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Ever since ornamental plants started moving into natural areas, some people have questioned the wisdom of trying to curb these vegetative incursions. In some areas, such non-native or "invasive" plant species are now displacing native plant communities. People concerned about the impacts of invasive plants on ecosystems have been accused of being too emotional about these plant incursions, using biased terms like "invasion," "alien," and "bad," while ignoring the beneficial side of introduced plants and trying to return our ecosystems to some pristine state that never really existed.

Criticism of invasive plant control efforts is understandable. We spend billions of dollars a year trying to manage invaded ecosystems, and though local success is common, wide-scale control of invasive plants is extraordinarily difficult, particularly when such plants continue to be sold at nurseries. Fighting what many see as a losing battle might be a fool's errand if there were not compelling reasons to do so. And research has consistently shown that there are good reasons to keep introduced plants off our properties.

Critics of invasive control efforts agree that invaded communities become different, but claim those differences are not "inappropriate." They argue that all ecosystems are in a constant state of flux, and humans should not challenge this "natural process," even if an introduced plant changes the diversity and abundance of native species. They further argue that plant invasions are not "bad" for ecosystems because there are no records of a plant invasion causing a continent-wide extinction of a native species, and such invasions actually increase the number of species in North America.

Though plant invasions have not yet caused the global extinction of a native plant, that does not mean ecosystem function has not been compromised at the site of the invasion. The number of species may be greater on a continental scale after the influx, but ecosystems don't function on a continental scale; they function locally. Using global extinction as the only indicator of harm is like saying death is the only symptom that warrants a visit to the doctor. When invasive plants like autumn olive, buckthorn, barberry, bush honeysuckle, or Phragmites invade a plant community, they replace the local native species at that site, potentially causing complete local extinction or a decline in plants that contribute to ecosystem function. There are dozens of published records of significant native plant declines within invaded sites.

If introduced plants were the ecological equivalents of the native species they replace, ecosystems would look different, but would be just as productive (though less stable). In fact, introduced plants may produce ecosystem benefits equal to natives, but they pale in comparison in perhaps the most critical role plants play in nature-they are poor at providing food for the animal life that runs our ecosystems.

Through photosynthesis, plants capture energy from the sun and store it in the carbon bonds of sugars and other carbohydrates, which are the basis of every terrestrial food web. But animals can only access this energy by eating plants or something that previously consumed the plants. And here is the rub: the group that is best at transferring energy from plants to animals is insects. Unfortunately, most insects are very fussy about the plants they eat.

Because plants don't want to be eaten, they protect their tissues with species-specific chemicals that prevent most insects from eating them. How do insects circumvent these formidable defenses? They specialize. Over time, a particular insect species develops the physiological, behavioral, and life-history adaptations necessary to overcome the protective compounds produced by a particular plant lineage. These adaptations enable the insect to eat those plants without being poisoned--but simultaneously prevents the insects from eating plants they did not specialize on.

Monarch butterflies, for example, specialize on milkweeds, which are toxic to most insects. But if we replace milkweeds with an introduced plant like spotted knapweed, the monarch disappears because it cannot develop on spotted knapweed. My student Melissa Richard and I recently conducted a survey that illustrates the degree to which invasions by introduced plants reduce insect populations. When we compared caterpillars in hedgerows invaded by autumn olive, multiflora rose, porcelain berry, and Japanese honeysuckle to caterpillars produced in hedgerows comprised of native plants, the results were stark. In the invaded hedgerows, we found 5 times fewer caterpillar species, 22 times fewer individual caterpillars, and 23 times less caterpillar biomass. This result follows the trend. Our research corresponds to other scientific studies conducted during the past decade that found populations of native insect herbivores are reduced 50-75 percent, on average, by non-native plant invasions, depending on how closely related the introduced plants are to local native plants.

So why is this important? What difference does it make whether we have any insects in our invaded ecosystems? In fact, the size and diversity of local insect populations makes an enormous difference to the complexity--and thus stability--of local food webs, because so many species of animals eat insects to obtain their protein and energy. This includes nearly all terrestrial birds, especially when they are feeding their young. The loss of insects means the collapse of local food webs. This was depressingly evident when we surveyed the invaded hedgerows this summer. "Not a creature was stirring, not even a mouse!" They weren't stirring, because they weren't there. What is lost when introduced plants replace native plants is a myriad of interactions among species: the very interactions that run our ecosystems. If we want to understand the real impact of plant invasions on ecosystems, we need to understand their impact on coevolved food webs and other forms of specialized relationships.

Nature entails a series of specialized relationships, from the resplendent quetzal's uncompromising need for wild avocado fruits in Central America, to the ability of adult bolas spiders to mimic the sex pheromones of moth prey, to the 11 species of native bees that only forage for pollen on goldenrod in New England, and the thousands of insect species that can develop only on particular plant lineages. Specialization that took millions of years to evolve is the rule in nature, and introduced plants, by virtue of being newcomers to local ecosystems, cannot support specialized interactions.

One observation that has complicated our understanding of the ecological impacts of invasive plants is the fact that many birds readily eat the berries of introduced plants. Noting this, land managers deliberately planted berry-makers like autumn olive and bush honeysuckle, thinking these plants were 'good for the birds.' Unfortunately, this conclusion is incorrect.

To determine whether an ecosystem is benefiting from an introduced plant, we must compare what is gained from adding that plant to the community to what is lost. If the losses exceed the gains, the net change in ecosystem health will be negative. When bush honeysuckle invades a forest, for example, it soon replaces most of the native understory, forming a near-monoculture of bush honeysuckle. The young oaks, hickories, maples, and beeches, as well as the viburnums, witch hazels, blueberries, and wood asters, that once comprised a diverse and productive stratum of deciduous forest disappear from invaded areas. However, bush honeysuckle supports almost no insects. You will find that nearly all its leaves are perfect: not a bite has been taken. The insects, particularly caterpillars that contribute the most to avian food webs, cannot thrive and reproduce in such invaded habitats.

The loss of insects matters because 96 percent of terrestrial bird species in North America rear their young exclusively on insects and the spiders that eat insects. And it takes many thousands of insects to nourish a single clutch of birds. Berries in the fall do not help the local birds if those same birds couldn't find enough food to raise their young the previous spring. And don't forget all the spring and fall migrants, birds like warblers and kinglets, that rely on insect protein all year long. They too lose valuable foraging sites when bush honeysuckle invades the local woodlot. Bush honeysuckle supports frugivorous birds for a brief period in the fall, but starves the vast majority of our birds, including the fall frugivores, the rest of the year.

Research has also shown that bush honeysuckle, buckthorn, autumn olive, multiflora rose and other invasives from Asia produce fall berries that are high in sugar, at a time when birds need berries high in fat. Migrants need high-fat berries to fuel their migrations, while winter residents rely on high-fat berries to build fat stores for cold winter nights. Why then do birds eat high-sugar berries when they need fat instead? For two reasons. First, after an invasion, berries from the introduced plant are often the only berries present, because the native berry plants have been outcompeted and forced from the site. But even if native berries are present, birds often eat high-sugar berries from Asian plants for the same reason we humans eat sugar when we shouldn't: it tastes good! So to conclude that we gain ecosystem services from Asian invasives is misleading at best. Yet hardly a week goes by without another gardening article encouraging people to plant bush honeysuckle and other Asian berry-makers because they are good for birds.

It is true that ecosystems are dynamic and always in a state of change. At issue, however, is not whether ecosystems change, but at what rate they change. The rate of natural change in ecosystems is so slow that resident organisms can often adapt to the changes. When human actions change ecosystems rapidly, however, adaptation through natural selection is impossible; either the organism already possesses the genetic capacity to handle the changes that have occurred, or it dies. Trying to limit the amount of non-native plants in our ecosystems is not an attempt to return ecosystems to a pristine state; it is an attempt to maintain the coevolved relationships that run our ecosystems. Such relationships are essential to ecosystem function.

So, are introduced plants bad? The preponderance of evidence suggests that because they have not been here long enough to develop the specialized relationships that define nature, introduced plants can't perform one of the most critical jobs in our ecosystems: they are unable to support large and diverse insect populations. And, if they are "bad" at supporting insects, they are also "bad" at supporting the animals that eat insects and thus the complex food webs that support our ecosystems.

Noted biologist, researcher, naturalist and author, Edward O. Wilson has argued throughout his career that insects are the little things that run the world. Humans cannot survive their loss, so it is in our best interest to consider building and protecting landscapes with the plants that support them best.

Dr. Doug Tallamy is a professor of entomology in the College of Agriculture and Natural Resources at the University of Delaware, and the author of the book Bringing Nature Home: How Native Plants Sustain Wildlife in our Gardens.

Caption: The pandorus sphinx moth specializes on plants in the grape family.

Caption: A white-eyed vireo feeds its young the drab prominent caterpillar, a specialist on Sycamore.

Caption: The double-toothed prominent caterpillar specializes on elm and resembles the edge of an elm leaf.

Caption: A bolas spider draws male moths to her snare by releasing pheromones that mimic a female moth.

Caption: A halictid sweat bee is one of many native bees that rear heir young exclusively on goldenrod pollen.
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Author:Tallamy, Doug
Publication:New York State Conservationist
Geographic Code:1U2NY
Date:Feb 1, 2018
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