Originally published August, 2011.
There’s some curious reasoning in “The invasive species war,” an “Ideas” piece on plants and attitudes that ran in last Sunday’s Boston Globe. After spending two paragraphs detailing the valiant efforts Charles River Watershed Association volunteers make to remove water chestnuts from the Charles river, writer Leon Neyfakh states:
“The reasons to fight invasive species may be economic, or conservationist, or just practical, but underneath all these efforts is a potent and galvanizing idea: that if we work hard enough to keep foreign species from infiltrating habitats where they might do harm, we can help nature heal from the damage we humans have done to it as a civilization… If we’re going to help restore a more natural environment, how do we decide what in the world is ‘natural’ and what is the result of artificial forces? ”
Those are rather odd statements to make in the context of water chestnuts. The Charles River Watershed Association doesn’t use the word “foreign” in its page on water chestnuts. They use the word “invasive,” which is description of the plants’ behavior; water chestnuts invade new areas, grow more quickly than other aquatic plants, and eliminate their competitors by shading or strangling them out. Is there another polite word for what these plants do? “Total war” seems a bit extreme.
As for “artificial forces”—I’d say that water chestnuts produce damage done by nature to nature. In case you’ve never seen water chestnuts in (very slow) action, here’s a brief description of their charming growth habits from the National Park Service.
“Water chestnut can form dense floating mats, severely limiting light – a critical element of aquatic ecosystems. Once established, it can reduce oxygen levels, increasing the potential for fish kills. It competes with native vegetation and is of little value to waterfowl. Water chestnut infestations limit boating, fishing, swimming and other recreational activities. Further, its sharp fruits, if stepped on, can cause painful wounds.”
In short, left to themselves, water chestnuts can transform the Charles River, the Arlington Reservoir, the Sudbury River, and pretty much any other fresh-water body in Massachusetts into water-chestnut farms. They’ll make is so you can’t boat or swim, fish die off from lack of oxygen, and ducks have nothing to eat–with no help from humans at all. Granted, humans brought them to the U.S. in the first place; these natives of Asia and Europe were first observed near Concord, MA in 1859.
Once he’s done with the water chestnuts (if only the Charles River Watershed Association could be done with water chestnuts!), Neyfakh goes on to describe claims that the division of plants into “native” and “non-native” is roughly the product of anti-immigrant bias. He quotes Hugh Raffles, an anthropologist at the New School, saying, “We choose to designate some plants and animals as native because they fit with the way that we want the landscape to look.”
That’s a fine explanation—if all you do is look at the landscape. If you examine the ecology, though, you’ll find that there are real, important, and measurable ways that non-native plants affect life around them.
According to Douglas Tallamy, chair of the Entomology and Wildlife Ecology department at the University of Delaware, when you replace native plants with non-native plants, you destroy the food chain. Here’s how it works, from Tallamy’s Bringing Nature Home web site.
“With few exceptions, only insect species that have shared a long evolutionary history with a particular plant lineage have developed the physiological adaptations required to digest the chemicals in their host’s leaves. They have specialized over time to eat only the plants sharing those particular chemicals. When we present insects from Pennsylvania with plants that evolved on another continent, chances are those insects will be unable to eat them. We used to think this was good. Kill all insects before they eat our plants! But an insect that cannot eat part of a leaf cannot fulfill its role in the food web. We have planted Kousa dogwood, a species from China that supports no insect herbivores, instead of our native flowering dogwood (Cornus florida) that supports 117 species of moths and butterflies alone.”
Tallamy provides lists of the species that support the most insects. Oaks, black cherries, goldenrods and asters top the list.
But when was the last time you saw goldenrod for sale at Home Depot?
Given that 95% of the land in the United States is now used for cities, suburbs, agriculture, or harvested for timber, those insects aren’t getting a lot of food. Every acre of lawn—and there are more than 40 million acres of lawn in the U.S., according to Tallamy—means that there are fewer leaves that caterpillars can eat. Birds rely on a good supply of caterpillars to feed their chicks; fewer caterpillars means fewer chicks survive. If the bird population is already low to begin with, that could mean extinction.
And frankly, Massachusetts birds need help. According to the North American Breeding Bird Survey, more than one-third of the inland birds surveyed in Massachusetts showed “significant negative trend” between 1966-2009. The population of bobwhites went down by more than ten percent a year. Massachusetts Audubon refers to bobwhites’ “virtual disappearance.”
What’s more, non-native species are getting more and more land every day. According to Massachusetts Audubon’s report Losing Ground, between 1999-2005, 40,000 acres of land in the state were developed; 30,000 acres from former forest, 10,000 acres from farms. Those home and business owners probably aren’t planting choke cherries and swamp white oak to feed the butterflies. They’ll put in lawns with non-native grasses, New Guinea impatiens, hostas, an Asian rhododendron or two and forsythia. None of these plants are native, and none of them will support the local web of life. As far as butterflies and birds are concerned, they’re green plastic.
Studies on “species richness,” or how many different species live in an area, bear Tallamy’s theories out. For example, Michael McKinney’s analysis of studies of urban and suburban areas showed, unsurprisingly, that center cities reduce species richness. Creating suburbs, though, increases the number of types of plants (since you’re adding marigolds to the milkweed), but reduces the number of types of insects and other animals. Presumably, they’re starving.
In short, you can make all the declarations you like about how words like “invasive” and “non-native” express human values, but it won’t make a spicebush swallowtail butterfly eat your petunias. That’s the difference between non-native and native plants. Humans value the imports; everything else prefers their plants home-grown. (Doug Tallamy just shows what’s on the menu.)
And yes, it is a human value to decide that it’s better to have more different species around rather than fewer. What can I say? I’m not a botanical moral relativist. I want enough edible plants around for the swallowtails, and fritillaries, and hairstreaks, and checkerspots, and mourning cloaks, and painted ladies. I’m willing to go to war with Massachusetts’ invasive non-native plants to make sure they do not starve. I think in this case, war is justified. I suspect our local skippers and admirals would agree.