Saturday, August 25, 2007

Aliens among us


There's a great early scene in Orson Welles' 1939 broadcast of War of the Worlds : a policeman quietly holds up a white flag of truce to the Martian pod. The commentator suggests that the Martians probably don't recognize the white flag as a sign of defeat. The creatures in the pod respond by sending out an enormous ball of fire to the dry field across the road. The field quickly erupts into flames and the policeman stands there, dumbfounded, holding a handkerchief tied to stick.

Land managers across America resemble the policeman, defeated, holding backpack sprayers of herbicide while nonnative invasive plants quickly rip across the landscape with no end of their invasion in sight. Invasive plants are those which are not native to a certain region but nonetheless colonize ecosystems. Some invasives, like honeysuckle, the mimosa tree and multiflora rose, were brought over to the New World as ornamental plants. They escaped cultivation and now choke forests from Louisiana to Illinois. Others, like sericea lespedeza, were introduced by my sister agency for erosion control. Now, Missouri agencies spend literally thousands of dollars spraying sericea. The U.S. Forest Service names 33 nonnative invasive plants whose populations have dramatically altered southeastern forests. The upland sand forests of Crowley's Ridge are not exempt. 13 of the 33 species have a stronghold on Missouri's Crowley's Ridge.

Invasives move in primarily through bird and mammal seed dispersal. Automobile tires are another important vector for the spread of invasives, which is why roadsides are more often than not thick with invasive species. When invasive plants move into an area, they choke out native plants by using the nutrients, moisture and light that the natives have depended on for a millenia. Honeysuckle, for example, begins to leaf out in March, weeks before most spring ephemeral wildflowers poke their first leaves out of the soil. Honeysuckle leaves persist through November, thereby blocking light throughout the entire growing season.

Invasive plants dramatically lower the biodiversity of an area. Aside from development or conversion to agriculture, invasive plants pose the greatest threat to biodiversity in America. Invasives often grow quickly, creating thick mats along roadsides, in prairies, covering trees in woodlands, savannas, glades, forests and streambanks. Essentially, invasive plants have adapted to thrive in any habitat that supports plant life.

As the biodiveristy of an area decreases, so, too, does wildlife habitat. Grassland birds, in particular, are negatively impacted by the pervasive, impenetrable swathes of sericea; the seeds provide no nutrients, the plants supplant grasses and forbs that grassland birds require for nest building, and young chicks are unable to navigate through sericea's thick stalks.

Bush honeysuckle (pictured), a bushy version of the vine that is literally choking the entire state of Arkansas and Crowley's Ridge, produces sugary bright red berries in the fall. The fruits have no protein and no fat, the two nutrients most important to wintering bird populations. A recent study has found that the feather color of certain birds(cedar waxwings, yellow breasted chats, common yellowthroats, for example)changes if the birds have subsisted on a diet of bush honeysuckle berries. Rather than yellow feathers, the birds develop orange feathers. A recent photograph of a cedar waxwing reveals jagged orange-tipped tail feathers (instead of smooth yellow tipped feathers). The jagged edge indicates that they developed under intense stress. Breeding behavior of these traditionally yellow birds has been interrupted by the honeysuckle diet; feather color is an extremely important aspect of bond pairing.

To combat nonnative invasive plants, an integrated approach is required. Manual removal can slow the spread of certain species, but for most, regular applications of herbicide and prescribed fire is required. Every plant has a particular prescription for removal. Small infestations of vining honeysuckle can be eradicated with a series of successive spring fires. Bush honeysuckle, on the other hand, must be cut down and stump-treated with a glyphosate-based chemical for several years in a row to stop resprouting. Sericea, the scourge of Missouri's prairies and savannas, actually likes fire. Infestations that are burned grow back stronger every year. Repeated herbicide treatments seem to work on sericea, but great care must be taken with the application; sericea leaves close up if they sense a strong chemical concentration. Because most herbicides are absorbed through the leaves, sericea's defense mechanism can block the treatment altogether.

An informal poll conducted in Jefferson City last year determined how much time land managers in one agency spent treating invasive species in the course of a year. Over 38,000 man hours and over $242,000 were spent spraying, disking, chainsawing, stump treating, and manually pulling invasive species in the public lands owned by one agency in Missouri. States like Arkansas and Louisiana have been decidedly passive towards invasive control; The Nature Conservancy chapters in both states are responsible for any aggressive eradication projects. As neighboring states refuse to treat their nonnative invasives, Missouri will always be threatened.

(Do your part. Remove your honeysuckle, autumn olives, mimosas, wintercreeper and trailing vincas. Don't buy purple loosestrife, burning bush, Japanese privets, English ivy, nonnative wisteria. Contact your state conservation agency about which native plants can be used in your landscape.)

No comments: