by Daniel Weber, Ph.D. University of Wisconsin-Milwaukee
Species come and species go. Throughout geological time, the barriers between plant and animal populations form, are broken down, and reform. Strong, long-lasting barriers can isolate populations and cause other populations of the same species to become distinct. When these barriers are broken the populations mix and new species can invade habitats that used to be off-limits. As humans migrated, they changed their surroundings to fit their perceived cultural and economic requirements. With the increased transport of goods between habitats, there are opportunities for non-native plant and animal species to infiltrate new ecosystems at a rate much faster than that experienced previously, which can cause great ecological damage.
Although species transportation is certainly a natural phenomenon (even to previously barren locations), the accidental or purposeful introduction of non-native species, or exotics, can cause serious disruptions to native ecosystems. Not all invasive species are harmful. In fact, most invaders never establish a foothold in their new environments. Those that do, increase in abundance and have several common attributes. These characteristics make them successful and difficult to eradicate. Critical to their success are characteristics inherent to the species: a fast growth rate, short reproductive cycles, large numbers of offspring, and efficient means of transporting offspring to new locations (called pioneer species).
In addition to the above mentioned species characteristics, successful invasions of non-native species depend upon two ecosystem structure characteristics: corridors of invasion and presence of vulnerable habitats. Corridors of invasion are routes through which species can travel long distances to particular regions. These routes can be physical (e.g., highways, rail lines, logging roads, hiking paths, or urban sprawl) or biological (i.e., humans carrying seeds, eggs, or mature individuals). Sometimes, these two process merge. Commercial transport (boats/ships, trucks, planes, etc.) is certainly a physical method but is an aspect of human (biological) culture and commerce, as well. At any one time, a few thousand species are traveling in ship ballast tanks.
The presence of vulnerable habitats suggests ecological disruption (usually due to human activity) or low species diversity. This allows new species to occupy unfilled niches. For some species, such as horticultural ornamentals, repeated, widespread introductions are necessary before the invading species can “take root.”
Case Studies: Purple Loosestrife and the Asian Horned Beetle
Purple Loosestrife and the Asian Horned Beetle illustrate the process of invasion, the damage done to biological communities, and the limited potential for control. The table below outlines other serious and destructive U.S. examples, albeit a very small sampling of the 4,000+ invasive species of plants and 500 invasive species of animals. The U.S. Office of Technology Assessment has classified many invasive species as noxious (see Table for a few examples and these web sites for more detailed information: http://www.nps.gov/plants/alien/ and http://www.invasivespecies.gov/). The U.S. has exported species to other countries as well, for example the American wine aphid, Colorado beetle, and the fall webworm.
Purple loosestrife, a wetland plant, first appeared in the United States in the early 1800’s and likely used two routes of invasion from its European homeland: the accidental transport of seeds in ship ballasts and purposeful introductions as a medicinal herb for diarrhea, dysentery, bleeding, wounds, ulcers, and sores. Its beautiful late summer inflorescence provided both food for bees and a reason for humans to encourage its spread. By the 1830’s this plant was well established in New England and rapidly spread to the Midwest and Canada through newly constructed canals. Today, it has infested 13 states and is listed as a noxious weed.
As with many successful invaders, purple loosestrife has a high reproductive capacity-a single plant can produce over 2.5 million seeds each year. The plant has a tremendous lifespan and is nearly indestructible. No known native species feed upon it, few plant species are able to successfully compete, neither fire nor herbicides harm its extensive root structure, and cutting it down merely encourages resprouting. Its existence threatens many endangered species of bulrush and turtles. In agricultural regions, the inedible purple loosestrife has caused problems on pasturelands and clogged irrigation canals.
While purple loosestrife is likely to remain as a constituent of American wetlands, its numbers can be controlled. However, this will require the purposeful introduction of insect species native to Europe that naturally feed upon various life stages of loosestrife. For nearly 20 years, researchers have investigated the life history, distribution, impact, and host-specificity of biological control agents. These studies have yielded six potential species of beetles that attack various parts of the plant’s structure and life cycle. While studies have proven the efficacy of these agents to be highly successful, there is great fear about introducing a new species into the environment. Fortunately, to date, these insects seem to be species-specific, i.e., they only feed on purple loosestrife.
The potential solution to purple loosestrife is not echoed for the Asian Horned Beetle. While many Midwesterners complain about the cold winters, those conditions keep many pests, such as the fire ant, from establishing themselves. However, the Asian Horned Beetle, a native of China, does very well in the frosty north. In 1992, inspectors examining solid-wood packing materials from China first noticed the insect, although it did not become established until 1996. The two-inch long grub can bore several feet through the sapwood of a tree, disrupting nutrient travel. Continual infestations will weaken the tree and leave it susceptible to wind damage and disease. The usual victims of this insect are maples, box elders, elms, willows, poplars, and horse chestnuts. This is a major concern for the $20 million/year maple syrup industry of New England and for the many species of flora and fauna that depend upon these trees’ biological community and the microclimatic conditions they create. Entomologists and forest ecologists are very concerned that if this beetle becomes widespread one-fourth of all shade tress in the US could be wiped out.
As trade with China increases, more introductions are likely-over 14 states have found these beetles in wooden crates from China. U.S. Department of Agriculture officials now require all wooden crates to be made of treated lumber; but how are the established insects treated? Although biological control seems appropriate, there are very few species that prey upon this insect-even in China. Predators and parasites have little impact on the population dynamics of the species and sprays are ineffective since the beetles spend much time deep in the tree. Although research into biological controls is ongoing, the only control is to cut down, chip, and burn infested trees. In 2000, this process cost taxpayers of New York (state) $10 million and taxpayers of Chicago $1.4 million. Unfortunately, the devastation to forests, their ecosystems, and residential neighborhoods goes beyond monetary damage.
Invasive species, or biological pollutants, are a microcosm of the consequences of human activity and have bequeathed to us a monumental dilemma of how to protect our native ecosystems.
Examples of Invasive Species Introduced in the U.S. and Their Harmful Effects
Effects of Invasion
– Pointsettia whitefly (Pakistan, Iraq)
– Rainbow smelt (Coastal N. America)
– Africanized Bees (Hybrid: European/African bees)
– Attacks melons, lettuce, broccoli, cauliflower, citrus, grapes, carrots, cabbage
– Caused decreases in herring and extinction of blue pike in Great Lakes
– Potential devastation of crops dependant on honey bee pollination
– Dutch Elm Disease (Via European Bark Beetle)
– Garlic Mustard (Europe)
– Gypsy Moth (Asia)
– Starlings (Europe)
– Kudzu (China)
– Fungus caused local extinctions of American Elm
– Out competes native forest floor flora
– Devastates hardwood forests
– Crowds out native bird species
– Vine covers forest vegetation, decreases timber production
– Leafy Spurge (Eurasia)
– Crowds out native grasses and rangeland plants by using more water and nutrients
– Carp (Asia)
– Sea lamprey (North Atlantic and Lake Ontario)
– Walking catfish (Southeast Asia)
– Spiny Water Flea (Europe)
– Zebra Mussel (Southwest Russia)
– Preys on eggs of other fish, uproots aquatic plants used as food and cover by native species
– Parasitic adult stages major cause of near extinction of Lake Trout from Great Lakes
– Pet store escapee out competes native species for food
– Eats food needed by native fish, spiny tail makes it inedible
– Filters food needed by native species, clogs water intake pipes
– Red Imported Fire Ant (Paraguay)
– Cockroaches (Europe)
– Rats (Europe)
– Buckthorn (Eurasia)
– Major health concern due to severe bites
– Major health concern due to vector for disease
– Major health concern due to vector for disease
– Common ornamental out competes native shrubs and saplings
Originally posted in “On Eagles’ Wings” June 20th 2004