It’s often noted that native plants and animals depend on each other because the two evolved specialized relationships together over thousands of years, but that’s a basic explanation that doesn’t offer any details. I’ve often wondered about individual species and to what extent native plants are essential to them. I watch ladybugs devouring aphids on native perennials, warblers foraging for insects in various shrubs and trees, and black-capped chickadees bringing squirmy larvae to their hungry nestlings, but how much do they really benefit when we choose to grow natives?
To my delight, a new study that focuses on one insectivorous bird species—the Carolina chickadee (Poecile carolinensis)—was recently released in Biological Conservation. Chickadees—whether they’re Carolina, Black-capped, or Chestnut-backed—are fairly common backyard species that, like most birds, don’t reproduce on seeds and fruit but instead eat and feed insects to their young. The study’s authors evaluated native plants, but also those that originated outside North America to see if they were a limiting factor for this particular species’ ability to effectively raise young. Their results prove that non-native plants reduce the quality of habitat for Carolina chickadees by not providing food for breeding.
Insects are crucial
It is the living environment—including insects—that sustains us and every other species. Herbivorous insects make up more than a third of the world’s animals, and their role is indispensable: By converting plant material to protein, they are nature’s only way of getting plants’ energy into animals who don’t eat plants directly, and into animals who eat the ones who feed on insects.
Most herbivorous insect species are called specialists, meaning they can’t choose what they eat. Their menu is short; they must rely on only certain types of plants (that they evolved with) which have certain chemical compositions that support them, and can’t exist where those plants don’t exist. A well-known example is the monarch butterfly, an insect whose larvae can only feed on native milkweed plants, but there are countless others. If you already recognize the charms of regional native plants and have witnessed how growing them attracts more wildlife to your yard, all of this comes as no surprise. Native plants host and support more native herbivorous insects and, consequently, more birds and other wild ones.
During the study’s two-year survey in the Washington, D.C. area, the research team correlated the birds’ diets to the plants they forage in. Using 97 suburban yards, they determined the species and origin of each tree and shrub, then checked the leaves of 16 plants at each site for caterpillars while tracking which plants received the most foraging visits from chickadees. Nest building in and near each yard was also examined through- out the chickadees’ breeding period, roughly April to early June on the east coast. Data revealed that these birds were more likely to nest in yards with native trees and shrubs than in yards with ornamentals that evolved outside North America. The native trees used the most included oaks, elms, cherries, and maples due to their ability to support the larvae of lepidoptera (butterflies and moths) and sawflies, which are essential for rearing young chickadees. Baby chickadees (and other birds) need a lot of food to survive: Previous research has shown that these busy parents need to collect 5,000 to 9,000 bits of food (depending on the clutch size of the brood) per nestful of chickadees, plus feed themselves! According to the Cornell Lab of Ornithology, “during a lodgepole needle miner [an insect that can kill trees] outbreak in Arizona, one chickadee was found with 275 of the tiny caterpillars in its stomach at one time.”
The native connection
Chickadees are generalist foragers, meaning they’ll look for food nearly everywhere, not just on certain plants. They will forage in non-native plant species but won’t find much, since few host the food they need. In my experience, black-capped chickadees also feed their babies a few adult insects and the occasional spider (which may be found almost anywhere), but in native trees such as oaks, a diversity of larvae can be found, and large numbers of them can often be found quickly. Douglas Tallamy’s “Lepidoptera index,” which ranks different types of plants by the diversity of caterpillars they promote, corroborates this. That same research found that woody plants apparently support many more Lepidoptera species than herbaceous species do. Whether that is because “woody plants in general are both longer lived and larger than most herba-ceous plants and thus may be easier targets for insect herbivores to exploit,” or because “herbaceous plants are underreported as lepidopteran hosts because they are more difficult to identify and less conveniently searched by collectors,” we ought to grow more woody plant to maximize biodiversity, if only to give the benefit of the doubt (and provide birds more cover!). And, as I reported two years ago, another study confirmed
that relatives of native trees (i.e., scarlet oak, a distant cousin of the west coast’s Oregon white oak) host and support fewer species of insects than the native counterpart, and that non-native trees that have no native relative in a region provide next to nothing. Yard after yard of ornamental, introduced species effectively destroys insect diversity.
So, now we have more compelling evidence that growing natives can improve the human-dominated landscape by supplying numerous ecological advantages, beauty, and the ability to support the entire life cycle of insectivorous birds. Whatever benefits the chickadees will also benefit other species, and increase biodiversity overall. The Douglas-firs in the back of my yard and the towering elms in the parking strip on my street nearly always have birds in them. Besides chickadees, I see woodpeckers, nuthatches, warblers, bushtits, and more. The chickadees simply tell us what they all need.