What Do Dandelions, Earthworms & Honeybees All Have In Common?

January-February 2011

THE DEBATE BETWEEN THE USE of native and non-native plants in the landscape has been a pretty hot topic in Canada and the United States. No doubt you’ve been having your own discussions with clients and colleagues.

I remember when the word “exotic” used to mean something new and exciting. Nowadays there are a lot of negative feelings associated with non-native plant material. Of course it’s because we’ve had too many bad experiences with exotic invasive plants. Purple loosestrife (Lythrum salicaria) and common reed (Phragmites australis subsp. australis) are some prime examples of plants that were accidentally introduced and are choking out all other species in wetland habitats, changing them dramatically and the organisms they are able to support. Garlic mustard (Alliaria petiolata) was introduced as a culinary herb in the late 19th century and now dominates shady ecosystems to the point where virtually no other herbaceous plant can complete its life cycle.

Ecosystem Dynamics
To really understand the impact of non-native plants on our ecosystem, one must have a thorough knowledge of plant identification, biology and reproduction, ecology, climate, soil science and well – basically the environment as a whole. Do we really know what we are talking about? Do we have all the facts? Many of us are concerned that so many non-native plants are established in Canada and we think someone needs to put a stop to it. The fact is, most of the exotic plant introductions have been made because of human activities, largely through immigration and trade. Some of these introductions were carried out by accident but some were deliberate. We want to be able to travel and trade freely across the world but we aren’t willing to accept the consequences of our actions.

Every ecosystem is elastic, every action has a reaction. It’s the survival of the fittest and the competition never ends. Where a habitat changes, some organisms will die off or relocate. “Nature abhors a vacuum” and so other organisms will move in to take the place of the ones who left or didn’t make it. Non-native organisms have the advantage of being introduced without any of the predators and parasites that keep them in check in their native land. If they happen to end up in an exotic ecosystem that supports their needs, they can be outrageously successful.

Example #1: The Earthworm
Some of us are adamant about planting only natives, yet we feel really bad when we accidentally kill an earthworm. As gardeners we are taught that earthworms are a sign of a healthy, active soil. They help break down organic matter (e.g. dead leaves) and recycle nutrients; they also improve soil aeration. This can be a great service if you have a heavy, compacted urban soil. Of the various species of earthworms found in southern Ontario do you know how many are native? None. North American earthworms were wiped out during the last glacier event.

The Great Lakes Region evolved without earthworms, they were introduced during the last few centuries via European settlers. Earthworms are some of the most successful organisms we know. They can move and colonize a field at the rate of just over 1.5 metres a year. They are so successful that the once permanent layer of leaf litter (a.k.a. duff) found on the forest floor in southern Ontario barely lasts a year without becoming worm food. The loss of the duff layer has many researchers concerned over the forest’s ability to buffer against extremes such as temperature and drought.

Example #2: Honeybees
As Canadians we have a huge and healthy respect for bees. Every pesticide that is registered in this country needs to demonstrate its toxicity to honeybees, which helps to determine their risk to the environment. We devote significant research dollars on apiculture and for good reason; honeybees are significant pollinators in the plant world. But was it always that way? No. Honeybees (Apis mellifera) are native to Europe, Asia and Africa and were introduced to North America in colonial times. No doubt they must have displaced the role of other, pre-existing pollinators (e.g. flies) but our civilization has developed right along with them and so we don’t know anything different. Several species of flies are significant pollinators in the plant world, so how come we don’t feel guilty about killing a fly?

The Soil Problem
Most of us have a real passion for trying to conserve and support the native species in our landscape. There is nothing quite as fulfilling as seeing a newly planted landscape of native trees, shrubs and perennials. But native plants grow in native soils, many of which require a humus topsoil to some adequate depth. Of course many of you know that most of the soils in residential and commercial landscapes are often based on compacted subsoil with a top dressing of stockpiled topsoil. Cityscapes are some of the most unnatural environments in the world. Yet we expect plants to survive in these sites and sometimes get upset when some of them survive too well.

There aren’t a lot of native plants that can grow and survive in these soil conditions and much of the plants that are surviving are not native, especially the trees (e.g. Ailanthus altissima, Tree of Heaven). Does it make sense to plant only native plant species when the outlook for their survival is so poor? Can we not consider some non-native plants for these tough sites where natives won’t survive?

Are They Really So Bad?
Non-native plant species are believed to contribute very little to our native ecosystem. Plants play many important roles, two of which are food and shelter. Norway maples, an introduced species, produces seed that is not a desirable food source for herbivores. But the trees themselves will often grow in poor, compacted, dry urban sites when little else will. The trees provide shelter (nesting sites) for wildlife while using atmospheric CO2, shade the earth’s surface against extreme temperatures and filter the air.

How about the lowly, introduced, invasive dandelion (Taraxacum officinale). We try so hard to exterminate them from our little lawns. What could they possibly contribute to our ecosystem? Next time they are in bloom, sit down near a patch of dandelions and watch how many insects visit the flowers. Dandelions are actually a huge pollen and nectar source for several species of pollinators in spring, including bees and butterflies. The leaves are edible and may provide a significant nutrient and medicinal source to animals.

Is The Glass Half Empty or Half Full?
A colleague of mine was telling me recently of how she and her husband were standing over a research plot observing the results of a solarization experiment. Plastic was laid tightly over a diverse group of weeds growing in soil. After six weeks, the only species left standing was the dandelion. The researcher remarked, “I can’t believe we couldn’t kill it.” But her husband was quite impressed and commented: “Wow, what an incredible plant! We need to harness those genes!”

Contact Information

This column is written by Jen Llewellyn, Ontario Ministry of Agriculture and Food and Rural Affairs (OMAFRA) Nursery Crops Specialist

Email any questions you have directly to Jen and we'll publish her response.

P: 519-824-4120 ext. 52671 • F: 519-767-0755

OMAF website: www.ontario.ca/crops

Nursery Landscape Agriphone: 1-888-290-4441

Our mission is to enhance and promote the care and benefit of trees for present and future generations in Ontario through education, research and awareness.