Doug Tallamy’s latest book, Nature’s Best Hope, continues his crusade against non-native plants. He now calls invasive plants “ecological tumors.” You might be tempted to respond that invasive plants are a small subset of non-native plants until you realize that Tallamy calls 3,300 plant species in North America “invasive.” There are approximately 6,500 species of native plants in California, which reminds us that introduced plants are often a significant portion of our urban landscapes. The title of Tallamy’s book is a misnomer. Nature is not confined to native plants, as Tallamy wishes it to be.
Tallamy makes no meaningful distinction between “invasive” and “non-native.” The classification of berry-producing non-native plants as “invasive” is a case in point. Although Himalayan blackberries are invasive, most other berry-producing non-natives in California are not. Cotoneaster, pyracantha, and holly are a few examples of berry-producing plants being eradicated in the Bay Area that are not inherently “invasive.” They spread because birds eat the berries and deposit the seeds elsewhere.
Eradicating berry-producing plants deprives birds of an important source of food. If herbicides are used to kill the plant, the birds are also exposed to harmful chemicals, known to reduce reproductive success and cause other sub-lethal health issues in wildlife. In the case of Himalayan blackberries, they are frequently eaten by children and adults, who are then exposed to the herbicides used to kill the shrubs that are often widespread in our parks and open spaces. San Francisco’s Recreation and Park Department sprayed blackberries in San Francisco’s parks and open spaces 23 times in 2019.
Tallamy and his nativist allies claim that native plants are beneficial to wildlife, especially birds. How can they claim that eradicating berry-producing plants benefits birds? They do so by claiming that native berries are more nutritious than non-native berries. In particular, they claim that native berries contain more fat than sugar and that migrating birds require berries with high fat content. Tallamy cites one study in support of that claim, a study that compared fat and sugar levels in the berries of 9 species of plants in the Northeast, 5 native species and 4 introduced species. They found that the native species they analyzed had more fat content than the introduced species they analyzed. (1)
Generalizations unsupported by evidence
From that single study of nine plant species, Tallamy generalizes that berries of plants that are considered native in Asia are less nutritious for migrating birds than the berries of native plants in North America are. (None of the nine plant species studied occurs in California.) Does that generalization make sense?
- Tallamy does not provide any evidence that there are fewer migratory birds in Asia, or that the nutritional needs of migratory birds in Asia are different than those in North America. In fact, looking at the migratory patterns of birds confirms that migratory routes of birds span several continents. The intercontinental flights of birds sometimes span both Asia and North America. There is no logical or evidentiary explanation for berries of native plants in Asia being uniformly less nutritious than native plants in North America.
- However, Tallamy offers evidence of the similarity between plants in Asia and closely related plants in North America. Wooly adelgids quickly made a transition to native hemlocks when they arrived in North America from Asia because its native host in Asia is closely related to the American native. The adelgid has “all but eliminated hemlocks” in America, according to Tallamy. The emerald ash borer has killed millions of ash trees in America when it arrived from Asia, where its native host was closely related. On one hand, Tallamy claims that native plants in America are unique, completely different from plants in Asia, yet he recognizes that insects from Asia rapidly adapt to closely related host plants in America.
- Asian species are not so foreign to America as Tallamy wishes us to believe. There are relicts of vegetation that extended completely around the Northern Hemisphere about 50 million years ago that were broken up by a combination of mountain-building and climate change. Tree of Heaven, Gingko, and Dawn Redwood, now considered introduced trees from Asia, occurred here naturally during that geologic period. Tallamy says we must confine our choices to plants that “share an evolutionary history.” In fact, many plants now considered non-native shared an evolutionary history with plants now considered native. Trees are time travelers, marching to the beat of the Earth’s geologic and climate drum. Now they must be on the move to survive our changing climate. We should not stand in their way.
Such generalizations unsupported by evidence are typical of Tallamy’s work. In “Native plants improve breeding and foraging habitat for an insectivorous bird,” Tallamy and his collaborators conclude, “We demonstrate that residential yards dominated by nonnative plants have lower arthropod abundance…that function as population sinks for insectivorous birds.” The data provided do not support such a broad generalization. They studied one species of bird, in one geographic location, in a short period of time. They inventoried arthropods for two years in a single month time-frame. They quantify only one variable (plant foliage biomass) in addition to the nativity of plants, the abundance of insects, and the breeding success of one bird species. They have not taken into consideration intervening variables such as variations in temperature, rainfall, pesticide use, etc. The bird species studied is abundant within its range. Its conservation status is “Least Concern.” The abundance of this bird species does not justify the dire predictions of Tallamy’s study.
I have focused on just one of the many controversies discussed in Doug Tallamy’s new book. I haven’t touched on the two most fundamental errors in Tallamy’s work:
- Tallamy underestimates the speed of adaptation and evolution. There is ample evidence of rapid adaptation to non-native vegetation, including Tallamy’s examples of wooly adelgid and emerald ash borer making a quick transition to North American native trees after arriving from Asia.
- He exaggerates the degree of specialization among insects. For example, he claims that 30% of native bees are “host-plant specialists,” yet Bees of the World (Michener, Johns Hopkins University) estimates a global average of 9% of bee species use plants within the same genus and it is “exceedingly rare” for bee species to be confined to only one plant species.
We have explored those issues in Tallamy’s work in previous articles:
- Doug Tallamy claims that insects eat only native plants, yet his own study proves otherwise: HERE
- Doug Tallamy claims that non-native plants are “ecological traps for birds.” HERE is an article that disputes that theory.
- Doug Tallamy claims that native and non-native plants in the same genus are not equally useful to wildlife, but he is wrong about that. Story is HERE.
- Doug Tallamy advocates for the eradication of butterfly bush (Buddleia) because it is not native. He claims it is not useful to butterflies, but he is wrong about that. Story is HERE.
- Doug Tallamy publishes a laboratory study that he believes contradicts field studies, but he is wrong about that. Story is HERE.
- Doug Tallamy speaks to Smithsonian Magazine, Art Shapiro responds, Million Trees fills in the gaps: HERE
(1) B. Smith, et. al., “The value of native and invasive fruit-bearing shrubs for migrating birds,” Northeastern Naturalist, 2013, 20(1): 171-84.