The caterpillar test — why a Toronto oak is worth a hundred ginkgos

An oak tree, in eastern North America, is the host plant for the larvae of 534 species of butterflies and moths. A ginkgo tree, on the same continent, is the host plant for three. Both trees grow well on Toronto's streets. Both produce shade, sequester carbon, slow stormwater, and look beautiful in fall. But the ecological work they do — feeding the city's actual living animals — is wildly, almost incomparably different.

This is the framework Doug Tallamy, an entomologist at the University of Delaware, has spent the last twenty years quantifying. The argument runs roughly like this. Insects, especially Lepidoptera (butterflies and moths), are the primary food of most North American songbirds and a substantial fraction of mammals, fish, and other birds. Insect populations depend on host plants — typically the genus or species the larvae evolved to eat. Native plants support far more native insect species than introduced plants do, because the insects haven't had the evolutionary time to adapt to chemistry from another continent. Therefore: the native-plant cover in a landscape sets, by a fairly direct chain of inference, the carrying capacity of that landscape for songbirds.

If you want to know how many caterpillars a tree feeds — and by extension, roughly, how many baby chickadees its block can support — you look at the genus and the species' nativeness. There is a lot of variation, but the differences are not subtle.

Toronto's street trees, ranked by what they feed

Here is Toronto's planting palette, by genus, with the typical number of native Lepidoptera species each genus supports in eastern North America. Counts are from Tallamy's research and the National Wildlife Federation's host-plant database; they are means, with substantial regional variation, but the order is robust.

You can see the gap immediately. The native oak — already a focus of Toronto's contemporary planting program — does between 100 and 200 times more ecological feeding work than the engineered, cosmopolitan, low-maintenance trees the city planted heavily through the late 20th century.

The Norway-maple wrinkle

The single most important caveat in the table above is that Acer as a genus is a North American native — but Toronto's most-common Acer is Acer platanoides, the European Norway maple, which is functionally non-native and supports a small fraction of the insects native maples do.

The 297 figure for Acer applies to native sugar maple, red maple, silver maple, Manitoba maple. Norway maple supports closer to 5–10 species. The 99,931 Norway maples on Toronto's streets — 14.5% of the entire street-tree canopy — are, ecologically, much closer to the ginkgo end of the table than the maple end. They look like maples; they aren't acting like them.

Apply the same wrinkle to Tilia: the European littleleaf linden the city plants supports far fewer insects than native American basswood. To Fagus: same with European beech versus the rare American beech. The genus-level numbers are upper bounds; the actual figures depend heavily on whether the city planted the Ontario-native or the European cultivar.

Why this matters in birds

Tallamy's most-cited number is the chickadee figure. A pair of breeding black-capped chickadees, raising one normal brood of four to six young, needs to deliver between 6,000 and 9,000 caterpillars to the nest in the two-and-a-half weeks before the young fledge. The parents collect those caterpillars overwhelmingly from trees within 50 metres of the nest. If the trees within that 50 m do not produce caterpillars — because they are Norway maples, honey locusts, ginkgos, or any of the other low-host-count species the city planted heavily — the brood doesn't fledge.

This is the mechanism by which a downtown street with a thick canopy of mature, beautiful, ornamentally-perfect honey locusts supports approximately no nesting songbirds, while a similarly-canopied street of mature red oaks supports a viable bird population. They look about the same. The chickadees know the difference within a single nesting season.

What the city is doing

Toronto's current planting list is markedly more native-weighted than the 1960s–80s palette that produced Toronto's mature canopy. Native red oak, white oak, bur oak, and swamp white oak are now in regular rotation; Kentucky coffeetree (Ontario-native, once endangered) is a featured species; hackberry has been brought back from underuse; serviceberry and redbud are placed where smaller trees fit. The Norway-maple-and-honey-locust era is, slowly, ending.

The replacement is not perfect. Honey locust is still on the planting list — operationally too useful to drop entirely. Many of the "native" oak plantings are 5 cm saplings; their 534-caterpillar contribution scales with biomass and won't be material for thirty years. The post-EAB replanting wave overrepresented Freeman maple ('Autumn Blaze'), an engineered hybrid that, like Norway maple, is a maple in name more than in caterpillar function.

But the trajectory is clearly toward higher ecological capacity. The 2010s and 2020s are putting trees in the ground that, by 2070, will have shifted Toronto's bird-supporting capacity meaningfully.

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