and plant a bunch of non-native invasive plants on the basis of one person's observations in her yard. Yeah, that sounds scientific. This guy is no better than a climate change denier."

"no better than a climate change denier"!! Never been called that before, that really is an insult.

[As the comment was not made here, I'll not give the name.]

Tracey Gelner

I think the point of native plant proponants is not that generalist lepodoptera can't utilize non-native species, but that many lepidoptera are species specific such as the Monarach, swallow tail, baltimore checkerspot, etc. It is these species we are concerned about going extinct as their host plants become rare. There is already plenty of the non-native plants growing in peoples gardens. The native plants are disapprearing, and species specific lepidoptera with them. Any encouragement to plant non-native plant species just puts these butterfly and moth species further from a population recovery as the non-native species take over valuable habitat. But hey- if you want to encourage practices that will ultimately lead to the extinction of nurmerous species, its a free country.

Graham Rice

I agree entirely, Tracey. Specialist lepidoptera require specific food plants and planting such plants in gardens is a valuable way to provide more host plants and support struggling populations.

But to be against "any encouragement to plant non-native species" is to be against most of the country's horticultural industry and against what most people do in their gardens. This is rather sweeping. It's long been recognized that gardens, where there is of course a preponderance of non-native plants, are invaluable havens for wildlife of all kinds - especially in suburbs and in areas of industrial agriculture. But to insist that nothing but natives is, frankly, rather astonishing.

Of course I'm not suggesting planting non-natives in the wild, few people do, although this is still done as cover and food for game - on both sides of the Atlantic. But I am suggesting that the sweeping generalization that non-natives are of no value to insects is dramatically misguided and that Dr Owen has proved as much by her exhaustive research.

Vincent Vizachero

Graham,

I'm sure that many of your readers are inclined to believe what you write merely because you write it.

I think I can speak for those of us who are more interested in truth than propaganda would appreciate it if you contained yourself to writing only things that are as they are instead of writing as if they were as you wish they were.

"Alien plants are better for insects than natives" is deliberately misleading, and you should be well educated enough to know it.

Dr. Owen's research (if you can call it that) might be better summarized as "non-native plants not completely useless to insects".

That's a headline we could all get behind!

Graham Rice

Well, Vincent, "Alien plants are better for insects than natives" is not at all misleading insofar as it relates to the moths on which which Dr Owen reports. And I doubt if anyone, anywhere, has counted and identified so many invertebrates in one location over such an extraordinarily long period as Dr Owen has done. To me, that counts as invaluable research.

It is, of course, not a universal truth that "Alien plants are better for insects than natives" in the sense that not all non-natives are better for all insects than all natives. Far from it. But it's certainly seems to be true that non-natives are better in some cases - and the phrase has also provoked a valuable and lively discussion. [Two comments, I should say, I have removed as their language would be likely to offend most readers.]

The phrase not only reflects an aspect of Dr Owen's work but caught the attention of a record number of readers who were all made to think afresh about the issue. It also prompted many interesting and thoughtful responses (plus the crude and offensive ones).

Had the piece been headlined "non-native plants not completely useless to insects" - no one would have read it or discussed the issue.

Vincent Vizachero

Graham,

I'm holding off comment on Dr. Owen's work itself until I have read the book. It's not been quite as easy to get in the U.S. as I'd hoped. I'm sure this comment thread will be completely dead by the time I receive and read the book, so it may well be that we never have a chance to have a substantive discussion about the data in the book (and the implications thereof).

However, people (even smart people) are prone to all sorts of cognitive errors that can adversely impact study design. The tone of your review suggests that Owen has found something to be true which decades of research by hundreds of competent biologists have found to be untrue, and that leads me to be extremely doubtful of her (alleged) findings. And curious about your choices in reviewing those findings.

If your primary interest is in generating website traffic, selling books, or defending the horticultural industry then I agree that the sensational headline may be more appealing than the accurate one.

And I am grateful to you for calling this book to my attention.

But the fact remains that the horticultural industry in the U.S. has worked very hard for a long time to promote plants with no regard whatsoever to their ecological value. That same industry has a powerful motive (profit) to ensure that every piece of information that can possibly be twisted into making natives look pointless or even bad is used thusly.

So I hope you will forgive me for being suspicious that Owen's work was published in a book by the RHS instead of (for example) a peer-reviewed journal of ecology. As I said, the final arbiter in my mind will be her methods and data. I hope the book arrives soon.

Genevieve

I think Tracey and Vincent have said it right. I'm by no means suggesting that non-native plants are evil; but perhaps you and the author of this book could take a look at the actual facts behind the drive to plant natives (as Tracey said, it's not the generalists we're worrying about) and not just set up straw men to tackle.

I really don't like the tone of this post, Graham, much as I respect you and your work - I think that you have missed the mark on this one. We all want validation that what we want to do is the right thing to do; but unfortunately even if you can find one person (of dubious credentials) to back up your case, it doesn't make it correct. I'm disappointed to see this.

Graham Rice

Thanks Vincent. I've thought about this and I'd say my primary aims here are to bring people information of which they might be unaware, to make them think, and to entertain. So over the years since I started this blog I've often tried to suggest an alternative to conventional wisdom when something valid turns up to challenge it.

So when I come across something that challenges the sweeping assumptions at the extreme end of the native plant movement, then it seems like a good thing to write about and against which to set an alternative view. And the more people who come here, and read what I and my readers have to say, the more I hope they will think about the issue and reflect on the fact that it's not as simple and clear cut as it's made to appear.

So, for example, it's good to point out to people who universally decry buddlejas and say that none should ever be planted that not only are they exceptionally good for lepidoptera (adults and larvae) - but, I might add, that there are actually six US native species, one of which is called Buddleja utahensis, which may well have insects which depend on them.

Graham Rice

I'm sorry you're disappointed, Genenvieve, but as I've said this looks to me to be a notable piece of research based on long and thorough observation in the field. And all the more important because it challenges the conventional wisdom.

And I have to say that I would wholeheartedly support energetic promotion of the planting in gardens of specific food plants upon which individual lepidoptera species depend. [As mentioned above.] And also, perhaps, of the creation of garden varieties of these plants with more ornamental appeal.

Even more important, perhaps, is planting them along freeways where the mania for mowing regularly seems utterly ludicrous. In such situations the local native flora should instead be encouraged, perhaps with intervention planting of specific local species on which insects and other creatures depend.

Genevieve

Graham, I appreciate the more nuanced views you're sharing here, and I think we'd likely find much to agree on within the topic.

I still do feel, though, that your headline yielded to a sensationalist urge and did not effectively capture the nuance of those beliefs.

In fact, I'd go so far as to guess that when a famed horticulturist such as yourself proclaims that "aliens are better", most people don't even read the whole article, much less the comments. They just walk away thinking that there is no appropriate place for planting native plants, and little reason to bother with them. That is a destructive idea to spread, even if it's done unintentionally.

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