History of plant invasions

Humans have always traveled with plants. Plants have hitch-hiked along with human migration and they have been intentionally introduced to new places. New migrants bring along plants that supply them with the food they like and the environment that they're comfortable in. The pace of plant introductions has paralleled that of the technologies of human transportation because plants are sessile and are completely dependent on outside forces to transport their offspring. Without human-aid, plants use wind, water, animal, and, in a few cases, their own limited mechanical power (Impatiens capensis, or touch-me-not, shoots its seeds off). But as humans breakdown the barriers of world trade, they also break down the natural barriers that maintain plant ranges (Turning Point Project 1999, Muth and Hamburg 1998, D'Antonio and Vitousek 1992).

In 1824, Nathaniel Bagshaw Ward, a surgeon from White Chapel, Egland, discovered a small plant growing in a glass jar in which he'd left a caterpillar to pupate (Orlean, 1998). His discovery sparked his invention of the Wardian Case, a glass container that could be used to safely ship plants across the globe. He built the first prototype in 1834 and used it to ship ferns from England to New South Wales. The trip took six months, but the ferns thrived along the way. Ward wrote an article in 1839 on his cases, and then a book in 1842 titled On the growth of plants in closely glazed cases. Ward's invention sparked a boom in the plant exploration business. His cases pushed the survival rate of shipped plants up from 0.1% to 90% and meant that explorers no longer had to send huge shipments in order to guarantee a significant number of survivors would arrive in the hands of their patrons. But the shipments still burgeoned. Exotic plants spread the globe like never before and the dissolution of biogeographic boundaries became even more rapid.

The allure of exotic plants has sent people all over the globe on dangerous adventures motivated by greed and beauty (Orlean 1998, Klinkenbourg 1999). Plant introductions have traditionally been the business of university arboretums, public gardens, and public agencies, but these groups tend to do self-serving research without sharing their plants by selling them to the public. Because of this, commercial nurseries have had a long-standing rivalry with public gardens for discovering new plants. Nurseries felt that they were battling exclusivity while the gardens felt they were fighting the noble cause of science against trade. Today, however, this has changed, and plant explorations often are shared by nurseries and public gardens (Kilnkenbourg 1999). The search for successful, beautiful plants has often lead to the introduction of very successful invaders. According to the Turning Point Project (1999) and Janet Marinelli (1996), at least half of all of the plant invasions in the United States have been by garden plants. Sarah Reichard and Faith Campbell (1996) found that of 235 woody invasive plants in the United States, 85% were originally introduced as ornamental or landscape plants, while an additional 14% were introduced as agriculture plants.

Unintended arrivals accompany cultivation, commerce, tourism, travel – contaminants of bulk commoditiies, packing materials, shipping containers, ship’s ballasts, contaminants of seed shipments, stow away on vehicles, diseased fishing and plant stocks (Mack 1990, Reichard 1997, OTA 1993). Intentional introductions are often for horticultural/ornamental purposes and sometimes for agicultural ones.

Some horticultural invasive plants have been promoted by government agencies for erosion control – some still are being promoted. One of my survey respondents sent a brochure to me from the Livingston Conservation District in Michigan. For the Spring 2000 plant sale it offered Eleaegnus (Russian olive), Vinca minor (periwinkle), and Lonicera tartarica (Tartarian honeysuckle) – all plants considered invasives by experts (see Faith Campbell's list of invasive plants). According to the Office of Technology Assessment (1993), Pueraria lobata (kudzu) was first introduced as a forage crop and was later promoted for erosion control. Cytisus scoparius (scotch broom) is still used as a highway planting in western states. It was originally introduced as an ornamental and then promoted by the US Soil Conservation Service for erosion control. It has now spread to over 500,000 acres in the state of California, where it displaces native flora and fauna (OTA 1993). Carpobrotus (highway iceplant) is still widely used along the freeways of Southern California.

Seed catalogs have been significant vehicles for the spread of horticultural invasive plants within the United States since after the Civil War (Mack 1990) In some cases, historical analysis of seed catalogs has shown that particular invasive plants were introduced much earlier than previously thought (Mack 1991). Eichornia crassipes (water hyacinth), Lonicera japonica (Japanese honeysuckle), and Gysophila paniculata (baby's breath) are just three serious invasive plants that appeared in seed catalogs in the late nineteenth century (Mack 1990). Eichornia crassipes is invasive throughout the tropical regions of the world, beyond its native region in South America - it clogs waterways, inhibiting navigation, and creating habitat for malaria and encephalitis carrying insects (Mack 1990). Lonicera japonica shades out forest understories and prevents forest succession, homogenizing the invaded landscape (Clark et al 1998). Gysophila paniculata has deep tap roots that stabilize dunes around the great lake, destroying the shifting-dune habitat for the federally threatened thistle Cirsium pitcheri (pitcher's or dune thistle) (Randall and Marinelli 1996).

The nursery industry is still an important vector for the spread of invasive plants throughout the United States. Of the 454 plants listed in Campbell's list of "plants that hog the garden" (1999), 292 – 60% - were still being sold in nurseries. Of the 16 invasive plants that I used in my survey, 11 were sold or planted by at least one respondent. Of the 77 respondents, 40 admitted to selling or planting one of the invasive plants in question (Q11).

While natural invasions have always occurred, large-scale human-induced plant invasions, in the United States, are relatively recent and they are much more common (OTA 1993, Invasive Plants Fact Book, Pimentel 1999). According to Reichard and White (2000), "As with so many other 'natural' environmental processes, humans change the scale…there is no way that the rate of introductions carried out by humans over the past several hundred years around the world now can be considered to be close to the 'natural' rate of dispersal."