Biodiversity and Conservation: A Hypertext Book by Peter J. Bryant  

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EXOTIC SPECIES

A major contributor to depletion and extinction, second only to habitat loss, is the unnatural introduction of species into new environments. Species have sometimes invaded new habitats naturally (e.g. when land bridges have become established) but human exploration and colonization has dramatically increased the spread of species. Whenever man has settled far away from home, he has intentionally introduced his familiar animals and plants.  Many other species (e.g. rats) have been accidentally transported around the world.  These forms that have been transplanted as a result of human actions are called exotic, alien, introduced or nonindigenous species.  

The first cases in the U.S. were from European explorers, who often released goats and pigs so that later colonizers had an abundant source of familiar animal protein, and colonizers then brought more of the same.

Some of our most abundant wild animals and plants, especially those that do well in urban or disturbed areas, are introduced species that have become established. For example, the starling, cabbage-white butterfly, eucalyptus tree, mustard, many grasses, etc. Most insect and plant pests are exotic species. It is estimated that at least 4,000 exotic plant and 2,300 exotic animal species are now established in the United States.

Many exotics have disastrous effects on native flora and fauna. They often leave behind the factors that have evolved with them and that control their population and spread. In their new habitat there may be fewer predators or diseases, so their populations grow out of control.  They are then called invasive exotics. Prey organisms may not have evolved defense mechanisms and native species may not compete successfully for space or food, so are often pushed to extinction. Since exotic species are self-perpetuating, they can be more permanent problems than other threats to biodiversity including overexploitation and habitat loss. Exotics are a factor contributing to the endangered or threatened status of 42% of animals and plants on the U.S. endangered species list.

The spread of exotics replaces healthy, diverse ecosystems with biologically impoverished, homogeneous landscapes. For example, places with a Mediterranean climate in southern Australia, the U.S. west coast, Chile and South Africa previously had few plant species in common (although they did show many examples of convergent evolution, leading to similar landscapes). They now share hundreds of weedy exotic species, mainly from the Mediterranean region.  Exotic plants often develop a monoculture landscape in which one species completely or almost completely predominates. 

Introduced Species in U.S. Coastal Waters

Plants

Most exotic plants causing environmental problems today were introduced accidentally, either as the result of "hitch-hiking" seeds or of "escaped" ornamental plants.  There was once, however,  a United States Office of Plant Introduction, which claimed to have introduced nearly 200,000 species and varieties of plants from all over the world.

CNPS EXOTIC PEST PLANTS PAGE
California Noxious Weed Control Projects Inventory - Information Center for the Environment

Exotic, unwanted plants in agriculture are called weeds.  The national losses in agricultural production plus the costs of their control were recently estimated at over $6 billion annually.  Many insect species accompanied these plant introductions and subsequently became pests. Between 1800 and 1980 the number of introduced insect species in the United States grew from about 36 to more than 1200.

Black mustard, introduced by the Spanish missionaries throughout coastal California, now colors the hillsides yellow in springtime.

Large areas of California are dominated by Argentinean pampas grass, and in many places it is choking out native species.

Another very common introduced plant is tumbleweed, otherwise known as Russian thistle. This native of the Ural mountains in Russia was introduced in 1877 in South Dakota, apparently as a contaminant in flax seed imported by Ukrainian farmers.  It quickly spread, partly because it can survive drought very well and needs practically no soil.  It became a serious agricultural pest.

Cattle grazing in Southern California encourages spread of Artichoke thistle.

Giant Reed (Arundo donax).  This huge, bamboo-like reed from the Mediterranean region was brought to Southern California by the Spanish missionaries in the 1700's for use in construction. Now it covers thousands of acres of riverbanks, eliminating cottonwoods and other native plants, and providing natural habitat for rats but very little else. It grows 4-5 inches per day and reaches a height of 25 feet. It is estimated it will cost $20 million to remove it.

Purple loosestrife (Lythrum salicaria) is a beautiful but prolific plant introduced into the United States in the 1800s, probably for use as a medicinal herb and ornamental plant.   It has spread rapidly and is now found in all the contiguous states except Florida and in all Canadian provinces, being especially suited for growth in freshwater marshes and stream margins.  Once established in those areas, loosestrife outcompetes and eliminates native plant species.  It covers approximately 400,000 acres and costs about $45 million a year in control costs and lost forage.  The U.S. Fish and Wildlife Service has developed a Purple Loosestrife Management Plan to deal with the problem.  A combination of traditional approaches, such as flooding, with novel biological control techniques involving loosestrife-specific, leaf-feeding beetles are being tested.

Kudzu vine, a Japanese species introduced in 1876 to shade porches on southern mansions and widely planted in the 1940's to control erosion, grows so rapidly (up to one foot per day) and luxuriously that it kills forests by entirely covering trees and shrubs. It covers between 2 and 4 million acres in the southern U.S., and costs an estimated $50 million yearly in lost farm and timber production.

Water hyacinth, a South American floating plant, is clogging 2 million acres of lakes and waterways, and displacing native vegetation. Florida, Louisiana and Texas spend $11 million each year trying to restrict its growth. Manatees help by feeding on it.

Invasion of water systems by water hyacinth is a global problem. Enormous mats of the plant are growing along the shores of Africa's Lake Victoria, where the vegetation blocks the intakes of water treatment facilities and power plants, and interferes with access to boat docks.  Kenya's government is contracting with an American company to try to remove the weed.   The same weed is overwhelming Lake Malawi, home to the greatest number of fish species of any lake in the world.

An invasive strain of the marine alga Caulerpa taxifolia  ("killer algae") appeared in the Mediterranean Sea in 1984, and is now rapidly covering thousands of acres of sea bed.  It smothers and kills all native vegetation, and the animals that depended on that vegetation are displaced or die off. In the summer of 2000, the alga was discovered in Agua Hedionda Lagoon (San Diego County) and Huntington Harbor (Orange County). This plant could have a disastrous impact on the local ecology if it is not quickly eradicated.  Legislation is being introduced to ban importation of the plant, which is used as a saltwater aquarium ornamental.

Great Plains Exotic Plants Bibliography

Invertebrates

Seastars invade Australian waters

Some of the most common insects in this country are introduced species:

The cabbage white butterfly was introduced from Europe.

The Argentine ant, which is now the most common ant in many urban areas of this country, was first noticed in 1891, in Louisiana. It probably got there on ships bringing coffee from Brazil. In less than fifty years it spread to many of the southern States, and it reached California by 1905. This ant species drives out native ants entirely wherever it becomes established. In coastal regions of California, the ant is contributing to a sharp decline in the population of coastal horned lizards.  It has also spread rapidly in Australia and South Africa, eliminating native ants in those countries as well. In the slide show it is shown tending aphids, a plant pest. The ants move the aphids to fresh parts of the plant, and they protect them from attack by other insects. In return they feed on a secretion made by the aphid, called honeydew.

Two particularly nasty exotic insects have recently invaded Southern California:

The imported fire ant, a vicious stinging insect from Brazil, entered the U.S. in the 1930's through Mobile, Alabama, probably in soil used for ships' ballast. They have been spreading ever since and have now reached California. They make colonies of about 300,000 individuals, each one mostly underground and about 6 feet deep and 2 feet diameter, topped by a mound of soil.  There may be 200 or more mounds and 40 million ants per acre.  They have a very painful sting and they attack in large numbers, but more importantly for biodiversity they eat just about anything. In some areas these ants have killed off 40% of native insect species.  Biological control methods are being tried.

The "killer bee," or Africanized honeybee, earned this fearsome nickname because of its aggressive, though not necessarily lethal nature. It's venom is not more toxic than that of its European counterpart, it cannot sting more than once, and it does not attack or hunt down prey without provocation.  Like normal honeybees, it stings in self defense or in defense of the colony and dies in the process.  However, it does react more quickly, it attacks in swarms, it pursues its target for a longer time and it takes longer to calm down.  The bees were accidentally released in the mid-1950s, when a swarm escaped from a hybridization research project in Brazil.  The bees have since been moving northward at about 2-300 miles per year.  Five people are known to have died from the killer bee strings since 1990.  They reached Los Angeles in 1998.  The bees have colonized more than 34,000 square miles of Southern California, including Orange, Imperial, Riverside and San Bernardino counties and parts of San Diego County. 

A very destructive introduction from Australia was the cottony cushion scale, which appeared in California citrus orchards in 1868 and almost wiped out that industry. It was completely controlled in a couple of years by the introduction of one of its Australian enemies, the vedalia. This is probably the most successful example of biological control, and it has also worked in many other countries where this pest became established. But efforts to control other pests by introducing their enemies have been less successful.

The gypsy moth, was brought from France in 1869 by an entomologist who hoped to interbreed them with silkmoths to establish a new textile industry. They escaped and established a colony that invaded all of the New England states, defoliating trees of many different kinds. In 1953, state and federal officials began spraying DDT to try to stop the spread of these moths. The spraying was not stopped until after it had been shown that there was so much DDT in the soil and plants that detectable levels began showing up in cows' milk.

A new pest in this country is the wood-boring Asian longhorn beetle, (called the starry sky beetle in its native China because of its markings). It was first discovered on trees in Brooklyn, N.Y., in October 1996 but has since been found in shipments of forest products in California, South Carolina, and Canada. It is difficult to detect because the larvae can be inside deep burrows in the wood.  It could cause millions of dollars worth of damage to ornamental trees and to the maple syrup and lumber industries in the United States.
Aliens Are Boring; The Dark Side of Trade

Zebra mussel. In the Great Lakes, the 1.5 inch long zebra mussel from the Caspian Sea was introduced accidentally in ballast water from ships in about 1986. It was first seen in 1988, and is now extremely abundant and present in all the Great Lakes as well as several river systems and lakes in the eastern U.S. It clogs major water pipes, smothers populations of native clams, and encrusts the spawning grounds of fish. It is also an extremely efficient consumer of plankton, removing the food supply for native mussels and fish. Not many native animals feed on zebra mussels, so its population is thriving. A 1997 distribution map shows zebra mussels have spread to 19 states in less than 10 years. Two species of goby from the Caspian Sea have been introduced to feed on the mussels. Passage of the Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 called for the establishment of a national nonindigenous program to control and reduce the risk of further introductions of aquatic nuisance species. This legislation specifically addressed the zebra mussel problem. 

BALLAST Case | California may be next! Zebra Mussels | Zebra Mussel (Dreissena polymorpha)

A similar problem has arisen with the Asian clam and other species introduced into San Francisco Bay. The clam was introduced in about 1986, and since then it has become one of the most abundant organisms in the entire bay. In some places it literally coats the seabed with populations of up to 20,000 per square meter. It depletes small plankton species and thereby reduces the food supply for other organisms in the food chain (note that this is the opposite problem to that faced in the Chesapeake Bay as a result of depletion of oyster beds).

Fish

Exotic fish have been introduced intentionally all over the world to support either recreational or commercial fisheries, and there have been many accidental introductions as well.

The California Department of Fish and Game has an active program to annually stock San Francisco Bay with Striped Bass, an exotic species from the east coast that is very popular for recreational fishing.  But Striped Bass is a predator that may be impeding the recovery of listed species including steelhead trout, chinook salmon, delta smelt and splittail.

The Sea Lamprey. This is a parasitic fish that has no jaws but has a mouth like a suction cup and a tongue armed with rasping teeth. It attaches to fish with its mouth, rasps through scales and skin with its tongue, and feeds on the body fluids of the host fish, often killing it. During its life as a parasite, one sea lamprey can kill 40 pounds or more of fish.

Sea lampreys are native to the Atlantic Ocean, not the Great Lakes. For many centuries, the lamprey's access to the Great Lakes was blocked by Niagara Falls. Then the Welland Canal was built in 1883, allowing the lamprey to enter Lake Erie. During the period from 1921 to 1940 it spread through the remainder of the lakes, feeding on several kinds of fish including the lake trout which had been the basis of a multi-million dollar fishing industry. The lamprey destroyed 97% of the trout population in the Great Lakes in this period. After many years of research, a poison was discovered (TFM or 3-trifluoromethyl-4-nitrophenol) that killed lamprey larvae but was not toxic to other fish. Over a two-year period (1960-62) the lamprey population was reduced by 85% but not eliminated. About 250 Great Lakes tributaries are now treated at regular intervals with the lampricide, and this method is being supplemented with barriers across streams to prevent the lampreys reaching their spawning grounds, and with release of sterilized males that reduce the number of fertile eggs laid by the females.

Other exotic species introduced into the Great Lakes during the 1900s were the alewife and rainbow smelt. These and the sea lamprey contributed to the decline of the blue pike through predation and competition, leading to its extinction in 1970. The blue pike once made up about 50% of the commercial fishery on Lake Erie.  Other Great Lakes species that have been lost are the deepwater cisco in the 1950's, the blackfin cisco in the 1960s, and the longjaw cisco in the 1970's. In addition to predation and competition by exotic species, these fish were also negatively impacted by overfishing, pollution, siltation and other forms of habitat degradation.

The Nile Perch, a predatory fish, was intentionally introduced into Africa's Lake Victoria, the largest freshwater lake in the world and the source of the Nile river, in 1962 in order to establish a new fishery. It is now eating its way through the lake's hundreds of endemic fish species. It is estimated that 300 species of indigenous fish became extinct in the lake in the 1980's; many of these fish were favored by the local people because of their taste. By 1985 as many as 95% of the fish caught in Lake Victoria were Nile Perch. But in recent years they have been getting smaller as they use up their food supply. The population is now expected to decline rapidly and the fishery will probably crash. This exotic species has eliminated a major food source for 30 million people.

Atlantic salmon have escaped from aquaculture pens in the Pacific Northwest, raising concerns for the wild salmon that are already struggling against over-exploitation and habitat loss.

Other exotic fish that are causing problems include the walking catfish in Florida, the mosquito fish all over the world, and the red shiner in desert streams of the southwestern U. S.

Over 126 species of exotic fish have been caught in open waters of the U. S., and 46 of these have established breeding populations. At least half of these cases have resulted from the release or escape of pets.  Most of them are tropical fish, which have become established in warmer states including Florida, Texas, and the Southwest. Examples include the oscar, Jack Dempsey, jewelfish, convict cichlid, Midas cichlid, and spotted tilapia; and livebearers, such as swordtails, platies and mollies, and armored catfishes. A new example (2002) is an Indo-Pacific species of lionfish that has been found near two shipwrecks off the coast of North Carolina. This could be an especially harmful exotic because its venomous spines are dangerous to humans and other fish.

Don't release unwanted pets!  They may eliminate native species by competition, predation, or spreading parasites or disease.  They may alter the genetics of natural populations by hybridizing with them.

Amphibians

In 1935, the central American cane toad Bufo marinus (9" long, weighs 4 pounds) was introduced in Australia to protect sugar cane fields against a beetle pest. Now, the toads are an ecological disaster. They have spread 2000 miles and are established in about 1/2 of Queensland. They are spreading at 17 miles/year. The toads feed on many native animals, from frogs to bees. The toads are poisonous, so nothing will eat them. Many native species are getting wiped out by this animal.

The Cuban treefrog was accidentally introduced into Florida in 1931 in a shipping crate. It is now established throughout southern Florida, where it preys on the native green treefrog and the squirrel treefrog.

The African clawed frog (Xenopus laevis) used for pregnancy tests, has become established in many areas in this country including some local reservoirs. 400,000 of them were taken out of the San Joaquin reservoir by a biological supply company a few years ago.

Reptiles

(Graphic from Phil's Eye on the Web) Problems caused by the introduced Brown Tree Snake on the island of Guam are discussed in the Chapter on Islands

Birds

Some exotic birds, which were originally introduced intentionally, have caused immense damage to wildlife and have become serious agricultural pests. In the late nineteenth century, homesick and chauvinistic European immigrants around the world had the idea of introducing familiar European birds in order to improve their new surroundings.
 

One misguided Shakespeare fan decided, in the 1890s, to introduce into the U.S. all of the birds mentioned in the works of William Shakespeare.  One of the Shakespeare birds (mentioned only once in all of his works!) was the European starling, which started out from a few pairs released in Central Park in New York City and now is one of the most abundant and widespread of birds on this continent, reaching from the east to the west coast and from Alaska to southern Mexico. They form vast flocks, which can be aviation hazards and have caused at least one plane crash. They cause problems in agriculture by feeding on fruit, and on the grain that is spread out for cattle and pigs. They also do serious damage to native bird populations - they evict bluebirds and swallows from their nests and tree holes, and often destroy eggs and young in the process.

Another problem bird that dates from the acclimatization era is the European house sparrow. These birds were introduced in many parts of the U.S. as well as other countries and are now well established and common in North and South America, Australia, New Zealand and Hawaii. They cause serious damage to crops by eating the grain on wheat and corn stalks before it ripens.

Mammals

Many of the animals intentionally introduced to new habitats have been herbivorous mammals, including goats, rabbits, pigs, sheep, cattle, horses, donkeys, monkeys, deer, wallabies, opossums, and squirrels.  23,000 exotic goats were recently removed from Santa Cruz Island!

Browsing or grazing can lead directly to extinction of plant species, but more often the effect of introduced herbivores is to reduce the habitat quality for indigenous species by turning habitats dominated by shrubs and trees into grassland. This has happened in many parts of the world as a result of sheep and cattle-grazing. Examples are the western U.S., Australia, and New Zealand. This becomes a form of habitat destruction. Many Southern California hillsides become brown in the summer because the introduced grasses that have been planted for cattle have replaced the natural vegetation (usually Coastal Sage Scrub) but are not well adapted to the low summer rainfall.

The red fox is an exotic mammal causing some problems in Southern California. It was introduced earlier this century, and since the early 1980's, it has been gradually exterminating several endangered species of birds in coastal wetlands. It eats the eggs of the light-footed clapper rail, which is an endangered species. Ecologists have recognized the importance of maintaining healthy populations of coyotes, because these are fox predators and keep the fox population from exploding.

Diseases

Exotics in Australia and New Zealand

Australia and New Zealand provide a huge number of examples of the destructive effects of introduced species. The first exotic to be introduced by man into Australia was the dog, brought more than 3,500 years ago by Asian seafarers. Later the dogs went wild and became the ancestors of the dingo, now considered an Australian native dog. Since the native predators, the Tasmanian devil and the Tasmanian wolf, are extinct from the Australian mainland, the dingo is the largest land carnivore and the top predator, feeding on kangaroos and sheep. The early settlers also accidentally introduced rats and mice. For hundreds of years these, and bats, were the only placental mammals on the continent. Then, between 1840 and 1880, more than 60 species of vertebrates were released in Australia. Acclimatization Societies were formed, and introduced dozens of species of exotic plants and animals (see table). Most of these introductions have been disastrous for Australian wildlife. 

The most destructive and most expensive transfer ever of an animal from one country to another was that of the European rabbit (similar to a cottontail) into Australia. It was introduced by a wealthy landowner, Thomas Austin, who had become homesick for the animals of his native England. Rabbits were not native to England either; they were introduced from Europe after the Norman conquest in 1066. Mr. Austin brought in a shipment of two dozen rabbits in 1859, and turned them loose on his estate in Victoria. They bred like rabbits, and provided some good hunting for Mr. Austin. Six years later he estimated that he had killed 20,000 rabbits and still had 10,000 left. They spread out across the continent, rabbit hunting became popular, and rabbit meat and fur became a major export for Australia. They did so well because their populations were not kept in check by weasels and foxes as they are in Europe. The only possible predators - the dingo and the Tasmanian wolf - were already being shot and kept in check by the sheep ranchers. And many of the rabbits' potential competitors, like kangaroos, were also being exterminated by the settlers.

In 50 years rabbits had spread all over Australia except the tropical regions in the north, and their populations were so dense that they would eat every blade of grass, and kill shrubs and trees by stripping them of their bark. They were denuding the sheep pastures of grass, turning once successful ranches into wastelands and reducing wool production by half. Finally they were declared vermin and were hunted, trapped, and poisoned. The government offered a bounty for rabbit tails, and millions were collected. But it is very difficult to catch every one. In 1902-1907 they built a 2,000-mile long fence, costing more than a million dollars, to try to stop the rabbits entering the cereal-growing area in the southwest. Rabbits starved to death and carpeted the ground with their bodies on one side of the fence, while the grass grew green on the other, for a while. Then a few rabbits got through and started the whole cycle again on the other side of the fence.

Today the "Rabbit Fence" marks a clear and straight boundary between the southwest, where all native vegetation was cleared to make way for agriculture, and the area to the east where forest still survives.  This man-made alteration in the vegetation pattern is the most conspicuous man-made feature of Australia when viewed from space, and it appears to be causing a change in rainfall patterns.

A potential solution to the rabbit problem was found, in the form of a virus that causes a rabbit disease called myxomatosis. It was found in Brazilian rabbits, where it causes only mild illness, but it was lethal to European rabbits. It is transmitted by mosquitoes and rabbit fleas. It was introduced into Australia in 1950, and it spread like wildfire. Millions of rabbits died, and the land started to turn green again.

Later on, in 1952, some French farmers got hold of some of the virus and introduced it on their estates. It spread throughout France, Germany and England, nearly wiping out the rabbit populations in those countries.

The myxomatosis victory was short-lived.  Myxomatosis-resistant rabbits have been spreading in Australia and the population built up to about 300 million in 1997.  The rabbits are now being affected by a new virus (rabbit calicivirus disease virus) which was accidentally released  from a testing site in South Australia and has spread in both Australia and New Zealand.  Already the rabbit population has declined dramatically in some areas, and preliminary data suggest that native plants and animals are making a comeback.  However, there are fears that the virus may also harm native species such as the short-tailed bat and New Zealand's national emblem, the kiwi, as well as cattle.

While exotic rabbits were destroying Australia's vegetation, introduced foxes were consuming the country's native animals. The foxes were introduced in the 1870's and by 1917 had spread all the way across the continent. They ate birds, birds' eggs, mice, frogs, fish, lizards, bats, and decimated at least six native species of small mammals, many of which were simultaneously being driven from their burrows by the rabbits.

In spite of the failure of the rabbit fence, the Australians have also built a dingo fence, separating off the sheep-farming southeastern part of the country to protect the sheep from dingo attacks. The fence is 6 feet high and, at 3,307 miles, the longest fence ever built anywhere. It was completed in 1960, but parts of it are nearly 100 years old. It achieves its main purpose better than the rabbit fence did, partly because there is a bounty to encourage hunters and farmers to shoot any dingoes found on the wrong side of the fence. But it has caused another problem - a population explosion of kangaroos on the sheep-farming side of the fence because of the absence of predators. The kangaroos are now competing with the sheep for grass and water (O'Neill, T. "Traveling the Australian Dog Fence". National Geographic, April 1997).

Australia was the source of one of the most destructive and troublesome of the exotic species in New Zealand - the bush-tailed opossum.  This marsupial was first released in 1837 in order to establish a new fur trade, and by 1930 it had been released at 450 locations.  Its populations burst out of control so that now the country has 70 million of them and they have been named public enemy number one.  Land management agencies are spending most of their budgets trying to control them, mostly by spreading poison-laced carrots.  The "possums" destroy canopy vegetation, causing loss of habitat and decimating many native bird species.  If you want to buy real fur, buy some fur or other products from this species and help reduce its numbers!

Detrimental Effects of Exotic Species

1.  Direct Effects.  The chestnut blight fungus was introduced into this country in the late 1800's along with some Chinese chestnut trees and was first recognized by groundskeepers at the Bronx Zoo in 1904. The Chinese species, which had evolved with the fungus, was hardly affected by it, but almost the entire U.S. population of chestnut trees was wiped out in about 50 years.

Dutch Elm disease entered the U.S. from Europe in about 1930 in elm logs imported for the veneer industry. This led to the destruction of over 4 million elms between 1933 and 1940. Insecticide (DDT) spraying of elm trees to control elm bark beetles, which spread the disease, led to widespread poisoning of birds, especially robins, in the 1950's.

2.  Indirect Effects.  Quite often, introduced species cause the spread of other harmful organisms. The Indian Mynah bird spread lantana seeds all over the Hawaiian islands, causing the spread of this plant as an invasive weed. Pigs, introduced into Hawaii, eat the fruit and thereby disperse the seeds of guava, an aggressive weed that has displaced large areas of native vegetation on many islands.

New Pathways for Invasion

Increased travel and trade are providing many new opportunities for spread of exotics: 

Government Agencies Concerned with Exotic Species

Several government agencies are concerned with trying to prevent importation of exotics, mainly to protect agriculture. These are:

Officials from these agencies are responsible for inspecting baggage and vehicles at airports and border crossings, and for setting up quarantine areas to prevent spread of pests such as the Medfly.

February 1, 1999, Babbitt, Glickman and Baker Will Announce Administration Effort to Deal with Invasive Species, News Release: U.S. Fish and Wildlife Service

At the same time, increased trade brings along greater opportunities for exotic species to spread, and the World Trade Organization and other free trade movements are making it more difficult for governments to keep out exotics.

Because it is difficult or impossible to completely inspect everything, new pests still keep arriving. For example, in Southern California the recent arrivals include:  

Additional links

Impacts of Introduced Species in the United States
Introduced Species The Threat to Biodiversity & What Can Be Done by Daniel Simberloff, Ph.D.