In the studies that have come out of Tallamy’s lab over the past decade, many variables in the plant/animal relationship were investigated.
- Comparing the insect herbivore’s attraction to native plant species vs. non-native plants that are closely related to the native plants (congeners) and other non-native plants that are not closely related to natives (non-congeners) produced telling statistics: the native plants were hands down favorites of insects. The non-native congeners attracted 50% fewer herbivores (mostly generalist feeders) than did the natives; the non-native, non-congeners attracted 70% fewer herbivores than did the natives.
- Insect feeding guilds (how they feed): Insects that mine into the layers of leaf material and those that produce galls have some of the most specialized host plant relationships known—they overwhelmingly prefer native plants. The insects that have chewing mouthparts (e.g., caterpillars) are primarily specialists who prefer native plants. Among insects that suck plant fluids, the ones that suck from the phloem (which carries fluids down from the leaves), prefer native plants. Those that suck from the xylem (tissue that carries fluids up from the roots), and spaces between plant cells, will feed from natives as well as some nonnatives congeners.
- Native plants supported significantly more species of herbivorous insects than did both non-native congeners and non-native, non-congeners.
Ten times more eggs were laid on native plants, and produced healthy immature life stages. The 1 out of 10 eggs that was laid on a non-native congener produced smaller, weaker, immature life stages—many of which died.
This is a particularly important finding as it refers to the sustainability of these life forms ‘with and without’ native plants in the ecosystems they inhabit.
No eggs or surviving immature life forms: No next generation. That species of insect is moribund.
- Conservation implications: “Plant origin is a good surrogate [indicator] for measuring immature herbivore abundance and species diversity; native plants support the most biodiversity, followed by non-natives with a close native relative, while non-natives that are unrelated to local flora produce a species-poor, uneven herbivore community.” (Tallamy)
There is no question that native plants attract and sustain larger numbers of individual herbivorous insects.There is no question that native species of plants attract a much greater number of species of insects.In order to consider the nature of the species diversity that is attracted to native and non-native plants, a statistical analysis (Beta analysis) was also done on the data collected from each of the individual experimental plots. This was done because it is important to know whether the herbivores that are able to use non-native plants represent a unique set of insects or merely repeated subsets of those insects that are attracted across all sites. Results of the analysis showed that, checking from garden plot to garden plot, the same few species of herbivorous insects were attracted to the non-native plants, in all separate plots.
In contrast, a much greater number of species was consistently found in each native plot. The herbivores attracted to the non–natives represented a small, redundant subset of the ones attracted to the native plants.
It is extremely important to understand that the DIFFERENCE lay in both the number of species, and in the overall number of individuals attracted to native plants; far more of both were supported by the natives.
Maintaining Biodiversity: Native Plants DO Provide Critical Ecosystem Services
By Maryann Whitman (with gratitude to Wild Ones Honorary Director Douglas Tallamy, Ph.D.)
One of the most critical roles plants play in nature is supporting food webs—that is, all of the animal life as it exists and interacts in our ecosystems; it is in performing this essential ecosystem service that plants introduced from other parts of the Earth, do not measure up to our native species, in serving local ecosystems into which they have been introduced.
In an area that I’ve been ‘letting go’ because it looked like it was producing some interesting seedlings—flowering dogwood and some wild cherries that I hadn’t differentiated yet, all vying for space from spreading Virginia creeper, trumpet creeper, and bottlebrush grass—I found a stranger.
Unlike all the other growth that looked a little worse for wear at the end of the season, this sapling was pristine. As everything else was half eaten, tattered, turning brown, its leaves, still shiny, turned attractive shades of red. Dead giveaway: it had evolved in a climate with a different growing season: I was nose to nose with my first ever Callery pear! Originally from China, Callery pear has become a rampant invasive on the east coast. It has arrived in Michigan. This plant’s pristine condition told me that no one was interested in munching on parts of it; its rapid growth told me ‘be wary’.
Before spring, I’ll remove this introduced, non-native plant, whose “evolutionary history happened elsewhere” (Tallamy’s words. —mw)
As members of the first trophic level of a food chain (the organisms that create their own food in an ecosystem, the autotrophs), plants perform a miracle of life. Through the process we call photosynthesis, plants capture energy from simple sugars (carbohydrates). Plants thereby enable animals to “eat” sunlight! These sugars, along with water and minerals that plants take from the soil, are the basis of almost every food web on the planet. Animals (the second, third and fourth trophic levels) in a food web benefit from the energy captured by plant photosynthesis— only if they can eat the plants available, or eat something that itself has eaten plants. And that’s the rub. In almost all ecosystems, the group of animals that is best at taking energy from plants and passing it to other animals, in the form of their own bodies, is insects. Unfortunately, most insects are very fussy about the plants they eat.
Each plant has a primary metabolic system, which has to do with the plant’s own life support. To defend against being eaten, plants can load their tissues with nasty-tasting secondary metabolic chemical compounds—feeding deterrents—that are very effective at preventing most insects from eating most plants.
No two plant lineages rely on the same combination of chemicals for protection.
So how do insects circumvent these formidable defenses? They specialize.
Over long periods of exposure, a particular locally native insect lineage develops the enzymes and physiological mechanisms necessary to deactivate the secondary metabolic compounds produced by a particular locally native plant lineage. This feat of adaptation enables the insect to eat the plant on which it has specialized, without being poisoned.
That’s the upside of specializing: native insect specialists can eat native plants that are distasteful or toxic to most other insects. Approximately 90% of the insect herbivore species in any given local ecosystem are specialists that can only eat the few plant lineages that share the particular chemical defense to which they have adapted.
The downside of specializing is that specialists become locked into eating only members of the plant lineage to which they have adapted. If these native plants become locally rare, the specializing insects do too. And the repercussions travel through the energy transfer to higher trophic levels of the food web (herbivores and the animals that eat them).
While the generalist insect species constitute only 10% of insect herbivores, in absolute numbers they are much more frequent.
Trying to manage invaded native ecosystems, our efforts to control invasive plants in this country, cost billions of dollars each year. Tallamy’s research over the past decade consistently indicates that there are indeed good reasons to keep introduced plants off our properties; our efforts need to focus on restoring ecosystem function.
From an article written by Tallamy and published in WILDONES JOURNAL (May/June 2011):
“There is strong evidence that, in terms of the most important contribution plants make to the diversity of life on Earth, non-native plants are not the ecological equivalent of native species. I am speaking of the role plants play as the first trophic level….The suggestion that we should embrace, rather than fight, the wide-scale replacement of locally native plant communities by plant species from other lands ignores the impact of non-native plants on local food webs. In view of the role that insects play in running the ecosystems that support humans and other biodiversity, we allow food web collapse at our peril.”
Burghardt, K.T., D.W. Tallamy, and W.G. Shriver. (2009) Impact of native plants on bird and butterfly biodiversity in suburban landscapes. Conservation Biology 23: 219-224
Burghardt, K.T., D.W. Tallamy, C. Philips, and K.J. Shropshire (2010). Non-native plants reduce abundance, richness, and host specialization in lepidopteran communities. Ecosphere 1(5): 1-22
Burghardt, K.T. and D.W. Tallamy (2013). Plant origin asymmetrically impacts feeding guilds and drives community structure of herbivorous arthropods. Diversity and Distributions 19:1553-1565
Burghardt, K.T. and D.W. Tallamy (2015). Not all nonnatives are equally unequal: Reductions in herbivore β-diversity depend on plant phylogenetic similarity to native community. Ecology Letters 18: 1067-1098
Ballard, M., J. Hough-Goldstein, and Douglas Tallamy (2013). Arthropod Communities on native and nonnative early successional plants. Environ Entomol 42(5): 851-859 (2013)
Chapin III, F.S. et al., (2000). Consequences of changing biodiversity. Nature 405, 234-242 (2000)
Eiseman, Charley and Noah Charney Tracks and Sign of Insects and Other Invertebrates: A Guide to North American Species. Stackpole Books ISBN: 978-0-8117-3624-4 Available at Amazon.com
Hooper, D.U. et al., (2005). Effects of biodiversity on ecosystem processes: Implications for ecosystem management. A position statement for Ecological Society of America
Tallamy, D. (2004). Do alien plants reduce insect biomass? Conserv Biol 18: 1689-1692
Tallamy, D.W. (2009). Bringing Nature Home: How you can Sustain Wildlife with Native Plants. Portland, OR: Timber Press Available at Wild Store
Tallamy, D.T. et al. (2009). Can alien plants support generalist insect herbivores? Biol Invasions 12: 2285-2292
Tallamy, D.W., et al., (2009). Ranking Lepidopteran Use of Native Versus introduced plants. Conserv Biol 23: No 4, 941-947
Wild Ones Journal, (2011). Vol 24 No3 A Friend to Aliens pgs 1-5