One argument that Native Plant Advocates make for their attempts to convert San Francisco’s open spaces to native plants (despite the habitat destruction and pesticide use) is that it’s better for wildlife. In fact, it’s the basis for the Natural Areas Program.
Insects, they believe, prefer native plants. That in turn, according to this theory, provides sustenance for insect-hunting birds and animals, providing a richer and more bio-diverse ecosystem.
Only, it isn’t, and it doesn’t.
Professor Douglas Tallamy (University of Delaware) proposed this theory, and his book Bringing Nature Home has been extremely influential. He speculated that most insects depended on native plants (owing to co-evolution). Since non-native plants wouldn’t attract insects, they would erode the base of the ecosystem.
Being a scientist, Prof Tallamy noted that this theory wasn’t based on actual observation. Being a scientist, he had his student collect data on insect use of both native and non-native landscapes. And being a scientist, even though the data did not support his theory, he published it. He wrote: “…there was no statistical difference in the amount of damage on either landscape type.”
The article below, republished with permission and minor changes from the website Death of a Million Trees, provides details and references (and is a fascinating read).
In our debates with native plant advocates, the scientist who is most often quoted to support their beliefs is Doug Tallamy who wrote an influential book, Bringing Nature Home: How Native Plants Sustain Wildlife in our Gardens. Professor Tallamy is an entomologist at the University of Delaware.
Professor Tallamy’s hypothesis is that native insects require native plants because they have evolved together “over thousands of generations.” Because insects are an essential ingredient in the food web, he speculates that the absence of native plants would ultimately result in “ecological collapse” as other animals in the food web are starved by the loss of insects. (Reference #1 below.)
Professor Tallamy freely admits that his theory is based on his anecdotal observations in his own garden, not on scientific evidence:
“How do we know the actual extent to which our native insect generalists are eating alien plants? We don’t until we go into the field and see exactly what is eating what. Unfortunately, this important but simple task has been all but ignored so far.” (Reference #1 below.)
This research has now been done to Professor Tallamy’s satisfaction by a Master’s Degree student under his direction. The report of that study does not substantiate Professor Tallamy’s belief that insects eat only native plants. In his own words, Professor Tallamy now tells us:
“Erin [Reed] compared the amount of damage sucking and chewing insects made on the ornamental plants at six suburban properties landscaped primarily with species native to the area and six properties landscaped traditionally. After two years of measurements Erin found that only a tiny percentage of leaves were damaged on either set of properties at the end of the season….Erin’s most important result, however, was that there was no statistical difference in the amount of damage on either landscape type.” (Reference #2 below)
This finding that insects are equally likely to eat native and non-native plants may be new to Professor Tallamy, but it isn’t new to the readers of Million Trees. We have reported many studies which are consistent with this finding.
- Professor Arthur Shapiro (UC Davis) reports that 82 of 236 (35%) total species of California butterflies have been observed either laying their eggs or feeding on non-native plants.
- Professor Dov Sax (Brown University) compared insects living in the leaf litter of the non-native eucalyptus forest with those living in the native oak-bay woodland in Berkeley, California. He found significantly more species of insects in the leaf litter of the eucalyptus forest in the spring and equal numbers in the fall. Professor Sax also reports the results of many similar studies all over the world that reach the same conclusion.
- The California Academy of Sciences finds that several years after planting its roof with native plants, it is now dominated by non-native species of plants in the two quadrants that are not being weeded, replanted and reseeded with natives. Their monitoring project recently reported that there were an equal number of insects found in the quadrants dominated by native plants and those dominated by non-native plants.
- Jennifer Owens (Ph.D., University of Michigan) reports 30 years of observing her garden in Wildlife of a Garden (Royal Horticultural Society, 2011). She found that non-native species were better as food plants for moth larvae than native species. Moth larvae used 27% of the native species in the garden as food plants, and 35% of the alien plants.
SPECIALISTS VS. GENERALISTS
When debating with native plant advocates, one quickly learns that the debate isn’t ended by putting facts such as these on the table. In this case, the comeback is, “The insects using non-native plants are generalists. Insects that are specialists will not make that transition.” Generalists are insects that eat a wide variety of plants, while specialists are limited to only one plant or plants in the same family which are chemically similar.
Professor Tallamy offers in support of this contention that only “…about 10 percent of the insect herbivores in a given ecosystem [are not specialists],” implying that few insects are capable of making a transition to another host plant.
However, categorizing insects as specialists or generalists is not a dichotomy. At one extreme, there are some insects that choose a single species of plant as its host or its meal. At the other extreme, there are insects that feed on more than three different plant families. It is only that extreme category which has been estimated at only 10% of all phytophagous (plant-eating) insects. The majority of insects are in the middle of the continuum. They are generally confined to a single plant family in which the plants are chemically similar.
Putting that definition of “specialist” as confined to one plant family into perspective, let us consider the size of plant families. For example, there are 20,000 plant members of the Asteraceae family, including the native sagebrush (Artemisia) and the non-native African daisy. In other words, the insect that confines its diet to one family of plants is not very specialized.
Professor Tallamy offers his readers an explanation for why specialist insects cannot make the transition from native to non-native plants. He claims that many non-native plants are chemically unique and therefore insects are unable to adapt to them. He offers examples of non-native plants and trees which “are not related to any lineage of plants in North America.”
One of his examples is the goldenrain tree (Koelreuteria paniculata). This is the member of the soapberry (Sapindaceae) family to which the soapberry bug has made a transition from a native plant in the soapberry family in less than 100 generations over a period of 20 to 50 years. Professor Tallamy’s other examples of unique non-native plant species are also members of large plant families which probably contain native members. Professor Tallamy is apparently mistaken in his assumption that most or all non-native plants are unique, with no native relatives.
THE PACE OF EVOLUTION
Even if insects are “specialists” we should not assume that their dependence on a native plant is incapable of changing over time. Professor Tallamy’s hypothesis about the mutually exclusive relationships between native animals and native plants is based on an outdated notion of the slow pace of evolution. The assumption amongst native plant advocates is that these relationships are nearly immutable.
In fact, evolution continues today and is sometimes even visible within the lifetime of observers. Professor Tallamy provides his readers with examples of non-native insects that made quick transitions to native plants:
- The hemlock wooly adelgids from Asia have had a devastating effect on native hemlock forests in the eastern United States.
- The Japanese beetle introduced to the United States is now eating the foliage of over 400 plants (according to Professor Tallamy), some of which are native (according to the USDA invasive species website).
These insects apparently made transitions to chemically similar native plants without evolutionary adaptation. If non-native insects quickly adapt to new hosts, doesn’t it seem likely that native insects are capable of doing the same? That is both logical and consistent with our experience. For example, the native soapberry bug mentioned above has undergone rapid evolution of its beak length to adapt to a new host.
Although Professor Tallamy tells us that the relationship between insects and plants evolved over “thousands of generations,” he acknowledges much faster changes in plants when he explains why non-native plants become invasive decades after their arrival: “Japanese honeysuckle, for example, was planted as an ornamental for 80 years before it escaped cultivation. No one is sure why this lag time occurs. Perhaps during the lag period, the plant is changing genetically through natural selection to better fit its new environment.” Does it make sense that the evolution of plants would be much more rapid than the evolution of insects? Since the lifetime of most insects is not substantially longer than the lifetime of most plants, we don’t see the logic in this assumption.
BELIEFS DIE HARD
Although Professor Tallamy now concedes that there is no evidence that insects are dependent upon native plants, he continues to believe that the absence of native plants will cause “ecological collapse.” In the same book in which he reports the study of his graduate student, Professor Tallamy repeats his mantra: “…our wholesale replacement of native plant communities with disparate collections of plants from other parts of the world is pushing our local animals to the brink of extinction—and the ecosystems that sustain human societies to the edge of collapse.”
This alarmist conclusion is offered without providing examples of any animals being “pushed to the brink of extinction.” In fact, available scientific evidence contradicts this alarmist conclusion. (Reference #3 below)
(1) Tallamy, Doug, Bringing Nature Home, Timber Press, 2007
(2) Tallamy, Doug, “Flipping the Paradigm: Landscapes that Welcome Wildlife,” chapter in Christopher, Thomas, The New American Landscape, Timber Press, 2011
(3) Erle C. Ellis, et. al., “All Is Not Loss: Plant Biodiversity in the Anthropocene,” in PLOS one (A peer-reviewed open-access journal)