Several readers of the Takoma Park Native Tree Selection Guide have questioned whether it uses a valid or the best indicator of the contribution that a tree species makes to supporting ecosystem health through the food web (See What is the Biodiversity Indicator?).
Below, Dr. Travis Gallo gives the perspective of an urban wildlife biologist on the validity of, and alternatives to, the biodiversity indicator presented in the guide.
Dr. Gallo is a wildlife biologist and Assistant Professor, Department of Environmental Science and Technology, University of Maryland College Park. He studies how to sustain and restore biodiversity in urban ecosystems.
References for the citations in this Q&A can be found in the References webpage.
What do you think of Doug Tallamy’s indicator as a measure of the contribution of tree species to sustaining biodiversity?
I think it’s rather clever. I’m familiar with some of the science that backs it up since my colleague, Desirée Narango, has co-authored several articles with Tallamy and others on research in this area.
The question that animates this research is how to restore ecological functions in urban landscapes, where human beings have created such novel ecosystems. In a 2018 article, Desirée and her co-authors found that residential yards planted with less than 70% native plants were less abundant in arthropods. That situation forced chickadees to eat a different diet which in turn reduced their offspring numbers and made chickadee populations unsustainable. The conclusion was that landscaping with nonnative plants created population sinks for insectivorous birds, and conversely prioritizing native plants promoted sustainable food webs (Narango, Tallamy, and Marra 2018).
Desirée next looked at whether some native plants were more important than others in supporting food webs. Narango, Tallamy, and Shropshire (2020) focused on Lepidoptera (butterflies) as a food source because good data were available on plant-Lepidoptera interactions, and because this order of insects is unusually important in the diets of insectivores. The researchers found that a small number of plant genera supported the majority of Lepidoptera species. The conclusion was that ecological restoration had to include plant species from these genera in order to raise the carrying capacity of the land as effectively as possible.
BTW, Desirée was also part of a research team which found that birds prefer to forage for caterpillars on trees that the data show to have the highest interactions with Lepidoptera species (Piel, Tallamy, and Narango 2021). That’s pretty good support for using these data to measure the relative interactivity of tree species in the ecosystem.
Does the focus on birds limit the usefulness of the indicator?
There are not a huge number of wildlife species (mammals, birds, reptiles, amphibians, fish) likely found in Takoma Park yards. Birds, and their interactions with other species, will be a pretty important part of the ecosystem.
Other wildlife species likely to be found in Takoma Park, for example, squirrels, will eat caterpillars, too. Even other insects eat caterpillars.
Do you know of indicators that measure the contribution of trees or plants generally to biodiversity?
I know some work that quantifies the value of biodiversity to ecosystem services. Robert Fuller found that people received more psychological benefits from urban green spaces when they were richer in plant, bird, and butterfly species (Fuller et al, 2007). Sophie Gilbert has calculated the economic value of mountain lions reducing deer populations. Climate Action Plans should, but generally don’t, consider the value of biodiversity in climate solutions. But the above measures concern the value from biodiversity, not measuring the value to biodiversity.
So, I guess my answer is no, I don’t know any indicators that measure the relative contributions by different tree genera to sustaining urban ecosystems.
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