Chapter 10: Alien invaders
Edited by Peter Moyle & Douglas Kelt
By Peter B. Moyle and Steve Ellsworth, last revised September 2004
The introduction of non-native (alien) species to an area can result in a loss of biodiversity. This statement seems to contradict itself, but it is true. The reason is simply that when alien species invade an area they can drive native species to extinction. Multiple invasions of alien species thus often increase local biodiversity but decrease global biodiversity. Everywhere humans settle they bring along familiar animals and plants. As a result, a few hardy species are enjoying world-wide distributions while endemic native species—those that occur nowhere else—disappear. Species that are part of this elite, increasingly world-wide fauna include the Norway and black rats, house mouse, muskrat, feral goat, house sparrow, starling, common carp, brown trout, mosquitofish, Japanese clam (Corbicula), Argentine ant, and cabbage butterfly. In fact, invasion of alien species ranks right up there with habitat change as a major cause of loss of biodiversity. Usually the two factors work together: altered habitats favor alien invaders. This chapter deals with alien species and explains why they are introduced, why they are successful, and how they affect native species.
An alien species is one that invades as the result of human activities an area where it was historically absent and therefore where it did not evolve (unlike native species). Most often, humans help these species cross barriers such as mountains and oceans that had previously inhibited the species’ movement. For instance, the many plants and animals that humans brought from Europe to North America crossed a major barrier, the Atlantic Ocean.
The number of species being introduced worldwide is growing rapidly because of the development of fast modern transportation systems that move humans and other organisms over great distances. Tropical fish collected in the wilds of South America can be sold in a U.S. pet store three days later. Invertebrates sucked into the ballast water of ships can be dumped into a different ocean after a ride of two weeks. An insect pest in a piece of fruit can be carried thousands of miles in a single airline flight.
Often alien species are unsuccessful in establishing populations because the habitat is not suitable or because organisms already present prevent them from doing so. All too often, however, they are able to establish populations. In many cases these introduced species become pests because they alter habitats and/or become harmful competitors or predators on native species. They may also bring with them diseases and parasites that threaten humans and other species. In many cases, the combination of habitats alteration by humans and invasion of alien species have put entire native floras and faunas in danger of extinction.
WHY ARE SPECIES INTRODUCED?
As humans have spread, so have many other species, taking advantage of the ‘new’ environments created for them by the ultimate invaders, us. The reasons for introductions of aliens are as diverse as the plants and animals themselves.
During the European settlement of the New World, there were many plants and animals brought over because the colonists were unfamiliar with New World species and thought the familiar species were superior to the unfamiliar ones. This rather arrogant attitude has been called Cultural Imperialism by Crosby (1986) and went hand in hand with the conviction that Europeans were superior to native peoples as well. As discussed in Chapters 1 and 2, many European settlers feared the wildness of the Americas and introducing species familiar to them was a means of reducing their uneasy feelings. Even today, many of our most familiar animals and plants are European in origin, including the grasses along the roadsides and the house mice in our homes and gardens. The four species discussed in the case studies to follow, the European starling, house sparrow, common carp, and brown trout, were all brought intentionally from Europe as creatures superior to their local equivalents.
We humans like to eat tried and true organisms and so introduce them to be handy for harvest. Common carp are a good example of this. Carp are prized as food in much of the world and they were initially introduced as food by immigrants from Germany. They are now one of the most widespread fish in North America. Bullfrogs, introduced as a food source in the western United States, have become a dominant member of many aquatic systems, displacing native frogs.
During settlement of the North American continent, animals such as muskrats, nutrias, beavers, and arctic foxes were introduced to support the fur trade. Even though there were positive economic aspects of these introductions, there were many negative ecological aspects. Introduced beavers along the eastern slope of the Sierra Nevada have become pests because they gnaw away at cottonwood trees that line the stream, seriously weakening them. These trees are important in providing habitat for other animals utilizing the river’s edge.
This has been most commonly done with fish, mammals, and birds that are introduced as game species. In many cases, these introductions are beneficial to people because they provide opportunities for fishing and hunting. However, these species may end up doing harm because they threaten the existence of desirable native species. Thus red fox were introduced into California for fox hunting and now are a major predator on ground-nesting birds (including endangered species such as clapper rails); they also may displace the smaller and shyer native gray fox. Recent studies on the Cosumnes River in central California have shown that the alien redeye bass, introduced as a game fish for small streams, has now almost completely displaced the native fish fauna in the river system. Because the bass is so small (a 25 cm fish would be a whopper), few people fish for it.
When an alien species becomes so abundant that it creates problems, other organisms may be introduced to control it. Unfortunately, control species often become pests as well even if they are effective in their “job.” An example of this is the grass carp that has been introduced into many parts of the world to consume unwanted aquatic vegetation. The grass carp has been more successful than expected and in some instances has removed large amounts of vegetation that provided habitat for native fishes and food for waterfowl. Under some conditions, biotic control may be a useful management technique for controlling a pest species. For example, mosquitofish are widely planted in California rice fields and urban areas to control mosquitoes and gnats. In these situations, they are an effective and inexpensive alternative to pesticides. In less disturbed systems, however, they can do damage to native fish, amphibians, and invertebrate populations. In general, the effect of introducing vertebrates is highly unpredictable and dangerous to ecosystems, so using them for biological control is risky.
The release of pets, better called misdirected kindness, is a surprisingly common method of introduction. Hundreds of alien species are brought to North America to occupy aquaria, cages, and backyards and many escape or are released by owners tired of taking care of them. In the majority of cases, these pets die of stress, starvation or being eaten by predators soon after their release, and do not become a problem. However, there are numerous cases where the release of pets has resulted in the establishment of harmful populations. This is especially the case with exotic fish that have been released in large numbers by hobbyists or have escaped from fish farms. Because these fish are tropical, they are only able to inhabit warm waters in North America; hence, introduced tropical fish are found in waters of Florida, Southern California, Nevada, Arizona, New Mexico, and Texas. In many cases, these species have become pests and now are virtually impossible to eradicate. The combination of predation and competition from these species and habitat alteration has put many native species in these areas on the endangered list. Alien birds have also found their way into areas of North America and Hawaii by means of unwise hobbyists. The intentional release of colorful tropical birds such as parrots and parakeets has been quite common in Florida, Southern California, and Hawaii where they are destructive to agricultural plants, especially fruit trees. Another widespread problem is that of house cats which are abandoned by careless owners. Those that survive the first few weeks of release often move into natural areas where they drastically decrease the populations of nesting and migratory birds, as well as lizards and other creatures. It is likely, for example, that California quail are largely absent from Davis because of predation on nesting birds and young by cats. The absence of fence lizards probably has a similar cause.
Release of organisms into wild areas can change the ecosystems if they become established. This has been a reason for deliberate introductions by management agencies who wish to increase the numbers of desirable species. Thus organisms are brought in to enhance the forage base of a desirable species such as game animals. An example of this occurred in Flathead Lake, Montana, where the introduction of a small shrimp as a food organism led to the collapse of the runs of kokanee salmon up local streams. Kokanee were a major source of food for bald eagles, and when this food supply disappeared, the eagles started foraging more on road-killed animals—and often became victims of vehicles themselves. Like biological control, ecosystem manipulation is a risky venture because of the unpredictability in how organisms will respond to changes in their environment.
By product introductions
An increasingly common introduction is the result of a by-product of human activity, especially commerce. Such by-product introductions are often considered to be “accidental.” However, because we now know they are occurring (just not precisely when and what) and can be prevented, they must be considered to be deliberate and preventable. Two common kinds of by-product introductions are range expanders and hitch-hikers. Range expanders are animals that move “naturally” through habitats created by humans, invading new areas. These include species that are commensal with humans. Examples of such organisms are raccoons (native to North America but only in limited areas) and opossums that are able to eat almost any type of food and therefore thrive in urban and suburban areas. Range expanding fish and clams are those that can enter aqueducts and be transported to new places. Thus much of the fish fauna of northern California has been introduced into southern California through the California aqueduct.
Hitch-hiking is an increasingly common source of alien species. Many of our major pests, from Mediterranean fruit flies to star thistle to black rats were carried to North America hiding in ships and airplanes. At the present time, thousands of species of small fish and invertebrates are being introduced all over the world through ballast water of ships. Ballast water is pumped into an empty or loaded ship to alter its trim and stability. Modern cargo ships often carry millions of gallons back and forth across the ocean. Planktonic plants and animals, including the larvae of fish and mollusks, are pumped in with the water and dispersed as the water is released, often thousands of miles away. Clams, fish, and zooplankton brought in with ballast water are now causing major problems in the Great Lakes and in San Francisco Bay. In the Great Lakes, the tiny zebra mussel has become so abundant that it is clogging water intakes to towns and power plants, shutting some down completely. Keeping the clams under control is now a major expense. The zebra mussel also competes with native clams, helping to push them to endangered status.
Some of the most devastating introduced species are those domestic animals that have become feral. These animals were once domesticated before becoming wild and free-roaming. Examples include goats, pigs, cats, dogs, horses, and burros. Even though they are often introduced as they escape captivity, they are also deliberately released and are often able to exist in a wide variety of habitats and climates. In Northern California and Hawaii, feral pigs have become an extremely destructive force; they are often observed rummaging through soils uprooting native vegetation, an activity which often allows the colonization of introduced plants and the erosion of hillsides.
ATTRIBUTES OF INTRODUCED SPECIES
What are the attributes of introduced species that allow them to invade successfully and maintain their populations? To successfully live in an area, an invader must be able to tolerate the range of physical conditions (such as temperature and moisture) in the area. For example, an alligator could not invade the Arctic and a polar bear could not invade Florida because of inappropriate temperatures and other conditions for their survival. Therefore, an attribute of many introduced species is their ability to withstand a wide range of physical conditions or at least the conditions present at their point of introduction. The ability to subsist on a wide variety of foods is another important attribute of many introduced species. This ability allows the introduced organisms to find sustenance in areas that have different types and arrangements of food than found in their native ranges. Successful introduced species often have high dispersal rates, meaning they can reproduce and spread their offspring rapidly. The ability to compete well with similar species is another important attribute. However, the most important attribute of any alien invader is to be able to live in close association with humans (see next section). If a species can survive in urban, agricultural, or other areas impacted by humans, it has a good chance of being successful. Once established in altered areas, an alien invader can frequently move into more natural areas. Among the commonest mammals in the Putah Creek Riparian Reserve on the UC Davis campus, for example, are black rats, house mice, feral cats, and red fox. Given the increase in extent and speed of global trade and transportation and given the presence of humans in every habitat on the globe, really only two rules apply to introduced species:
1. Any species can be successfully introduced.
2. Any ecosystem can be invaded by alien species.
WHAT AREAS ARE MOST VULNERABLE TO INTRODUCTIONS?
Ecological systems are so complex that it is extremely difficult to predict just where particular introduced species are likely to succeed. Charles Elton’s (1958) landmark review of plant and animal invasions concluded that invaders are more likely to establish themselves in areas that have been altered by humans and in areas with relatively simple communities, such as on islands. It is now well established that human-disturbed habitats are more readily invaded than undisturbed areas. Disturbed habitats are more easily invaded because the disturbance has already reduced or disrupted the native populations. For example, introduced fishes in California can readily establish themselves in new reservoirs that fill after dams are built, but they often have a hard time invading the streams above the reservoirs. The streams are still dominated by native fishes that are unable to survive in the reservoir. Such streams are said to have high environmental resistance to invasion.
If disturbed habitats are easiest to invade, isolated habitats, such as islands and desert springs, often suffer the most damage from invaders. Such areas often have simple communities because few species have been able to find their way to them or can survive in such limited areas. The simple communities are easily invaded because the species in these areas have evolved in the absence of other species and may not have the necessary adaptations to combat competition or predation from unfamiliar species. Elton refers to this as a low degree of biotic resistance of an ecological community to invasion. In desert springs where there are no native fish predators, native pupfishes have bold and aggressive courtship displays. As a consequence they are easily caught and devoured when predatory largemouth bass are introduced into the springs. In the examples that follow, the major themes that have been presented so far will be illustrated in detail.
STARLINGS AND HOUSE SPARROWS IN NORTH AMERICA
Both the European starling and house sparrow were purposefully introduced to North America from Europe in the late 1800s. Originating from very small populations, they have both expanded over most of North America. The reasons for the success of these species are not completely known, but one thing is clear—the success of these species has come at a cost to native birds.
Starlings are pests in many senses of the word—their populations have expanded dramatically, they have harmed native birds, and they have eaten millions of dollars worth of crops. In 1890 the release of sixty starlings into Central Park in New York City marked the start of what was to become their amazing spread throughout the continent. The introduction was by a society that wished to establish in the New World all of the birds that were mentioned in the works of William Shakespeare and the starling is mentioned in Henry IV. From the original sixty birds, the population has grown to about 200,000,000, making it one of the most common bird species in North America. In California, the first starling was officially reported in 1942 and they have since become widespread in the state. In the 1960s, California initiated eradication programs to control starlings. The programs were successful in killing a great number of starlings. However, those that survived have been able to build the population back up to enormous numbers and most control efforts have been abandoned.
Why has the European starling been so successful? The key to any species success is that it is able to withstand climatic fluctuations, able to compete for food and space with similar species, able to eat a wide variety of foods, and able to coexist with humans. Starlings have all these characteristics. Not surprisingly, the expansion of starling populations has been associated with declines of native birds, especially ones that share similar nesting requirements, such as bluebirds, northern flickers, and woodpeckers. These species nest in holes in trees from which starlings actively displace them. In the case of woodpeckers, starlings will wait until they are through drilling a nesting hole in a tree and aggressively keep them from using it. Starlings are also able to use buildings as nesting sites, so they are able to live in many areas where native species cannot. Like many other introduced organisms, starlings have been implicated in transmitting diseases, most notably a fungal disease known as histoplasmosis that is harmful to humans.
House sparrows were introduced under the false assumption that they could control insect pests infesting city parks (they are largely seed eaters). House sparrows are thought to have become associated with human populations during the development of agriculture thousands of years ago, so it makes sense that their success in North America followed deforestation and planting of crops. House sparrows are widely recognized as aesthetic and agricultural pests and have been implicated in reducing populations of native birds that have similar food and nesting requirements. However, recent research contends that evidence for displacement of native birds by house sparrows does not hold up under scrutiny. The success of house sparrows provides an interesting case for ecologists to study because a similar species, the tree sparrow, was also introduced yet has not spread across the continent. Differences in size, competitive ability, and genetic variability have been proposed as possibilities for the disparity in success between the two species.
THE BROWN TROUT AND COMMON CARP IN NORTH AMERICA
Common carp and brown trout, like house sparrows and starlings, were purposefully introduced from Europe in the late 1800s. The reasons for both introductions were basically the same—to provide more fishing opportunities at time when wild fish were widely sought for food. Both species have been extremely successful in colonizing waters across the continent and the cost to native species has been high.
Common carp, native to the Danube River in Europe and Asia, has a long history of being introduced in various places in the world, starting with the Romans. In Europe during the Middle Ages carp were an important source of food for monks and were widely raised in monastery ponds, resulting in their spread throughout Europe. In the late 1800s common carp were introduced into North America because many people thought they were better than native fishes for both food and sport fishing. In fact, for a few years raising carp was the main job of the U.S. Fish Commission and congressmen vied to have carp shipped to their districts. Today, carp are found in nearly all major river systems in the United States and are commonly considered to be pests. One of the main reasons common carp have been successful is their ability to survive in a wide variety of habitats and conditions, including polluted areas where few other organisms can survive. They can be found in streams, rivers, lakes and estuaries and even in irrigation canals. If carp have influenced other species, it has been largely through the removal of aquatic vegetation by their feeding activities. Vegetation in streams and lakes is valuable as habitat for many species of native fish that use it for shelter, food, and spawning sites. Carp have also been implicated in changing the water quality where they reside. The activities of carp stir up mud and sand from the bottom, something that harms other fish because the decrease in water clarity that results interferes with vision that is critical for the sighting and capture of prey. Carp have also been implicated in the declines of ducks, such as canvasbacks, apparently by removing vegetation that they use for food. Another likely impact of the carp has been its transfer of harmful parasites to native fishes. The positive side of carp is that they grow rapidly to large sizes in altered waterways, are abundant, are easy to catch, and are good to eat. Because they eat ‘low on the food web’ they accumulate fewer pollutants, or at least lower concentrations of pollutants, than more favored game fishes such as bass.
Attempts to control and eradicate carp in North America have been largely unsuccessful. One management technique has been to apply a potent chemical called rotenone to kill fish. Carp populations have been able to recover from such treatments by being able to quickly recolonize areas and by having a high resistance to rotenone. However, when carp have been removed from some waters, native fish and duck populations recover to a significant extent. Because common carp are here to stay, we might as well enjoy them. They in fact are a fine food fish (as anyone who comes from an Asian or Eastern European cultural background can testify) and a fine sport fish, especially on light tackle (ask any warmwater angler in England). Few other game fish can reach 20 or more pounds in an urban setting!
Unlike carp, brown trout are much sought after as game fish in North America because they are rumored to be smarter than most anglers. They are native to many parts of Europe and western Asia and were first introduced into North America in Michigan during the 1880s and into California in the 1920s. Today they are occupants of rivers and lakes in many parts of North America. Brown trout are similar to many native trout species, especially brook trout, so it follows that they have similar requirements and hence compete for resources with native trout.
Declining native trout populations as brown trout invade demonstrate that brown trout have been successful competitors. A study done in Michigan indicated that brown trout are an aggressive species that actively excludes native brook trout from choice foraging habitats. Brown trout have high growth rates and grow larger than many native trout. Larger size is an important factor in being able to avoid being eaten as well as being able to eat smaller trout, including native trout. Predation by brown trout on golden trout (the state fish of California) in the upper Kern River has been implicated as a major threat to populations of golden trout. Other affected species in California include cutthroat trout and McCloud River bull trout. Another reason for the success of brown trout is that they are more difficult to catch by fishing than native trout so they are able to maintain populations in heavily fished waters.
Several states, including Michigan and California, have attempted to reduce brown trout populations in areas favored by native trout. Sometimes removing them has come through physical capture, an activity that is time consuming and costly. In the Sierra Nevada, the use of the pesticide rotenone has been successful in eliminating populations of brown trout from small streams in order to restore populations of native golden trout.
Common carp and brown trout, both purposefully introduced for similar reasons, have been problems for different reasons. They were introduced at a time when people had little concept of what an impact these species would have. Their introductions were part of a long tradition in western culture of tinkering with nature in order to “improve” it. With today’s knowledge of ecological systems, fishery managers rarely recommend fish introduction because of subsequent social, economic, and ecological impacts. However, ignorant anglers do move carp and brown trout (as well as other fish) around, creating problems for native fish populations and often for other anglers.
THE SEA LAMPREYS AND ALEWIVES IN THE GREAT LAKES
The Great Lakes system, comprised of five major lakes and their connecting channels, is a vast and intensively used body of fresh water. Each of the five major lakes (Superior, Michigan, Huron, Erie, and Ontario) are among the fifteen largest in the world and collectively they contain twenty percent of the world’s supply of surface fresh water. The lakes create more than 10,000 miles of shoreline, a total that surpasses the Atlantic coast of the United States by 3,600 miles. The lakes and the surrounding land have been highly altered by human activity during the last several centuries. The assemblage of organisms found within the lakes has undergone a major change, especially within the last century, as alien organisms found their way through canals or were intentionally introduced for recreation.
We will focus on the sea lamprey and alewife that have established large populations in the Great Lakes and have had major impacts on native species. The sea lamprey and alewife are species that originated in the Atlantic Ocean but are able to survive in fresh water as well. This indicates that they would have long ago had access to all of the Great Lakes if not for the presence of Niagara Falls, which is created as water flows from Lake Erie to Lake Ontario. Niagara Falls creates a natural barrier for entrance into the upper four Great Lakes (Lakes Erie, Huron, Michigan, and Superior). The falls also once provided a barrier to the movement of ships through the entire system. Early settlers realized what an enormous economic asset the lakes could be if they could be opened to shipping, and this led to the construction of the Welland Canal between Lake Erie and Lake Ontario in the 1800s. When the canal eventually opened in 1929, species in Lake Ontario, including the alewife and sea lamprey, had access to the upper four Great Lakes. Curiously, sea lampreys may not have come in through the canal but became established through the use of juvenile lampreys as bait.
The sea lamprey is an eel-like fish that preys on large fish; it attaches to their sides and extracts blood and other bodily fluids. The sea lamprey was first reported in small numbers in Lake Ontario during the 1830s. The lamprey apparently arrived there in large numbers during the 1870s from the Mohawk-Hudson River drainage in the state of New York, via the Erie Canal. As the populations grew into the 1900s, their presence was not felt to a large extent because the fish populations which it potentially could have harmed such as lake trout and Atlantic salmon had already been severely depleted by commercial fisheries and loss of habitat for reproduction. Once lamprey got around Niagara Falls and through Lake Erie (largely unfavorable for them) they arrived in Lake Huron and Lake Michigan where they found many prey species. The lamprey attacked the native lake trout, which was unable to survive attacks of this foreign organism. The abundance of lake trout had already been greatly lowered due to commercial fishery harvests, which made the attack of the lamprey especially devastating. By the mid 1950s, lake trout populations plunged to near extinction in Lake Michigan and Lake Huron. Lampreys, however, thrived, having lots of alternate prey as well.
An interesting question is why the sea lamprey was able to have such an impact in the Great Lakes considering that its effects are not greatly felt in areas where it is native. Native fishes in the Great Lakes were certainly not prepared in an evolutionary sense to withstand attack by lamprey. If these species had evolved with the lamprey, then they would have had the means to detect and withstand attacks by them. In other inland waters where the sea lamprey has been established for thousands of years, such as the finger lakes of New York, the sea lamprey has not devastated species that it attacks. This is because the lamprey is a natural part of these systems and other species have adapted to its presence.
In 1955 the lake trout was very close to extinction in Lake Michigan and Lake Huron, and the sea lamprey had become such a recognized problem that the governments of the United States and Canada formed a binational organization, the Great Lakes Fishery Commission, to develop ways to control or eliminate the lamprey. Research was directed towards developing a chemical that harms the larval stage of the lamprey that are hatched in streams and rivers. It was discovered that TFM (3-trifloromethtl-4-nitrophenol) destroyed many of the larvae and had no readily apparent effects on other aquatic organisms except other lamprey species native to the Great Lakes region. The chemical was widely used to treat lamprey spawning grounds and had substantial success. The ability to control the lamprey has allowed the stocking and establishment of lake trout and four other introduced game fish. It also allowed the recovery of populations of lake whitefish, a favored commercial fish species. Recently there has been concern over the effects of TFM on other aquatic organisms. The Fishery Commission has had very limited success in developing other means of controlling the lamprey and is finding the chemical treatments to be increasingly expensive.
Recently, sea lamprey appear to be making a comeback and there is concern that they will again decimate stocks of desirable fish. The lamprey may be developing a resistance to the chemical treatments and are reproducing out in the lakes where it is impossible to chemically treat. One thing is clear—the sea lamprey problem is not going to vanish and measures for its control will continue to cost millions of dollars. Due to human action, the lamprey was able to invade a system containing fishes that were not adapted to their style of predation. Given substantial evolutionary time, fish populations may be able to adapt to the presence of the lamprey. Interestingly, introduced Pacific salmon have proven to be resistant to lamprey attacks because the Pacific salmon evolved with the Pacific lamprey, which is similar to the sea lamprey. Chinook, pink, and coho salmon, as well as steelhead, are all abundant in the Great Lakes both because they can survive lamprey attacks and because they prey on another alien species, the alewife.
The invasion of the alewife into the upper Great Lakes from the Atlantic Ocean was also remarkably successful. The alewife is a herring-like fish that also gained access to the upper four lakes through a canal system. Unfortunately its success came at the expense of native fish species such as yellow perch and ciscoes which supported commercial fisheries. The alewife population reached enormous abundances in the mid-1960s in Lake Michigan and Lake Huron; in fact, it accounted for the vast majority of biomass in the lakes. At this time, there became a severe problem of alewife die-offs. Dead alewives literally covered beaches and plugged municipal intakes, often cutting off water supplies to cities and industries. In several extreme cases, the die-offs were so severe that they created public health problems for shoreline communities. The reasons for the die-offs are not completely certain, but a leading theory is that the alewife, having not evolved in the Great Lakes system, was vulnerable to temperature fluctuations in the lakes. Although millions of alewives died in this period, millions more survived and continued to be a problem. The alewife, then, had major economic costs by lowering stocks of valuable fish and by necessitating cleanup programs.
One of the reasons that alewives were able to establish such large populations is their ability to out-compete native species for zooplankton and because there was little threat of predation. The sea lamprey and commercial fishing had reduced those species that were likely to consume alewife. In 1966, coho salmon from Oregon were planted in Lake Michigan and Lake Erie. The following year, chinook salmon were introduced. The intent of these plantings was to provide fish that would eat the alewife and, as a possible side benefit, to develop a fishery for the salmon. The salmon thrived and grew rapidly to an extent beyond the wildest dreams of fishery managers. The salmon were successful presumably due to their ability to feed on the alewife. For the first time in the Great Lakes the alewife was a benefit: it was providing forage for salmon and trout that were becoming extremely valuable to the Great Lakes states as sport fish. Three more alien game fish, brown trout, rainbow trout, and pink salmon, were also introduced.
The alewife population apparently has been declining since the mid-1970s and currently is quite low. This has caused concern because the disappearance of the alewife may mean that coho and chinook salmon will not have any forage fish to consume, and the loss of these two species would hurt the valuable sport fishing industry. The cause for the decline of alewife has been debated by scientists and fishery managers, but clearly the alewife has difficulty living under variable conditions in the Great Lakes. This could be a classic case where an introduced species establishes a population that then undergoes wild fluctuations because it is not adapted to its new environment. It is not known whether the species that the alewife replaced, such as yellow perch, several minnows, and a cisco species (commonly known as the “bloater”), will return to former abundances. There are some indications that these natives are increasing in abundance, but it is not known if they can provide a substantial food supply for the new game fish.
The story of the species’ success in Lake Michigan and Lake Huron is an ironic one. The sea lamprey came in and virtually wiped out the only native trout species, the lake trout. This then paved the way for the alewife to establish itself because there were no large fish to control its population and it was able to efficiently use the available resources. In the 1960s the lamprey was controlled by chemical treatments and the alewife population became enormous. To control the alewife, four exotic predatory species were introduced. These alien species were very successful and an extremely valuable sport fishery developed for them. The alewife population had become highly valuable because it was supporting large populations of game species. However, predation from the salmon and other fish was so intense, the alewife population declined, which resulted in decreases in the fisheries. However, fewer alewives meant more food for other fishes, so populations of sculpin, burbot, and yellow perch increased (Madenjiaet al. 2002).
The populations of species in the Great Lakes are currently highly unstable. This is due to many factors and clearly one of the leading ones is that many of the species are not native to the lakes. This case study is a lesson of what can happen when human activities create a system dominated by alien species. Fish populations in the Great Lakes will continue to require intense management for the indefinite future.
INTRODUCED SPECIES ON OCEANIC ISLANDS
Although oceanic islands, such as Hawaii, Guam, Puerto Rico, make up a minute fraction of the Earth’s surface area, they are rich in endemic species adapted to their unique conditions. Unfortunately many of these species went extinct in recent years or are threatened with extinction. One of the main factors responsible for these extinctions is the vast number of which humans have introduced. In certain cases, the number of introduced species is astonishing. In the Hawaiian Islands, it has been estimated that 65 percent of all current plant species have been introduced, along with 20 mammals, 20 reptiles, 20 amphibians, and 50 bird species. There are as many introduced insects on the Hawaiian Islands as in the entire contiguous United States. Not surprisingly, most of the documented extinctions of plants and animals within the past 200 years have been native species associated with islands. The role introductions have played in these extinctions can be seen by examining the impact of an introduced snake on Guam, the loss of rock iguanas on Pine Cay in the West Indies, and the effects of feral animals on the Galapagos Islands.
Understanding why island ecosystems have been so vulnerable to species introduction and extinctions requires examination of several ecological principles. Because the islands are surrounded by oceanic water, they are difficult for continental plants and animals to colonize. The few that do make it often become ancestors to an array of strange and wonderful creatures, such as the brilliant honey creepers (birds) of Hawaii. Ecological communities on islands have fewer species so their structures are likely different from continental communities. They typically lack predatory land mammals, for example. This makes island systems relatively easy to invade. Island species are also highly susceptible to diseases and parasites introduced by humans and their domesticated species.
Brown tree snake on Guam
Guam is a small island in the Western portion of the Pacific Ocean about halfway between Japan and New Guinea. Recently it has been invaded by the introduced brown tree snake. The snake is native to Australia, New Guinea, and the Solomon Islands and was apparently introduced via military boats in the late 1940s, perhaps deliberately to control introduced rats. The native forest birds of Guam (a total of 18) have been declining steadily and several have become extinct. There are many possible factors contributing to the decline, but the brown tree snake is implicated for directly causing the decline by preying on bird eggs. The expansion of the range of the snake in Guam and the contraction of the ranges of birds has been highly correlated.
What accounts for the success of the brown tree snake on Guam? First of all, there are no significant predators or competitors to limit its population. This situation is similar to what we saw with the alewife and sea lamprey in the Great Lakes. Another reason is that because off the forest structure and degree of human development there were few places of refuge for the birds from the snake. Like many other successful introduced species, the snake has generalized food habits that allow it to maintain high population numbers. Lastly, it has the ability to go long periods without eating, a fact that allows it to maintain populations in the face of a low abundance of prey items. The future of the bird fauna clearly rests on whether the brown tree snake can be controlled. However, it will be very difficult and costly to control or eradicate these snakes, so it will likely be years before birds will be able to survive on the island.
The decline of rock iguanas on Pine Cay
Rock iguanas are a group of related species endemic to the West Indies. Their populations have been steadily declining since the arrival of humans and their associates: dogs, cats, pigs, and mongooses. Several rock iguana species are now extinct and others are approaching extinction. Pine Cay provided a unique opportunity to study the loss of iguanas in a relatively natural setting following the construction of a hotel and tourist facility in 1973 and subsequent release of cats and dogs at the facility (Iverson 1978). The iguana population was reported to have been near 15,000 and declined to the verge of extinction just three years after the hotel development. Apparently feral cats did live on the island in low numbers previous to hotel construction but were not a threat to the iguanas because they foraged on a large rat population. The cats brought in with the development wiped out the rat population, then switched to foraging on iguanas. Dogs may have been a more serious threat because they were often observed chasing and catching iguanas for sport. Clearly rock iguanas had little or no defense to the attacks of either cats or dogs. This is yet another example of the loss of a species which did not have the necessary evolutionary adaptations to survive the influence of introduced predators.
The Galapagos Islands are located in the Pacific Ocean about 1,600 km west of the South American continent and just below the equator. The islands draw attention from geologists because they are geologically young and of volcanic origin and from biologists because of their unique flora and fauna. The most famous biologist to study on these islands was Charles Darwin. The unique species and communities that Darwin and many others studied are now threatened by large numbers of exotic plants and animals. Humans have been introducing species since the early 1800s, and endemic members of several groups of plants have been reduced drastically and are in danger of extinction. Feral animals make up a large portion of the introduced animals and have been most destructive.
Goats on the Galapagos Islands are the most abundant and destructive of the feral animals. The first goats came in 1813 and soon spread throughout many of the islands. The speed at which they populated some of the islands is truly remarkable. For instance, on the island of Pinta a population of several goats increased to about 20,000 in only 15 years. One reason for the goat’s success is its ability to thrive at many different elevations on the islands and eat many different plants. Goats also have high rates of reproduction. The impact of goats on the native vegetation has been high. They have managed to remove a wide variety of plants including trees, ferns, shrubs, and herbs. An area on the island of Santa Fe was described as “so over-run by goats that grass and herbs were eaten to the roots during the dry season, exposing the soil to erosion. Bushes were torn up and even the tree-cacti were attacked” (Schofield 1989). Plants on the goat-infested islands are able to grow only in places inaccessible to goats. Several scientific studies documenting the impacts of goats convinced authorities to organize hunting efforts, which began in the early 1960s. Great numbers of goats have been killed and, in areas where they have been significantly removed, many plant species (except those driven to extinction) have begun to return (Schofield 1989).
Cattle, pigs, and donkeys have also been harmful to native vegetation, although their impact has been less than that of goats (Schofield). Each of these species had its own type of impact depending on the plant species and island considered. Efforts such as fencing, hunting, and poisoning have begun to control populations of cattle and pigs; there have been no efforts to control donkeys. Cattle have aided in the dispersal of an exotic plant, known as “guava.” They eat the fruits of guava and excrete the seeds indifferent locations. Guava out-competes native plants. There are also dozens of other exotic plants which have displaced native species.
The native organisms of the Galapagos Islands have been under siege from exotic plants and animals for nearly two centuries. It has only been recently that their declining numbers have been appreciated enough to encourage efforts for their protection. It is important for the scientific community to further study the impacts of exotic species there because essential data are missing that may prompt authorities to act to save the unique fauna and flora of the Galapagos.
It is worth noting that the problem with invading species in the Galapagos islands is not a remote and exotic problem. Feral goats, pigs, and sheep are a major problem on islands off the California coast, such as Santa Cruz Island, are their eradication (on-going) is regarded as essential to protect endangered species like the island gray fox and numerous other endemic plants and animals.
SAN FRANCISCO BAY AND ESTUARY: A DOWNHOME PROBLEM
San Francisco Bay and its estuary are arguably the most invaded aquatic system in the world. Nearly 300 species of non-native invertebrates and fish have become established there and new species are becoming established at the rate of one every 12 weeks. In San Francisco Bay, a vast majority of the invertebrates, from clams to zooplankton, are from some place else. Native species are rare. In the estuary, a majority of the fish are non-native, ranging from the striped bass to the shimofuri goby. Periodically, a new invader causes dramatic changes to the ecosystem. For example, in 1989 a new clam from Asia (overbite clam, Potamocorbula amurensis) was found. It quickly took over the bottom of the brackish parts of the estuary, with clams on top of clams creating densities in excess of 10,000 per square meter in places. This clam is now filtering the water column in Suisun Bay several times a day, removing most of the planktonic algae and animals. One result is a drastically reduced food supply for fishes, especially the larval forms that use the bay as a nursery area. Another problem is that because they filter the water so effectively, the clams are concentrating the heavy metal selenium in their flesh. This may ultimately create a toxic problem for animals which feed on the clams, such as sturgeon and diving ducks which feed on the clam.
The major source of recent introductions, such as the Asian clam, has been ballast water of ships, because San Francisco Bay is a major port. Millions of gallons of water containing millions of organisms are dumped into the bay every week. They continue to be introduced because the shipping industry does not have to pay for the damages caused by introduced species like the clam (or the zebra mussel in the Great Lakes). There are solutions to the problem but they involve changes to ship operations as well as continuous monitoring of the ballast water of incoming ships. As of 1997, legislation to regulate ballast water dumping on the west coast has failed to solve the problem. Stricter legislation has been passed, but it remains to be seen how well it will work.
Alien species have been introduced for many different reasons and have had multiple impacts on the places they invaded. Few alien species have been benign form the perspective of native species. Typically, we set the level of their impact on how much they have hurt our economy. For instance, sea lampreys and starlings have been considered major problems because millions of dollars have been spent for their control. There are numerous other introduced species that do harm that goes unnoticed because they are not economic threats or may even provide economic benefits (despite ecological damage). In the long term, to the benefit of ourselves and our environment, we need to think more in terms of ecology rather than in terms of economy. This means protection of species in their native habitats.
Introducing species can enhance local diversity in the short term, but the problems that exotic species can create (competition, diseases, unstable populations, etc.) threaten diversity over the long term and from a global perspective. Moyle et al. (1986), in a paper discussing the impacts of introduced fishes, puts the issue in perspective by the use of a parable:
“There is a long and honorable tradition in western culture, dating back at least to the Romans, of tinkering with fish faunas by adding new species. This tinkering is part of a much broader tradition of tinkering with nature, to “improve” on it. The moral and mechanical problems that are encountered when trying to improve on nature were dramatically illustrated in Mary Shelly’s famous novel (published in 1818), “Frankenstein, or the Modern Prometheus.” In this story, Count Frankenstein, a dedicated scientist, attempted to create an improved human being but soon discovered, to his mortal distress, that he had created more problems than he had solved. Most of his problems stemmed from focusing on the solution to a narrowly perceived problem without considering how the solution (the monster) would fit into society at large.”
This parable tells us that we should be concerned about long-term and broad-scale consequences of our actions. Although society perceives introductions of most exotic organisms as beneficial,these actions are collectively harmful to the planet and to society as a whole.
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Table of Contents
1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife
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