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Invasive Plants

2. Plant Invaders: How Non-native Species Invade & Degrade Natural Areas

by John Randall

Change in plant communities natural. New species move in as the climate changes and as soils build up and become richer or erode and become less fertile. The arrival of new species may be the result of a single catastrophic event like a hurricane, or of gradual change over thousands of years. Humans have vastly accelerated the movement of plants, carrying thousands of species that could not have crossed natural barriers like oceans, mountain ranges and deserts to new areas. Species that have flourished and spread on their own only after people transported them across barriers they could not otherwise surmount are considered non-natives. In many areas these plants have overwhelmed the native plants and animals. Non-native species are responsible for most damaging invasions, but a far smaller number of natives also have invaded and degraded new habitats.

How Plants Get Around

Unlike animals, plants are sessile, meaning individuals can't move from place to place. However, many plants have mechanisms that enable their seeds, spores or other propagules to move and colonize new areas far from the parent plant. Some seeds are shot explosively from their pods, some are carried by wind or water. Others are caught and carried on the fur or feathers of passing animals. Animals eat the nourishing fruits of some plants and may deposit the seeds, unharmed, at distant sites.

The distances seeds travel can be astonishing. Most native Hawaiian plants evolved from colonists that crossed thousands of miles of ocean from the Americas or Asia. Following the retreat of the glaciers roughly 10,000 years ago, many species moved northward, colonizing the newly available ground. Among them were several oak species which migrated, probably with the help of hungry blue jays, from the southeastern U.S. to southern Canada, an average of 380 yards each year.

But no animal surpasses humans in dispersing plants, and we've been doing it for a long time. The "Iceman," whose 5,200-year-old corpse was recently discovered on a glacier on the Italian-Austrian r, had stuffed grasses into his shoes to keep his feet warm and was carrying a sloe berry. Human transport of plants and animals—and disease organisms—increased exponentially as our transportation technology developed. People transported and introduced some non-native species intentionally, for food, fiber, medicine, ornament or scientific curiosity. European species were advertised for sale in the U.S. as early as the Colonial period, and by the late 1800s seed catalogs listed hundreds of non-native ornamentals. Seeds of other plants were introduced accidentally in sacks of seed grain, wool or cotton, in mud stuck to machinery or in ship ballast. Human activities like farming, irrigation, forestry and mining have made it easier for these non-native species to become established by removing native vegetation, disturbing the soil and altering the availability of water and nutrients.

Intentional and unintentional introductions continue at a rapid pace today, accelerated by the recent rise in international trade. For example, before humans arrived in Hawaii, new plant species are believed to have become established at the rate of one every 10,000 years. For the past 200 years, we have been bringing new species to the islands at a rate one million times higher.

The Damage They Do

In their new environments, a relatively small number of non-native species have reproduced and spread without further human assistance, and only a fraction of these have become invasive pests in their new habitats. Yet the economic damage to agriculture and the environment caused by a few hundred or so of these plant invaders is staggering.

Invasives reproduce rapidly and can form stands that exclude nearly all other plants. In the process they damage natural areas, altering ecosystem processes, displacing native species, hybridizing with natives and changing their genetic makeup, and supporting other non-native plants, animals and pathogens.

Plants that change fundamental ecosystem processes such as the frequency of wildfires, the availability of water or nutrients and the rate of soil erosion cause the severest problems. They "change the rules of the game" and many native plants and animals can't compete. Melaleuca (Melaleuca quinquenervia), introduced for forestry and as an ornamental, has invaded herbaceous wetlands in south Florida, forming solid stands and crowding out nearly all other species, moderating soil temperatures under their deep shade and drawing down the water table. The species has converted at least 450,000 acres of marsh into swamp forest, where native herbaceous plants can't survive. In the Southwest, tamarisks (Tamarix species) invade wetland and streamside areas and also likely lower water tables, shrinking or eliminating the surface water habitats of native plants and animals. Dense stands of tamarisk trap more sediments than those of native vegetation, altering the shape, carrying capacity and flooding cycle of watercourses.

Invaders that don't change basic ecosystem processes cause other problems. In forested areas, trees such as Norway maple (Acer platanoides) grow into the canopy, as do vines like Japanese honeysuckle (Lonicera japonica), where they shade out or topple trees; shrubs like the bush honeysuckles (Lonicera species) and buckthorns (Rhamnus cathartica and R. frangula) take over the mid-story, while herbaceous species such as garlic mustard (Alliaria petiolata) colonize and dominate the groundlayer. Prairies and other grasslands across the continent are severely infested by non-native species, many of them serious crop and rangeland pests like leafy spurge (Euphorbia esula) and yellow starthistle (Centaurea solstitialis). In wetlands in the northern third of the U.S. and southern Canada, purple loosestrife Lythrum salicaria) forms large, dense stands, eliminating the open water areas that waterfowl require and elsewhere displacing native plants that feed and shelter wildlife.

Some invaders hybridize with natives and with time could eliminate native genetic strains. Non-native white mulberry (Morus alba), now widespread in eastern North America, hybridizes with the native red mulberry (Morus rubra).

Some invasives reduce or eliminate the very species and communities that national parks and nature preserves were set aside to protect. Rare species appear to be particularly vulnerable to the changes wrought by non-natives. For example, 30 of California's 53 federally listed endangered plant species are threatened by non-native invaders.

Invasive plants can also promote invasions by non-native animals. Leaf litter from the Chinese tallow tree (Sapium sebiferum), a pest in bottomland forests and swamps along the U.S. Gulf and lower Atlantic coasts, alters rates of nutrient cycling, boosting populations of the non-native isopod Armadillium vulgare while depressing populations of native soil invertebrates.

The Most Vulnerable Areas

One question that has long intrigued scientists is why some areas appear to be more prone to invasion than others. Many hypotheses have been advanced but there are few solid answers. Nearly half of the plant species growing wild in Hawaii, which like many remote islands in temperate and tropical areas is vulnerable to invasion, are non-native; New York, with 36 percent, has the highest percentage of non-native species among the mainland states. One explanation for the vulnerability of remote islands is that they had no large native herbivores, and so native plants did not evolve spines or foul-tasting chemicals that would have made them unpalatable to the pigs, cattle, sheep and other grazers brought by humans. Islands and peninsulas such as southern Florida may also be vulnerable because they have relatively low numbers of native species or are missing certain distinctive plant groups. Some, but not all, researchers believe that this leaves "empty niches" that new arrivals can exploit. For example in Hawaii, where no mangroves are native, a species introduced from Florida in 1902 has become a nasty pest.

What Makes a Plant Invasive?

Researchers are also working to discover why some non-natives become invasive while others don't. According to one theory, some probably succeed because they aren't held in check by the predators and parasites that controlled their numbers in their original lands. And because they're not under attack, they may be able to redirect energy they would have invested in producing the toxic chemicals or spines to growth and reproduction. Native plants, by contrast, cannot lower their defenses or they will be attacked by the pests that evolved along with them. Two major studies found the best predictor was whether a plant was known to be invasive in another part of the world.

Recent work has found that species that share some or all of the following characteristics are most likely to be invasive:

Some evidence also indicates the following characteristics may be predictors of a species's invasiveness:

Preliminary data from one interesting study shows that invasive species are likely to have relatively small amounts of DNA in their cell nuclei. Apparently, the cells in these plants are able to divide and multiply more quickly and consequently the entire plant can grow more rapidly than species with higher cellular DNA content. This gives them a leg up in disturbed sites.

Many decades often pass between the first introduction of a plant and its eventual rapid spread. As far as we know, Atlantic cord grass (Spartina alterniflora) was present in small patches in a few spots on the Pacific Coast for 50 years or more before it began to spread aggressively. In other words, species that rarely spread today may turn out to be troublesome 40 or more years from now. And that's why it's so important that we find some way to determine which species are most likely to become invasive. Then we will be able to act while their populations are still small—and controllable.

John M. Randall is invasive weed specialist at The Nature Conservancy, where he provides leadership, technical support and advice on weed control to Nature Conservancy preserves nationawide. He is a founding member, past president and current board member of the California Exotic Pest Plant Council, and a board member of the National Association of Exotic Pest Plant Councils. He holds a PhD in ecology from the University of California, Davis.