Thursday, September 4, 2014

A Species Is a Species Is a Species (Except When It's a Cultivar)

Dr. Douglas Tallamy
Professor & Chair of Entomology and Wildlife Ecology at the University of Delaware
I recently had the privilege of speaking with one of my horticultural heroes - University of Delaware professor Dr. Douglas Tallamy - to discuss his ongoing research on cultivars conducted in partnership with the renowned Mt. Cuba Center.

In addition to his scientific research, Tallamy is a darn-good writer and author of the wonderful book Bringing Nature Home, which opened my eyes to the importance of biodiversity in our own backyards and showed us all of us can make a difference by supporting vibrant ecosystems in our gardens.

Tallamy is working with Mt. Cuba Center to research cultivars of native plants. What is a cultivar? I'll answer that question by quoting extensively from Mt. Cuba's Executive Director, Jeff Downing:

The newfound popularity of native plants seems like very good news for the environment. But it’s complicated. The thing is, the vast majority of native plants available in the trade are cultivars. A cultivar is a plant that has been selected for a particular attribute or combination of attributes that can be maintained through propagation. Cultivars are often selected (or created through hybridization) for characteristics like flower color, hardiness, disease and/or drought resistance, interesting foliage, or any other noteworthy feature. In order to perpetuate a distinguishing characteristic, most cultivars are propagated clonally. As a result, for many cultivars, every plant is genetically identical.

Sometimes funny things happen when plants are selected for particular attributes. When roses were bred for disease resistance and floral beauty, they lost their scent (which prompts the question: Would a rose, by any other name, smell like anything?). When the Red Delicious apple was selected for its attractive shape and color, we sacrificed flavor.

And that is where it gets interesting. Are native plant cultivars that have been selected for particular attributes as attractive to insects and pollinators as the naturally occurring species? As gardeners we hope so, since these ecological benefits are a big part of what we hope to gain by choosing native plants. But the reality is that we don’t know, because the ecological value of native plant cultivars hasn’t been widely studied. Until now.

Mt. Cuba Center has funded two graduate student fellowships to research the ecological value of native plant cultivars and Doug Tallamy is participating in part of that research. Along with Mt. Cuba Center Fellow Emily Baisden, he's looking at whether cultivars of native woody plants attract as many leaf-eating insects as naturally occurring species.

(But wait, I hear you cry, why on Earth would I want to attract leaf-eating insects? Well, I'll give you two reasons - some of those leaf-eating insects will turn into beautiful butterflies and moths. And some of them will provide food for birds.)

Here's a snapshot of my conversation with Dr. Tallamy:

Garden of Aaron (GOA): Can you tell me a little about your motivation for conducting this research?

Tallamy: Lots of people tell me that they would like to increase the percentage of native plants in their yard, but all they can find at nurseries are cultivars. So people ask me, "Are they as good as the species?"

The answer is that no one has compared straight species to cultivars in terms of their impact on pollinators or on caterpillars eating their leaves. If you are putting plants in your yard to encourage complex food webs, to support insects who will feed the birds, you want to know whether these plants will be as good in those ways. That's the motivation for the Mt. Cuba project. I've made some predictions for 5-7 years now about the impact of cultivars, but it will be nice to have some data to see what's really happening.

GOA: What are the parameters of your research?

Tallamy: It's impossible to test every cultivar, so we're looking at the typical types of genetic changes that create cultivars. Some cultivars take a green leaf and make it purple or variegated. Others take a fat plant and make it skinny or a tall plant and make it short.

Then there are lots of selections involving changes to the flowers, mostly changing the shape, the petal size, the colors and so forth. What will that do to pollinators?

Finally you have cultivars that are focused on disease resistance. If you important resistance into a a plant, does that also impact the insects that pollinate or feed on it?

GOA: Which plants are you investigating?

Tallamy: We are looking at five different cultivars of Cornus sericea [Red Osier Dogwood]. We are looking at flowering dogwood, blueberries, red cedar, red maple, sweetgum, stag-horn sumac, arrowwood, winterberry and a disease-resistant Princeton Elm.

GOA: How is the study structured?

Tallamy: All comparisons are within a species - that is, we are comparing the straight species to the cultivars. They're all growing at a common garden in Mt. Cuba Center with five plants of the parent species growing in a circle. Then we plant the cultivars in clumps around that circle. If an herbivore can find one plant, it should be able to find the others too. Essentially they're located in the same space and planted on the same day.

GOA: How did you choose which plants to include in the study?

Tallamy: We picked plants that we know have insects associated with them. So for instance, Itea [e.g., Itea virginica, Virginia sweetspire] does not have any caterpillar association. Elm trees are high in caterpillar associations. We are looking specifically for caterpillars and vacuuming for anything else. Actually, we're finding more insects than we had thought we would. We've conducted a couple of samplings so far, but it's still early in the study.

GOA: Do you care to make any predictions as to what the study will uncover? Do you have any hypotheses at this point?

Tallamy: If you make a green leaf purple, you're adding anthocyanins. These could affect feeding behavior.

Variegated leaves take away chlorophyll, so my prediction would be that those plants would support fewer lepidoptera [i.e., moths and butterflies].

In terms of changing a plant's habit - making it shorter or taller - unless that genetic change also alters leaf chemistry, I don't see why it should be a factor for herbivores, so I would predict no change in terms of the caterpillars it would attract.

Disease resistance could certainly affect herbivores. When a plant manufactures a chemical to protect itself from disease, there could be some crossover into deterring herbivores.

It's easy to predict how changing the shape or color of a flower might impact pollinators. Flower energy budgets are tight. When you make petals bigger, you're probably reducing nectar. Double flowers remove the flower's reproductive organs and turn them into petals [i.e., if the flower is sterile and does not produce pollen, there is no reason for pollinators to visit].

In terms of changing colors, I don't know how that will affect the UV spectrum [that many species of insects can visualize]. It could have an impact on pollinators.

GOA: Would it be possible for the horticultural industry to produce cultivars that might attract more pollinators than a straight species?

Tallamy: Sure, you could select for traits that enhance pollination or produce greater nectar load. It might not make the plant prettier, but you could advertise it by saying it would help gardeners to attract more butterflies. I'd say that it's almost certain it would sell for that reason alone.

GOA: What do you hope will be the impact of this research?

Tallamy: I hope that people to will start to think about selecting plants based on their function, not just on their aesthetics. If we can accomplish that goal, I'd be happy.

I'd also love to see traits moved along in ways that don't involve cloning. It would be good if we could have some kind of breeding program that would cross plants and still maintain desirable traits while preserving genetic variability. Red maples are naturally swamp plants that hate dry city climates, but if you're looking for a red maple that will do well under city conditions, you can go to the mountains of Pennsylvania and find selections that have already been made for you. There you'll find red maples growing that have been bred through natural selection to survive heat, drought, wind, cold and very little water. You could collect a number of those, interbreed them and produce plants that have the same survival traits without relying on cloning.

We need to do that kind of native plant exploration for harsh city conditions. It makes no sense at all to say that only plants from China will grow in urban conditions. You can look around city lots and see lots of native plants growing. Washington DC, for instance, is filled with oaks and elms, so it's just an urban legend that native North American plants won't grow in our cities.

GOA: To get back to a statement you made at the start of our interview, why is it so hard to find straight species plants instead of cultivars in the nursery trade?

Tallamy: It's a question of supply and demand. If nurseries thought they could sell the straight species, they would do it. But they have spent a century with the mindset that consumers will only buy plants with fancy names and that you have to introduce new plants every single year like the fashion industry.

Now there's growing consumer demand for natives that will do something in our yards instead of just looking nice. Of course, to find a plant that does both - that looks nice and supports the food web - would be the best option.

GOA: Thank you for your time, Dr. Tallamy. Good luck with your research!

Editorial note -- Dr. Tallamy has compiled a spreadsheet called "Host Plants" showing which tree genera host the most Lepidoptera species. According to the spreadsheet, Quercus (oaks) top the list, attracting more than 500 species of lepidoptera, most of them native. Presumably the spreadsheet is targeted for North American audiences when it characterizes genera and lepidoptera species as native or exotic.