In Nature’s Best Hope, Doug Tallamy concedes that there is no evidence of extinctions of native plants being caused by the introduction of non-native plants in the Continental US. However, he accuses non-native plants of something more nefarious: “There is one biological phenomenon associated with some plant invasions that is so pernicious, even continental scales are not protecting natives from invasive species. I speak of…introgressive hybridization, where the invasive species hybridizes with a closely related native, and then through repeated backcrosses and directional gene flow, the gene pool moves closer and closer to that of the invader.”
Jake Sigg calls this phenomenon, genetic pollution. Both Tallamy and Sigg consider such hybridization a loss of the native species and, indeed, it can be the end of localized variants of a species. However, hybridization is often instrumental in the creation of a new species, one that is often superior to its ancestors because it is better adapted to present environmental conditions.
In a recently published study of the evolution of oaks, scientists traced the 56 million year evolutionary history of roughly 435 species of oak across 5 continents where they are found today. Oaks are wind-pollinated, leaving pollen fossil records of their presence where they may no longer live. Using DNA analysis of fossil pollen, scientists tell us when and where oaks have lived. Their presence or absence was determined by changes in climate that created or eliminated land bridges between continents enabling movement of plants and animals, as well as providing the climate conditions in which oaks can survive.
Hybridization was instrumental in the formation of oak species and the ability of oaks to survive in different climate conditions. The article in Scientific American about the genetic study of oak species concludes: “A firm grasp of when, where and how oaks came to be so diverse is crucial to understanding how oaks will resist and adapt to rapidly changing environments. Oaks migrated rapidly as continental glaciers receded starting around 20,000 years ago, and hybridization between species appears to have been key to their rapid response. The insights we can gain from elucidating the adaptive benefits of gene flow are critical to predicting how resilient oaks may be as climate change exposes them to fungal and insect diseases with which they did not evolve.”
In fact, a recent study suggests that assisted species migration and intentional hybridization are necessary to prevent the extinction of plants in Arctic regions, where the climate is warming the fastest. Intentionally planting species from warmer regions into colder regions in anticipation of climate warming is called assisted migration. It is not a new concept. The study acknowledges that intentional hybridization is a radical suggestion that contradicts conventional wisdom: “Traditionally, hybridization is viewed as negative and leading to a loss of biodiversity, even though hybridization has increased biodiversity over geological times.” This study acknowledges the role that hybridization plays in increasing biodiversity.
In the Bay Area, we are surrounded by examples of hybridization, some intentional and tolerated and some natural, but not tolerated:
- Sycamores are the most common street tree in the United States and we have many here in the Bay Area. They are a hybrid of London Plane Trees and our native Sycamore. The California native was intentionally bred with the London Plane Tree to increase its drought tolerance. Sycamore street trees are one of the most popular because they are extremely hardy and tolerant of challenging conditions in urban settings. They are also the host trees of one of our native butterflies, Western Tiger Swallowtail. The Tiger Swallowtail probably used our native Sycamore in the past, but made a seamless transition to the hybrid.
Update: I learned about the hybrid origins of our local Sycamore street tree in an urban forestry class at UC Berkeley. Peter Del Tredici has sent me this correction: “The london plane tree, Platanus x acerifolia is generally considered to be a hybrid between the european species, P. orientalis and the eastern species, P. occidentalis. the west coast species, P. racemosa is not part of the mix.”
- Spartina alterniflora is a marsh grass that is native on the East Coast. It grows taller and denser than our native marsh grass, Spartina foliosa that also dies back in winter, unlike the East Coast native that does not. In other words, non-native spartina is superior protection from winter storm surges compared to native spartina. Yet, non-native spartina is being eradicated using herbicides along the entire West Coast of the country because it hybridizes with the native spartina species. The herbicide used for that purpose has been sprayed for about 15 years, which is probably why attempts to plant native spartina as a replacement have been unsuccessful. The result of the eradication project has been bare mud that provides no protection from erosion caused by rising sea levels and more intense winter storms. In other words, if non-native spartina were permitted to hybridize with native spartina on the West Coast, the result would be a new species that is better adapted to face the threats of rising sea levels and intense storm surges.
Fear of hybridization is akin to fear of mongrelization–the mixing of races–by racists and xenophobes. It is closely related to the fear of non-native plant and animal species, a short-step away from the fear of human immigrants. Concern about racial purity is not far from fear of “genetic pollution.” State laws in the US prohibiting interracial marriage were not repealed until 1967, when the US Supreme Court ruled in Loving v. Virginia that such laws were unconstitutional in the 16 states in which these laws still remained. These are cultural fears, not grounded in biological science.
Doug Tallamy’s intended audience is home gardeners. Although he urges his readers to remove invasive species, he does not endorse the use of herbicides. Unfortunately, his work is used by public land managers to justify their eradication projects that usually use herbicides. If Tallamy’s work stayed in its home gardening lane, it would do less damage to the environment.