Sunday, November 29, 2009

Does growing invasive plants help birds?

This past week I was very excited to pick up a beautiful book, Bird by Bird Gardening by Sally Roth (Rodale, 2006), about designing gardens to attract various families of birds to the garden. And in many ways it is an excellent book, with a wealth of information about the needs of different birds in terms of food, shelter, water, etc., with lists of plants and garden plans for all sorts of lovely birds (she's an American writing for American gardeners but of course a lot of it will apply here in Canada too). It's a beautiful book, lavishly illustrated with full colour photographs, and a wealth of information.

As I read the book more closely, however, I became troubled, as she recommended plants which may be invasive in parts of the US (and Canada too, though I can't fault her for that if she wasn't intending to cover Canada in the book). For example, she suggests

  • Berberis spp. (barberries, berbéris) without distinguishing between the native and invasive varieties (B. thunbergii is invasive in the northeastern US, B. vulgaris invasive in many states from coast to coast; they both are listed in the Invasive Exotic Species Ranking for Southern Ontario (PDF))
  • Eucalyptus spp. (Australian natives which are invasive in California, especially, but also in the Pacific north west including part of BC)
  • Euonymus alatus (winged euonymus, fusain ailé) which is invasive in the northeastern US (and southern Ontario)
  • Pyrus calleryana (Callery pear, poirier de Chine) which is invasive in the eastern US

(These are just the most notorious invasives I noticed flipping through the book; I didn't do an exhaustive investigation.)

Now, there are a lot of older wildlife gardening books that recommend various invasives, especially berry plants, but I was very unhappy to see them still being recommended in such a recent book.

I was horrified when I got to a three page essay, "Invasive Implications", on pp. 74-76.

First, she implies that the term "invasive" was created by the USDA and is based on agricultural concerns, and that all non-native plants are invasive. Now, even if invasive plants were only a problem for agriculture, that is still an extremely serious problem since virtually all people in North America are completely dependent on agriculture for our food. To me that would be reason enough to avoid gardening with invasives.

Of course, in reality, there are many ecologists concerned about the impact of invasive species on ecosystems. Invasives are definitely not only a concern for farmers.

Then, she says that invasives mostly affect disturbed areas, and so are not a problem for native plants. Even if invasives only affected disturbed areas, to me that is still a problem, for they are displacing the native plants which would normally colonize disturbed areas. Invasive plants can prevent native plants from returning to disturbed areas, so this habitat is permanently rather than temporarily lost.

(And unfortunately, there are invasive species which invade undisturbed habitat; witness the heartbreaking blanket of garlic mustard (Alliaria petiolata, herbe à l'ail) carpeting the forests in High Park and around the Ontario Science Centre, for example.)

She suggests that invasives are mainly a concern in warm areas (southeast US, California, and Hawaii) or near wetlands. In reality, while invasives may be worse in those areas, that doesn't mean they aren't a serious problem here.

Then she points out that sometimes authorities use environmentally questionable practices (herbicides). There is real debate on if/when pesticides should be used to control invasives, but surely if we don't want the authorities to feel driven to use pesticides we should avoid growing plants that are invasive in the first place! She also describes an instance she witnessed of a birds' nest being destroyed when the plant they were living in was removed. Again, all the more reason to prevent these situations by not growing invasives in our gardens.

Most upsettingly, she writes that since birds use some invasives for shelter or food:

I stick up for many invasives [because] we've destroyed so much bird habitat, so many native plants, and such multitudes of insects, that I believe birds deserve some extra help.


The sad truth is, these invasives she's defending are a leading cause of habitat loss. In particular, the multitudes of insects she mourns depend on a multitude of native plants for food. Most herbivorous insects can only eat certain species; even a closely related non-native plant often supports many fewer insects (in both number of species and number of individuals) compared to its native counterpart. Birds can seek cover and build nests in many locations, and fruit and nectar of many plants are palatable, but without a healthy insect population most birds won't be able to get the nourishment they need.

Invasive plants don't help birds, they harm birds.

If you want to grow plants to help birds, there are many native plants (many listed in Bird by Bird Gardening, and Bringing Nature Home by Doug Tallamy) which provide shelter, berries, and the insects which are such a crucial part of many birds' diets.

For more information about the impact of native vs. non-native plants on wildlife, please read Bringing Nature Home.


  1. Rosemary, An example of invasives displacing native forage is my dreaded dog-strangling vine. As it's a cousin of milkweed, it confuses Monarch butterflies, who lay eggs on leaves that will prove to be inedible by the caterpillars. Huge fields of it are now growing in the Don Valley. Reproducing three ways (seeds, proliferation on a single node and massive underground systems of runners), it's almost impossible to eradicate. Frightening!

  2. PS I'm going to link to this article in next week's best of the web.

  3. Rosemary,
    I, too, am very troubled by this book, which is why I've never recommended it to anyone from my blog. It's a shame that so many books for attracting birds continue to recommend invasive plants with no thought of their long-term impact on native ecosystems.

    I'm thrilled that people like you are being such great advocates for using native plants to attract wildlife.

    We must all learn to be skeptical of the plants recommended by the "gurus", and learn to do our homework before we plant. Look a plant up to see what its range is and determine if its appropriate for your garden before ever setting trowel to soil.


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