The Takoma Park Native Tree Selection Guide provides data on a biodiversity indicator, which ranks the relative contributions of various tree species to the food web.
Below is a basic introduction to this biodiversity indicator, drawn from the guide (specific pages indicated in parentheses).
In our discussion series, Doug Tallamy takes a deeper dive into explaining the science behind this indicator
What is the indicator?
(pg. 3, 11, 15)
Professor Douglas Tallamy and K.J. Shropshire, both at the University of Delaware, have developed an indicator for species interaction, namely, the number of caterpillar species (Lepidoptera) hosted by a plant genus.
The data were provided by Douglas Tallamy from an unpublished database for Prince George’s County MD.
For a description of the methodology used to develop this indicator, see Tallamy, D. W. and K.J. Shropshire. 2009. “Ranking Lepidopteran Use of Native versus Introduced Plants.” Conservation Biology, 23(4), 941–947. http://www.jstor.org/stable/29738829
What does this indicator accomplish?
Tallamy’s indicator serves to highlight the critical role of insects in supporting biodiversity. The attitude that all insects are pests that should be exterminated is incompatible with a scientific understanding of ecosystem health and the importance of biodiversity. Also, like all good indicators, Tallamy’s indicator simplifies a complex reality in order to make biodiversity an easier concept to apply.
Can you give an example of what comparing two tree species on the indicator tells us about their relative contributions to the ecosystem?
Flowering dogwoods illustrate how this indicator makes understanding and measuring the extent of species interactions easier. The native flowering dogwood (Cornus florida) and Asian flowering dogwood (Cornus kousa) provide many identical ecological functions, such as recycling carbon and water, enriching soil, etc. However, the two species interact with other species in our local ecosystem to markedly different extents. In this respect, the two dogwood species differ in their ecological contributions.
The native dogwood provides habitat for 114 species of caterpillars in Takoma Park, the first link in a chain of species interactions. Caterpillars provide an important contribution to the natural food web because they are an easily consumed and dense source of nutrients for so many species. Most terrestrial birds, for example, are wholly dependent on caterpillars to feed their young. Caterpillars that survive to become butterflies play an additional vital role for many plants through pollination. The pollinated plants have benefited in this way from native trees, as have all the species (including humans) who depend on pollinated plants.
In stark contrast, kousa dogwoods have not been documented as successfully hosting caterpillars native to our area and thus cannot reliably provide the cascade of benefits to other species provided by native dogwoods. Kousa dogwoods have not evolved together with these other species native to our ecosystem and therefore do not interact with other species to the extent necessary to protect native biodiversity.
How do I use this information when selecting a tree for my yard?
Plant a native tree. Both native and non-native trees help to address climate change. However, native species rank much higher on the biodiversity indicator than do non-native species. In other words, the native trees generally give a bigger environmental bang for the buck because they have evolved to provide benefits to other native species. These benefits form the foundation of the natural food web, pollination, and other positive impacts,
But how do I use the values for the biodiversity indicator in the guide’s lists of native tree species?
(pgs. 5, 7-10)
As you say, the Biodiversity Indicator column in these lists gives the number of caterpillar species hosted by each tree species. You will see that some native trees are better than others at supporting the food web through hosting Lepidoptera species. A ★ highlights these “superstars.” In the guide, we have defined superstar tree species as those which host over 200 species of Lepidoptera.
For example, the majestic native oaks that have historically defined Takoma Park’s tree canopy provide habitat to 498 species of native caterpillars (Lepidoptera) in our area. Native willows, cherry and plum trees, and birches host over 300 caterpillar species.
If you plant a superstar, it will deliver more benefits to the ecosystem by making a larger contribution to the food web.
However, there are other factors to consider in selecting a tree species, as outlined in the guide section, Five Steps in Selecting a Native Tree.
Note that the superstars are mostly very tall trees that fill the uppermost niche in the urban forest. If people only plant small trees, the urban forest will lose this ecological niche.
So, my native tree will only benefit the ecosystem if it’s a superstar?
The other native tree species provide different types of ecological value.
First, many tree species with lower biodiversity indicator numbers support different caterpillar species from those hosted by the superstars. For example, some caterpillar species have evolved to live in the unique habitats provided by various species of pines (Pinus) and cannot survive on, say, oaks. Similarly, sycamore, sweetgum, and tulip trees all host many specialist caterpillars found only on those respective tree species.
Second, having many different species of native trees helps provide food and shelter throughout the year. As a result, your yard thrives because beneficial insects pollinate your plants, help control unwanted insects such as aphids, and enrich the soil.
Third, a diversity of native trees provides more opportunities for good climate adapters to emerge. In other words, it is too soon for research to establish how different tree species respond to various climate change scenarios. By planting a diversity of native tree species, we hedge our bets that we will have included some good climate adapters. By increasing the number of trees of each species planted, we support evolutionary adjustment to climate change through natural selection.
Fourth, diverse urban forests also provide insurance in case an invasive species or pathogen, like Dutch elm disease, chestnut blight, or most recently the emerald ash borer, kills off a specific tree species in the City’s canopy. Diverse urban forests comprising native trees give this insurance and at the same time protect biodiversity.