Keystone plants for the biodiverse garden

The extreme decline in insect (and therefore bird) populations has us gardeners facing a paradigm shift. We understand increasingly the positive impact we can, as individuals, make on local food webs.  We can exponentially increase insect populations by removing invasive species, planting natives, and adopting thoughtful practices that create a healthy habitat for wildlife on our urban properties.  

Now for the paradigm shift- planting to increase the number of plant-eating insects in my garden. I no longer purchase the pest-resistant varieties from the garden centre (many being aliens from other countries). Instead, I want the larval host plants for our native moths and butterflies- and I want to include some of the ‘keystone plants’, those that feed the greatest number (90%) of caterpillars. 

You can have dozens of native plants, but without any keystone plants, your yard will not provide the insect abundance necessary to sustain viable food webs

Douglas Tallamy, The Nature of Oaks, 2021

This first group of keystone plants consists of natives which have co-evolved with the insects that eat them, and this group belongs to just a few plant genera which provide approximately 75% of the insect food birds and other animals require. (D. Tallamy, The Nature of Oaks). To decide which of these are best for our gardens, refer to Tallamy’s ranking list of woody plants supporting the greatest number of lepidoptera species, as well as his appendix of native plants by region ( the Mid-Atlantic and middle states being the closest to South Eastern Ontario) with wildlife value, and his appendix of the host plants of butterflies and showy moths (Bringing Nature Home, 2007).

Brown-hooded owlet caterpillar on Symphyotrichum. Image: Nancy Shepherd

The second group of keystone plants are for our native specialist bees (approx. 25% of our native bees). These specialize only on pollen from plants in a specific genus, a few closely related genera, or in a single species, and require this pollen to feed their young. (Bees do not specialize on nectar). Some keystone plants are especially valuable as they provide pollen for specialist bees, and are larval host plants for a variety of lepidoptera. As with group 1, group 2’s keystone plants are native to the area, having co-evolved with the specialist bees that depend on them for the survival of their species. 

So back to that paradigm shift. I want to purchase the plants with the greatest positive impact on local lepidoptera and specialist bees. So, rather than choosing plants by hardiness zone, I need to choose by ecoregion, and I can find a good list to start from in the following National Wildlife Federation’s online resource: 

The National Wildlife Federation, Keystone Plants by Ecoregion

At the top of the list for woody species are the oaks, followed by members of the Prunus family (our native Black Cherry, Chokecherry, Canada Plum being good examples), the Willow family (which also supports 14 species of native bees), and fourth, our native Birches. These trees support hundreds of species of caterpillars (oaks support approx. 450 species). Oaks have other advantages for the climate resilient garden: they are the best watershed managers and the best carbon sequesters. (D. Tallamy, The Nature of Oaks). If you’re looking for a small tree or shrub and have the right location, try an Alternate-leaf dogwood, or a dogwood shrub like the Red osier dogwood. The dogwoods support approximately 115 species of caterpillars, and 4 species of specialist bees.

Image: Nancy Shepherd

Our native wildflowers are also important contributors to a biodiverse ecosystem, The goldenrod genus supports over 100 species of caterpillars and approx. 42 species of specialist bees. Members of the Symphyotrichum genus support approx. 100 lepidoptera species, and 33 bee specialists. Other highly rated genera include Helianthus, Rudbeckia, and our native roses.

While the above resources are really helpful with plant choice in the Year of Biodiversity, we mustn’t forget the importance of other native species. For example, while milkweeds support only 13 species of caterpillar, they are the only larval host plant of the Monarch butterfly and the Milkweed Tussock moth. 

Additional resources if you’re interested in specialist bees: