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.
This is an excerpt from the Wild Ones Journal
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“In an area that I’ve been ‘letting go’ because it looked like it was producing some interesting seedlings— lowering 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 foodchain (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 the sun and store it in the molecular bonds of 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.
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 non-natives 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.
- 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.
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.
By Maryann Whitman
(with gratitude to Wild Ones Honorary Director Douglas Tallamy, Ph.D.)