Looking at Plant Invasions in the Past
Richard Enser, Coordinator
Rhode Island Natural Heritage Program
Richard Enser works in the Department of Environmental Management as the co-ordinator
of RINHP, which protects the state's native biological diversity through a program
of inventory, environmental impact review, restoration, and conservation planning.
Enser is also president of the Rhode Island Natural History Survey, a consortium
of the state's ecologists and naturalists who work to advance scientific knowledge
of Rhode Island's biota, ecological communities, and environmental resources.
"It is a curious fact that most of our weeds have a European
origin. Indeed, many of them grow here more vigorously than in the Old World,
their native home... The weaker native herbage is often crowded out by them."
---William Whitman Bailey
Among Rhode Island Wildflowers
W.W. Bailey, Brown University's first professor of botany, wrote this description of "weeds" in 1895, but the landscape upon which these plants proliferated had been prepared for them by the destructive forces of man many years before his observations. Today, although we are mostly concerned with those alien plants that invade natural habitats at the expense of native species, it is equally important to consider how historical land-use practices served to create an environment that supported their growth and spread.
Prior to the appearance of Europeans, Native Americans managed the land principally through the use of fire; but the degree to which the land was cleared accelerated once the first colonies were established in the 1600s. Clearing was at first concentrated around small settlements near the coast, but soon the demand for fuel, building materials, and farmland increased. By the mid-1700s, a large percentage of the region's virgin habitats had been decimated. Before European settlement, forest clearings and other open sites created by natural forces, such as hurricanes or fire, revegetated relatively quickly with native species through the process of natural succession. However, the more destructive forces wrought by the colonials resulted in disturbed lands readily populated by introduced weedy plants that held a competitive edge over their North American relatives. These new arrivals included some species that were purposely introduced for utilitarian needs (culinary herbs, medicines, dyes) such as tansy (Tanacetum vulgare), coltsfoot (Tussilago farfara), and timothy (Phleum pratense). Other plants appeared when their seeds or propagules were transported across the ocean in ships' cargo or ballast. They persist today as some of our most familiar weeds including common dandelion (Taraxacum officinale), chicory (Chicorum intybus), and common plantain (Plantago major) also known as whiteman's foot.
Introduced weeds began to retreat from interior locations during the 1800s when farmers began migrating to more fertile lands in the Midwest, and many of New England's abandoned farms were reclaimed by the native forest. By 1930, at least 70 percent of Rhode Island's landscape had been reforested. However, in coastal communities the land remained open as housing and infrastructure development kept pace with a population concentrating in the urban centers, and military installations were established on the islands and shores of Narragansett Bay. These highly altered areas served as fertile ground for the spread of a new suite of plants initially introduced from Europe and Asia during the late 1800s for ornamental purposes, but later found desirable for other uses.
Among this group are plants considered to be the most scurrilous exotic invaders: autumn olive (Elaeagnus umbellata), multiflora rose (Rosa multiflora), European bittersweet (Celastrus orbiculatus), Japanese barberry (Berberus thunbergii), shiny buckthorn (Rhamnus cathartica), Japanese honeysuckle (Lonicera japonica), morrow honeysuckle (Lonicera morrowii) white poplar (Populus alba), Japanese black pine (Pinus thunbergii), and winged euonymus (Euonymus alata). Selected for controlling erosion, providing windbreaks, and for wildlife food and cover, most were touted by state and federal agencies as a quick fix because they were easily established and quickly propagated to provide the desired effect. These same characteristics facilitated the escape of these plants from cultivation to dominate the vegetation of disturbed sites. Some of these plants, especially Japanese barberry and winged euonymus, also have the capacity to invade undisturbed natural areas.
One plant that clearly illustrates the success and ecological impact of invasives is autumn olive, which many Rhode Islanders call Russian olive. True Russian olive is a different species which has not naturalized in Rhode Island. Native to Europe and western Asia, this shrub was introduced to North America in the early 1900s and promoted for windbreaks and erosion control. Capable of tolerating drought and poor soil conditions, it was planted in medians and along rights-of-way of newly constructed highways. Today, it is especially prevalent along many Rhode Island roads. In addition, autumn olive was, until recently, promoted by conservation agencies as a wildlife food plant and distributed to landowners for backyard enhancement projects. Although it reproduces vegetatively, autumn olive is primarily spread by birds attracted to the abundant clusters of fruit that persist through the winter. During the early 1970s, several University of Rhode Island students surveyed wildlife using ornamental vegetation planted along Route 1 in South Kingstown and Charlestown. Many birds were recorded feeding on autumn olive, most notably the European starling which is, ironically, another exotic invasive species. (All North American starlings are derived from a flock of about 100 birds released in 1890 in Central Park, New York City, as part of an effort to introduce all of the birds mentioned in the writings of William Shakespeare.) Flocks of starlings numbering in the hundreds were commonly observed descending noisily into dense patches of autumn olive in the median to quickly consume the fruit, then flying en masse to nearby fields and forest openings to deposit the undigested seeds. The results of this bird-assisted dispersal are clearly evident in the monotypic stands of autumn olive that have developed throughout South County. The individual shrubs are so densely packed that understory vegetation is almost completely eliminated.
Although the visual impact of invasives is readily seen as these plants overwhelm roadsides, old fields, and even climb up and over abandoned buildings, the ecological consequences of this invasion are difficult to quantify. The Natural Heritage Program has documented the extirpation of more than 50 species of native Rhode Island plants since botanical record keeping began in the early 1800s. No single invading alien has been directly implicated in this demise, generally because exotics populate sites that are already disturbed. But some natural communities have suffered from direct habitat alteration and subsequent proliferation of invasives, most notably the estuaries and tidal marshes of Narragansett Bay and the salt ponds along the south shore. It is estimated that 50 percent of the original coastal wetlands of Rhode Island have been lost to filling and dredging, and many more acres have been degraded by the restriction of tidal flow which has benefited the spread of the common reed, Phragmites australis. Biologists agree that Phragmites is indigenous to North America, although it has been suggested that a more invasive genetic variety may have been introduced from the Old World. Aside from the uncertainty of its origin, it is clear that Phragmites is a primary example in Rhode Island of an invasive species that has usurped natural habitats and severely impacted the native species inhabiting them. Some of these wetlands were formerly extensive cattail stands that provided nesting habitat for a variety of marsh-nesting birds, such as rails and bitterns. Today they are carpeted with a choking growth of Phragmites that provides minimal wildlife habitat. The brackish upper edges of tidal marshes have also been affected. These areas supported a diverse flora adapted to the narrow salinity range in this zone, a feature that also appeals to Phragmites. Native plants lost in Rhode Island from the brackish zone include such specialized species as seaside buttercup (Ranunculus cymbalaria), saltpond pennywort (Hydrocotyle verticillata), and arrowgrass (Triglochin palustre).
Increased awareness of the history of alteration in the coastal zone has spurred widespread interest in restoring these habitats. Efforts such as the recent restoration of tidal flow at the Galilee Wildlife Sanctuary will help refurbish natural systems along the Rhode Island coast and foster the return of some species that were diminished by ill-advised past practices.