[Written in response to reviews of Jennifer Owen, Wildlife of the Garden: A 30 Year Study, claiming that insects prefer exotic plants]
What Dr. Owens (or the reviewer) is claiming is contrary to scientific consensus, since Ehrlich and Raven first published in 1964. That’s not to say she is wrong, but it does require that she support her claim through rigorous, repeatable scientific inquiry.
I have not read her book, so I can only comment on the reviews that were sent to me. There are several things I would need to see before I took her results seriously.
First, plant/ insect interaction theory describes how insect herbivores should respond to host plants. In essence, the theory claims — and is supported by thousands of studies — that most insects (an estimated 90%) adapt to plant chemical defenses by developing specific physiological mechanisms, behaviors, and life histories that enable them to circumvent a particular type of defense.
Once they have developed this ability, they can eat any plant that uses that particular chemical as a defense. Typically, a plant lineage (genus, sometimes family) shares a common defense. This means the lineage of insect that adapted to that particular defense specializes on the plant lineage that makes that defense. It also means that that insect cannot eat plants it did not adapt to.
Note, we are talking about insects that eat plants. Not pollinators, and not predators. So the first thing I would look for in her study is the type of insect she was counting on her native and alien plants.
One review says she found 533 species of parasitoids. I am sure she collected these when they were seeking nectar at flowers. That is, they are not herbivores, so they don’t count if we are challenging accepted theory.
She also counted 23 species of butterflies. If they were counted as adults, they don’t count. Same with the 375 species of moths. Did she survey them as larvae when they were eating leaves, or as adults when they were sipping nectar? Bees and wasps are also pollinators at flowers (wasps are predators when not at flowers), while lacewings and ants are predators. All of those insects were mentioned in the reviews, but are simply seeking sugar water. Nearly all flowers, alien or native make nectar.
When we focus only on insect herbivores, it turns out that Dr. Owens’s results are actually not contrary to predictions. She found that 46 species of moths fed on 40 native plants in the garden, while 75 alien plants provided food for 38 species of moth. That is, if she had planted a garden of only the 40 native plants, she would have supported 46 species of moths. If she had planted only aliens, she would have supported 38 species of moths.
So the review’s claim that aliens support more insects is incorrect. Note that if these plant numbers represent the abundance of plants in her garden, she had twice as many species of aliens as natives. In a true comparison of insect preference, you would need the same biomass of both alien and native plants.
The real confusion comes from her study of the 4 species of generalist moths. Remember, if 90% of insect herbivores are specialists with a narrow host range, 10% are generalists with a broad host range. Here she found that the generalists preferred alien host plants.
John Parker has found similar results here in the US, so there does seem to be something going on between generalists and well-defended native hosts and this has stimulated new research as to why one out of ten insect herbivores can circumvent alien plant defenses better than native plant defenses.
But let’s be careful with our overall conclusions. Missing from Dr. Owens’s garden are most of the specialist herbivores. She recorded 46 species of specialist moths, but how many never showed up at all, because their host plant has been eliminated from surrounding suburban ecosystems?
One review says she found ¼ of the species in Britain. If she only recorded ¼ of the caterpillar species in her area, ¾ of the species never made it to her garden, and my guess is that most of those were specialists on native plants that have been lost to her suburban ecosystem because of the bias toward alien plants in suburban gardens.
Through no fault of her own, her study was filtered to include only those species that could survive in a sea of alien plants: the generalists. What is surprising is that so many specialists were able to locate their native hosts when she did supply them.
© 2011 – 2012, Doug Tallamy. All rights reserved. This article is the property of Native Plants and Wildlife Gardens. If you are reading this at another site, please report that to us
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This is great information. My goal is certainly not to have a garden full of insect generalists at the expense of my favorite butterfly specialists.
Carole Sevilla Brown recently posted..Make Every Day Earth Day
Even my adult insects prefer the nectar of native plants over aliens. Is this something that’s fact, or am I simply imagining it? Sure seems to be true for me.
Benjamin Vogt recently posted..Its Spring- Think Fall – Get These Native Plants
It’s also probably worth noting that Britain is a very different environment than North America, where a lot of us are gardening. It has comparatively few endemic plants, owing to all those pesky Ice Ages, many of which are hard to find, (the plants, not the Ice Ages) and it has been under intense human cultivation for a very very long time comparatively, so that, as an author writing about a similar study (in response to this very book, I believe!) in “No Nettles Required” pointed out, nearly everything that was going to go extinct because it couldn’t live in a human-altered environment already HAS. There may have been a great many more insects that hosted exclusively on natives, which have already passed this mortal coil owing to loss of their host plants—and the survivors of so much human cultivation are going to be more weighted towards generalist species that can survive on an intensely altered and cultivated area. (The author of “No Nettles” suggested that while it’s possible that Owens is indeed correct in her garden—they didn’t find a substantial weighting either way in their study—places that DO have a significant number of endemic species damn well oughta grow them and be proud they’ve got them, as their local fauna is unlikely to be terribly impressed by introduced roses. I approve of this attitude enormously.)
UrsulaV recently posted..Flowers That Count
Once again, Doug Tallamy has focused our attention on the details that matter. To care for the planet, we need to explore and try to understand the deep, often hidden complexities of nature, not just the few obvious bits that we big, galumphing, near-sighted and short-sighted humans can see at a glance. Butterflies and moths are pretty, yes, and many gardeners love to provide nectar to support them, but if we fail to provide food for their progeny, what’s the point?
Thanks Doug for answering some of these studies with fact and careful methodology…since planting more native plants I have seen a growth in the number of insects that frequent the garden as well as birds…
Donna recently posted..Earth Day
Until five or six years ago my garden was planted almost exclusively with alien plants. It wasn’t by design, I didn’t decide I was only going to plant something from Asia, Africa or Europe, I just really didn’t pay attention to the plant’s origin. I was more interested in the color or shape. Add to that, native plants were absent in most nurseries, so my chances of accidentally adding a native to my plant pallet was slim. I had a pretty garden but a dull garden. There were few birds and an occasional butterfly. I never observed any caterpillar except gypsy moths. Then, Holy Moly! I found out about the research entomologists had been doing on insect herbivores when I heard Doug Tallamy’s talk on the native plant, native insect connection. All of a sudden I had a good reason to pay attention to what I planted in my garden. Now my back yard is nearly 95% native plants, beautiful and never dull! I’m not a scientist but I can say by anecdotal notes, the populations of native butterflies and moths are on the rise as well as a wonderful diversity of birds.
Catherine Zimmerman recently posted..WE ARE GROWING!
It would be interesting to see if Dr. Tallamy knows of any scientific research that provides a comparison of the nutritive value of flower nectar found between native vs. non-native plants. In years past, when we used to have a butterfly bush in our yard (because we didn’t know any better), we observed that fritillaries and yellow swallowtails showed a distinct preference for purple coneflowers. Whenever the purple coneflowers were in bloom at the same time as the bush, the butterflies were almost exclusively on the coneflowers, rarely the bush. Same goes for my oak-leaf hydrangea and the patch of common milkweed. Obviously these are all species from different plant groups, and maybe it’s invalid to compare the nectar between different genera. But it does seem intriguing that nectar seekers seem to “know” which plants have a higher food value, and I’ve always wondered if it’s possible that native flowers have a higher sugar content/ caloric value than non-native ones. And, if the nectar of a native and a non-native plant were shown to have the same food value, would the nectar seekers still favor the native plant?
I am sure that not only your native butterflies and moths are on the rise, but native bees must be very grateful for your native plantings. Many bees may be generalists when it comes to nectar but their pollen needs are very specific in many cases. It is hard to keep count of all the complex interactions in nature. That is why it is better to resort to a species assemblage that has already co-evolved for many thousands or even millions of years than to grow a mish-mash of plants from all the corners of the world, no matter how pretty they look together.
Beatriz Moisset recently posted..Where have all the spring beauties gone
The evidence in our own gardens going through the same changes from traditional horticulture offerings to a more indigenous planting has seen such an increase in insects and other creatures.
Our gardens are not the only place this increase is evident. Walk the restored ripairian corridors or prairies in your area, speak with the likes of Stephen Packard. Ask the stewards of this movement to regain our wild natural spaces if you want evidence that native plants bring a greater biodiversity.
Gloria recently posted..Native insects native plants
Thanks for this. I appreciate Doug’s careful framing of the questions we ought to be asking when presented with some dubious research (often by folks with little or no scientific training). I find it interesting that some folks are so eager to grasp *any* argument against native plants. I truly don’t understand the depth of the passion… it’s not like we’re saying we will take away their hostas and daylilies and daffodils; we’d just like to add to the mix available in the gardening world, which means encouraging growers and propagators to offer plants they wouldn’t have grown a decade ago, so gardeners have more access. P.S. I’m not giving up my daffodils
Over the years doing green roofs it has become apparent to us at MetroVerde the majority of wildlife coming to the roof are first attracted to native species. Certainly they check out others but they always end up back at the native species.
Hummingbirds are a good representative example. They check out all the plants but quickly end up at our natives, such as Coral Honeysuckle.
Kevin Songer recently posted..Biodiversity and Florida Green Roofs- Urban Core Wildlife
Doug, thank you so, so much for providing us some logical assessment of this data. This is the first scientifically-minded reaction I have read in response to this book, and I’m only sorry that the book has been getting so much attention from people who are unprepared to rigorously discuss the claims. Your informative discussion here is incredibly helpful.
Genevieve recently posted..Book Excerpt- Energy-Wise Landscape Design
Doug, As an avid gardener and landscape designer who tries to use and promote native plants as much as possible, but someone who is not a science major, I found your post very logical, helpful and simple to understand. The distinction between insect herbivores vs. insect pollinators and how they each use plants is one of the finer points of the native plant issue that can get lost in the ‘science’. I think that the constant reinforcement of that distinction will help to crystalize the benefits of incorporating native plants into exisiting gardens for both casual and experienced gardeners alike.
Debbie Roberts recently posted..Spring in my Connecticut Garden
I am thankful for Doug’s insightful and reasoned analysis. It sure beats “this is patently ridiculous”, which is my response to this silly book. I really don’t understand this controversy. Isn’t obvious that nature comes in self-supporting ecosystems that evolved as a whole and function best together without invasive alien intruders? I am not giving up my daffodils either, but I am certainly not planting Japanese barberry to attract native insects!!!
Carolyn @ Carolyn’s Shade Gardens recently posted..Chanticleer- A Pleasure Garden
In regard to “…why one out of ten insect herbivores can circumvent alien plant defenses better than native plant defenses.”
Some studies indicate that they do not fare “better” at non-native plants than native ones, but rather what is taking place is that as generalists who are able to partake from the aliens, they often seek them out because of the simple lack of competition. The native plants are preferred by a majority of native insects, so the competition is more fierce in that arena.