In Louisiana for heron nesting season, I found the image at right after returning from a morning on the water photographing nesting Cattle Egret. Getting out of the boat, I saw a lovely natural scene. Under a green canopy and patchy clouds that softened the mid-morning light, Spanish Moss hung above Nutria sleeping peacefully on a bed or exposed tree roots surrounded by Duckweed.
The calming scene belies, however, the toll this invasive species has taken on Louisiana and many other local ecosystems into which the Nutria has been introduced.
In addition to dislodging native Louisiana species (such as otter, beaver, muskrat, birds, and crustaceans), Nutria have caused significant ecological damage: eroding river and stream banks; destroying marshes (including up to 8,000 acres of marshland in the Blackwater National Wildlife Refuge in Maryland); ruining aquatic vegetation; consuming commercial crops; girdling fruit, deciduous, and other trees; and compromising irrigation systems. Rodents, the Nutria have chewed tires, wooden fascia, and flotation devices, and the burrows they dig have undermined buildings, piers, and other manmade structures. In addition, Nutria carry numerous pathogens and parasites (including nematodes, blood flukes, liver flukes, protozoa, and tapeworms) that can transmit disease to livestock, pets, and humans through contact with the water the Nutria inhabit.
If Nutria were the only invasive species in North America, the danger posed to native wildlife and the ecosystems they inhabit might be quickly contained. But Nutria are only one of hundreds (perhaps thousands) of invasive species currently thriving and creating serious issues in North America.
Some arrived naturally, like Aedes aegypti (the mosquito carrying and transmitting the Zika virus) and the Africanized honeybee, both of which flew here from South America. Other invaders arrived by ship, either as cargo (as did the Asian tiger mosquito and the Asian Citrus Psyllid) or in ship ballast (such as the Asian carp, zebra mussel, and snowflake coral).
But mankind introduced the majority of invasive species into the U.S.: intentionally, foolishly, accidentally, or in ignorance.
Consider just a few of the many examples of invasive-species introductions and their consequences:
Kudzu. A flowering plant in the Pea family, Kudzu “was first introduced to the United States during the Philadelphia Centennial Exposition in 1876 where attendees marveled at the sweet-smelling blooms, large leaves and sturdy vines of what was touted as a great forage plant and ornamental for the backyard. Then, in the 1930s through the 1950s, the Soil Conservation Service promoted it as a great tool for soil erosion control,” according to the Nature Conservancy. The Soil Conservation Service apparently did not know that the Japanese considered it a weed or that the hardy plant “grows at a rate of one foot per day.” In the south, Kudzu has quickly spread to thousands of acres (outstripping attempts to control it with pesticides) and in the process has devastated native vegetation, costing property owners, industry, and local governments millions of dollars in damage and attempts at management.
The African Clawed Frog. From the 1930s to the 1950s, long before you could order a do-it-yourself pregnancy test from Amazon, your doctor (or the lab she used) depended on the African Clawed Frog to determine pregnancy. When alternate methods became available, the African Clawed Frog became expendable, and they were released into the wild. The species survived, multiplied, and easily extended its range in its new habitats in North America (and elsewhere). But recently, scientists discovered that the Frog carries (but is apparently unaffected by) a pathogen—Batrachochytrium dendrobatidis (Bd, for short)—a fungus responsible for the “recent decline or extinction of 200 frog species worldwide” and considered “the worst disease in vertebrate history,” according to the New York Times.
Burmese Pythons. Store owners (and smugglers) bring such exotic species as Lionfish, Burmese Pythons, and, more recently, Nile Crocodiles (which prey on humans) into Florida as part of the pet trade. When these pets grow beyond the cute stage, owners often release them into the Everglades. First observed in the park in the 1980s, Burmese Pythons have proliferated handsomely, and today researchers believe that current population levels in the park range from as few as 30,000 to more than 300,000 pythons. One of the largest snakes in the world, Burmese Pythons can and will eat just about anything that moves, including alligators. As a result of their presence in the Everglades ecosystem, researchers have measured “declines from 88% to 100% in the frequency of raccoon, opossum, bobcat, rabbit, fox, and other mammalian species sightings.” And the invasive python has compromised a program to reintroduce the native marsh rabbit, which it took pythons just eleven months to extirpate. Of additional concern, evidence suggests that the Burmese Python has begun to migrate to Naples, FL, a populous area northwest of the Everglades.
Asian Citrus Psyllid. While one of the world’s largest predatory snakes devastates wildlife in the Everglades, a tiny invasive insect, the Asian Citrus Psyllid, has caused significant damage to the Florida citrus industry. An Asian species that apparently entered the country by ship—either carrying the disease themselves or on imported trees already infected by the disease—the Psyllid destroys groves in two related ways. It damages citrus plants as it feeds on new growth, preventing new shoots from developing properly and ultimately compromising the health of the tree itself. But it causes even more significant damage by spreading a bacterial disease, called Citrus Greening. The bacteria affects the fruit, causing it to turn green and fall to the ground before ripening. “In a 2012 report, University of Florida agricultural analysts concluded that between 2006 and 2012, citrus greening cost Florida’s economy $4.5 billion and 8,000 jobs,” according to Lizette Alvarez of the New York Times. And the Psyllid has enlarged its territory in the U.S.: “The disease has spread to Texas, California and Arizona, where officials are anxiously watching developments in Florida.”
To be considered invasive, a species needn’t be exotic (as the Kudzu, Clawed Frog, Burmese Python, and Psyllid are). Even creatures native to one ecosystem can become invasive by entering a nearby environment it had not previously inhabited and harming it or the creatures native to it.
In fact, not all exotic species entering new environments become invasive. Some fill niches not previously exploited. For example, before spotting the sleeping Nutria, I had been photographing nesting Cattle Egret (Bubulcus ibis), a species native to Asia, Africa, and Europe. Cattle Egret, came to the southern United States as part of an incredible feat of nearly worldwide migration and territory expansion. Its natural migration to South America and the U.S. has proved largely benign and often beneficial (particularly to cattle ranchers) as the birds consume large quantities of insects that live near, annoy, and occasionally infect cattle with disease.
You can read more about Invasive Species in the articles listed here:
• Invasive and Exotic Species of North America
• Aquatic Invasive Species [in Hawaii]
• Invasive Species [National Wildlife Federation]
• Invasive Species [New York Times]
• Invasive Species [Wikipedia]
• Everglades: Nonnative Species [National Park Service]
• Nutria destroy the very fabric of wetlands
• Nutria [in the Chesapeake Bay]
• Cattle Egret [Wikipedia]
• List of invasive species in the Everglades
• List of invasive species in North America
• Kudzu in the United States
• Journey with Nature: Kudzu
• Frog Once Used in Pregnancy Tests Spread Deadly Fungus
• Burmese Pythons in Florida
• Nonnatives – Burmese Python
• Nile Crocodiles Are in Florida, but No Need to Panic
• Citrus Disease With No Cure Is Ravaging Florida Groves
• Asian Citrus Psyllid
• Citrus Greening
• Citrus Greening [University of Florida]
• Nutria [Internet Center for Wildlife Damage Management]
• Nutria [In Washington State]
• Arizona: Bees Pose Menace
• Crayfish to Eat, and to Clean the Water
• Commercial harvesting of crawdads to help Tahoe’s ecosystem
• Cats as Invasive Species? The Less-known Facts about Their Wildlife Impact
• Cats and Foxes Are Killing Off Species in Australia, Study Says
• Fear of Ruin as Disease Takes Hold of Italy’s Olive Trees
• Tiny Flea Reveals the Devestating Costs of Invasive Species
• Importing Both Salamanders and Their Potential Destruction
• Exotic lice causing balding in California deer
• What We Do to Stop Ballast Water Introductions of Invasive Species
• Man-Eating Nile Crocodiles Are Now in Florida — Because Florida
• Zebra mussel