| CHAPTER 7 |
| 7.1 INTRODUCTION |
The introduction of new species to an area can result in a loss of biodiversity. This statement seems to contradict itself, but it is true. The reason: when a species invades an area it can drive native species to extinction. Introduced species 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 localized native species disappear. Species that are part of this elite, increasingly world-wide fauna include the black rat, house mouse, muskrat, feral goat, house sparrow, starling, common carp, brown trout, mosquitofish, Japanese clam (Corbicula), and cabbage butterfly. This chapter deals with introduced species and explains why they are introduced, why they are successful, and how they affect native species.
An introduced species is as one that invades, as the result of human activities, an area where it was not historically present and therefore where it has not evolved. 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 allowed these species to cross a major barrier, the Atlantic Ocean. An introduced species is opposed to a native species, one which has evolved in the area where it is found.
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 for sale 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.
Often introduced 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, introduced species 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 human alteration of habitats and introductions of exotic species have put entire native floras and faunas in danger of extinction.
6.2 WHY ARE SPECIES INTRODUCED?
Species can be introduced deliberately for a variety of reasons. One of these is for aesthetic enhancement of the surrounding environment. During the European settlement of the New World, there were many plants and animals brought over because of unfamiliarity with New World species. As discussed in Chapter 2, many European settlers feared the wildness of the Americas, and introducing species familiar to them was a means of reducing their uneasy feelings. 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 in part to satisfy the needs of familiarity. Nearly all trees, shrubs, and flowers in urban settings are introduced for aesthetic reasons. If you look around Davis, you will find few plant species that were here before settlement. Of the major street trees, only the sycamore (along Russell Blvd) was probably found here 150 years ago.
Organisms can be deliberately introduced as a food source. 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. Bullfrogs, introduced as a food source in the western United States, have become a dominant member of many aquatic systems. Domesticated animals such as goats, pigs, and cattle have been introduced throughout the world for food and often have enormous impacts on their environment when they establish feral (wild) populations. Nowhere is this more evident than on oceanic islands (see section 7.8).
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.
Another reason organisms are introduced is for recreation. 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. Examples include brown trout and ring- necked pheasants.
Species are introduced on occasion for biotic control. This biological management technique is used when a species, often an introduced plant or small animal species, becomes so abundant that it creates problems. To control the spread of the pest species, another species that is known to consume the pest is introduced. In the past, control species have become pests as well. 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. However, the effects of introducing vertebrates is highly unpredictable and dangerous to ecosystems, so using them for biotic control is risky.
Another reason for introduction, and one which is surprisingly common, is through release of pets, better called misdirected kindness . Hundreds of exotic species brought to North America to occupy aquaria, cages, and backyards sometimes escape or are released by owners tired of taking care of them. In the majority of cases, these transplants 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 establishment of populations. This is especially the case with exotic fish that have been released in large numbers by hobbyists or have escaped from fish farms. As 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 influence from these species and habitat alteration has put many native species in these areas on the endangered list. Exotic 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 apparently 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.
Release of cats into wild areas is a form of ecosystem manipulation, a reason for deliberate introductions by management agencies who wish to increase the biological productivity of a system to increase the numbers of desirable species. This is often done by introducing organisms to enhance the forage base of a desirable species such as game animals. An example of this occurred in Lake Tahoe. A small shrimp was introduced to provide food for lake trout (introduced as well). After this was done, the lake began losing its remarkable clarity as the number of algae in the lake increased. One of the reasons suggested for the clarity loss was that shrimp were eating zooplankton species that normally would have eaten the algae. This occurred at a time when increased nutrients were also entering the lake due to runoff and septic systems in the developed regions. Presumably the two factors worked together to reduce clarity. Ironically, the lake trout did not increase in response to the shrimp, and numbers of other species such as kokanee salmon declined. Like biological control, ecosystem manipulation is a risky venture because of the unpredictability in how organisms will respond to changes in their environment.
While the preceding types of introduction were all deliberate introductions of individuals of known species, an increasingly common kind of introduction is as a by-product of human activity, especially commerce. Such introductions are often considered to be accidental but since we now know they are occuring (just not precisely when and what) they must be considered to be deliberate. Two common kinds of by-product introductions are range expansion 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 canals and be transported to new places. Sea lampreys and alewife invaded the Great Lakes through canals constructed for transportation. Likewise, 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 introduced 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. It is also competing with native clams, helping to push them to endangered status.
Some of the most devastating introduced species are those that have become feral animals. These animals were once domesticated before becoming wild and free-roaming. Examples include goats, pigs, cats, dogs, 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.
6.3 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 North Pole and a polar bear could not invade the Everglades because of inappropriate temperatures for their survival. Therefore, an attribute of many introduced species is their ability to withstand a wide range of physical conditions. 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. The aggressive pursuit of food and places to reproduce are ways that an introduced species can become competitively dominant. This often occurs when the introduced species has larger or faster growing individuals. Possibly the most important attribute is being able to live in close association with humans. If a species can survive in urban, agricultural, or other areas impacted by humans, it has a good chance of being successful. In fact, altered habitats are often dominated by introduced species.
Although introduced species are likely to have the general characteristics listed above, in fact given the increase in extent and speed of global trade and transportation and given our increasing knowlege of biology, really only two rules apply to introduced species:
1. Any species can be successfully introduced.
2. Any ecosystem can be invaded by introduced species.
7.4 WHAT AREAS ARE MOST VULNERABLE TO INTRODUCTIONS?
Ecological systems are so complex that it is extremely difficult to predict just where 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 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 can readily establish themselves in new reservoirs that fill after dams are built, but they 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 simplified communities because few species have been able to find their way to them. The simplified communities make them easily invaded because the species in these areas have evolved in the presence of few other species and thus do 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.
7.5 CASE STUDY OF INTRODUCED BIRDS: 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 initiation of the introduction was through the efforts of 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 1960's, California initiated eradication programs to control starlings. The programs were successful in removing a great number of starlings. However, those that survived have been able to build the population back up to enormous numbers.
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 eastern bluebirds, northern flickers, and red-headed 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.
7.5.2 House Sparrows
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 impacting native species that have similar food and nesting requirements. However, recent research contends that evidence for displacement of native birds by house sparrows is anecdotal and 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.
7.6 CASE STUDY OF INTRODUCED FISH: 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. Both species have been extremely successful in colonizing waters across the continent, and the cost to native species has been high.
7.6.1 Common carp
Common carp, native to parts of Asia and Europe, has a long history of being introduced in various places in the world. In its native range, it is held in high esteem as a valuable game and food species. In Europe during the Middle Ages monks used carp as fish for their monastery ponds and included them in their diets. 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. Many Americans feel that carp are unsightly and unsuitable for consumption and that they harm the sought-after game species. 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.
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 have been able to withstand the treatment 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. Since 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) and a fine sport fish, especially on light tackle (ask any warmwater angler in England).
7.6.2 Brown Trout
Unlike carp, brown trout are sought after as game fish in North America. 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, uninformed anglers do move carp and brown trout ( and other fish) around, creating problems for native fish populations and often for other anglers..
7.7 CASE STUDY: 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 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 exotic 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 provides a barrier to the movement of ships through the entire system. Early settlers realized what an enormous economic asset the lakes could be if the lakes could be opened to shipping, and this led to the construction of the Welland Canal between Lake Erie and Lake Ontario in the 1800's. When the canal eventually opened in 1929, species in Lake Ontario, including the alewife and sea lamprey, had instant access to the upper four Great Lakes.
7.7.1 Sea Lamprey
The sea lamprey is an eel-like fish that is parasitic on large fish; it attaches to their sides and extracts fluids. The sea lamprey was first reported in small numbers in Lake Ontario during the 1830's. The lamprey apparently came in large numbers during the 1870's through the Erie Canal and Mohawk-Hudson River drainage in the state of New York. As the populations grew into the 1900's, their presence was not felt to a large extent because the fish populations which it potentially could have harmed such as the lake trout and Atlantic salmon had already been severely depleted by commercial fisheries and loss of habitat for reproduction. By the time the Welland Canal was opened, the lamprey population was a like a bomb ready to explode into the upper Great Lakes. Sea lamprey swarmed into Lake Erie, where they have not become abundant because of high water temperatures and poor habitat for reproduction. However, they continued into Lake Huron and Lake Michigan where they found many prey species and established large populations. The lamprey attacked the native lake trout species, 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 1950's, lake trout populations plunged to near extinction in Lake Michigan and Lake Huron.
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. 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.
Today, 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 activities (construction of the Welland Canal) the lamprey was able to invade a system containing fishes that were not adapted in any sense for 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 may prove to be resistant to lamprey attacks because the Pacific salmon evolved with the Pacific lamprey which is similar to the sea lamprey.
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 dieoffs. Dead alewives literally covered beaches and plugged municipal intakes, often cutting off water supplies to cities and industries. In several extreme cases, the dieoffs were so severe that they created public health problems for shoreline communities. The reasons for the dieoffs 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 outcompete native species for zooplankton and because there was little threat of predation because the sea lamprey and commercial fishing had reduced those species that consumed the 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 exotic game fish, brown trout, rainbow trout, and pink salmon, were also introduced.
The alewife population apparently has been declining since the mid-1970's 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 exotic species were very successful and an extremely valuable sport fishery developed for them. The alewife population had become highly valuable because it was supporting the fish which were supporting the game species. The alewife population has declined, which has caused concern for the sport fishery because the desired game species may not have substantial food. If the alewife disappears, it is likely that the large populations of game fish which were enjoyed for several decades will not be possible.
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 comprised largely of introduced species. The populations in the Great Lakes will continue to be unstable for the indefinite future.
7.8 CASE STUDY: INTRODUCED SPECIES ON OCEANIC ISLANDS
Although oceanic islands, such as Hawaii, Guam, and 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 became extinct in recent years or are threatened with extinction. One of the main factors responsible for these extinctions is the vast number of species 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 introductions 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 simplified. 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.
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 a recent study (Savidge 1987) strongly implicates the brown tree snake for directly causing the decline by preying on bird eggs. The study details how 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 of 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 allows 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 lack 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.
7.8.2 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 as 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.
7.8.3 Galapagos Islands
The Galapagos Islands are located in the Pacific Ocean about 1600 km west of the South American continent and just below the equator. The islands have drawn 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 1800's, 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 1960's. 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 1989). Each of these species has 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 in different locations. Guava has been reported to outcompete 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.
7.9 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 asiatic 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.
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. Presumably, strict rules will have to wait until the zebra mussel or a similar species invades, creating some new huge and expensive problems.
Species have been introduced for many different reasons, have used different means to maintain their populations, and have had multiple impacts on their environment. Few introduced species have been benign from 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 and we have spent millions of dollars 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. 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 Shelley'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.
Most techical literature on the effects of introduced species is contained in journals such as Ecology, Conservation Biology, and Biological Conservation (see especially Vol. 78(1- 2), 1996)
Elton, C. S. 1958. The Ecology of Invasions by Animals and Plants. Methuen, London. 181 p.
Iverson, J. B. 1978. The impact of feral cats and dogs on populations of the West Indian rock iguana, Cyclura carinata. Biol. Conserv. 14:63-73.
Long, J. L. 1981. Introduced Birds of the World. Universe Books, New York. 528 p.
Mooney, H. A., and J. A. Drake (eds.). 1986. Ecology of Biological Invasions of North America and Hawaii. Springer- Verlag, New York. 321 p.
Moyle, P. B., H. W. Li, and B. A. Barton. 1986. The Frankenstein effect: impact of introduced fishes on native fishes in North America. Pages 415-426 in R. H. Stroud, ed. Fish culture in fisheries management. American Fisheries Soc., Bethesda, Md.
Savidge, J. A. 1987. Extinction of an island forest avifauna by an introduced snake. Ecology 68(3):660-668.
Schofield, E. K. 1989. Effects of introduced plants and animals on island vegetation: Examples from the Galapagos archipelago. Conserv. Biol. 3(3):227-238.
The section headings (Abstract, Introduction, etc.) should be centered and the body of each section should follow immediately below the heading. Do not begin each section on a new page. If one section ends part of the way down the page, the next section heading follows immediately on the same page.
One important general rule to keep in mind is that a scientific paper is a report about something that has been done in the past. Most of the paper should be written in the PAST TENSE (was, were). The present tense (is, are) is used when stating generalizations or conclusions. The present tense is most often used in the Introduction, Discussion and Conclusion sections of papers. The paper should read as a narrative in which the author describes what was done and what results were obtained from that work.
Every scientific paper must have a self-explanatory title. By reading the title, the work being reported should be clear to the reader without having to read the paper itself. The title, "A Biology Lab Report", tells the reader nothing. An example of a good, self-explanatory title would be: "The Effects of Light and Temperature on the Growth of Populations of the Bacterium, Escherichia coli ". This title reports exactly what the researcher has done by stating three things:
If the title had been only "Effects of Light and Temperature on Escherichia coli ", the reader would have to guess which parameters were measured. (That is, were the effects on reproduction, survival, dry weight or something else?) If the title had been "Effect of Environmental Factors on Growth of Escherichia coli ", the reader would not know which environmental factors were manipulated. If the title had been "Effects of Light and Temperature on the Growth of an Organism", then the reader would not know which organism was studied. In any of the above cases, the reader would be forced to read more of the paper to understand what the researcher had done.
Exceptions do occur: If several factors were manipulated, all of them do not have to be listed. Instead, "Effects of Several Environmental Factors on Growth of Populations ofEscherichia coli " (if more than two or three factors were manipulated) would be appropriate. The same applies if more than two or three organisms were studied. For example, "Effects of Light and Temperature on the Growth of Four Species of Bacteria" would be correct. The researcher would then include the names of the bacteria in the Materials and Methods section of the paper.
The abstract section in a scientific paper is a concise digest of the content of the paper. An abstract is more than a summary. A summary is a brief restatement of preceding text that is intended to orient a reader who has studied the preceding text. An abstract is intended to be self-explanatory without reference to the paper, but is not a substitute for the paper.
The abstract should present, in about 250 words, the purpose of the paper, general materials and methods (including, if any, the scientific and common names of organisms), summarized results, and the major conclusions. Do not include any information that is not contained in the body of the paper. Exclude detailed descriptions of organisms, materials and methods. Tables or figures, references to tables or figures, or references to literature cited usually are not included in this section. The abstract is usually written last. An easy way to write the abstract is to extract the most important points from each section of the paper and then use those points to construct a brief description of your study.
The Introduction is the statement of the problem that you investigated. It should give readers enough information to appreciate your specific objectives within a larger theoretical framework. After placing your work in a broader context, you should state the specific question(s) to be answered. This section may also include background information about the problem such as a summary of any research that has been done on the problem in the past and how the present experiment will help to clarify or expand the knowledge in this general area. All background information gathered from other sources must, of course, be appropriately cited. (Proper citation of references will be described later.)
A helpful strategy in this section is to go from the general, theoretical framework to your specific question. However, do not make the Introduction too broad. Remember that you are writing for classmates who have knowledge similar to yours. Present only the most relevant ideas and get quickly to the point of the paper. For examples, see the Appendix.
MATERIALS AND METHODS
This section explains how and, where relevant, when the experiment was done. The researcher describes the experimental design, the apparatus, methods of gathering data and type of control. If any work was done in a natural habitat, the worker describes the study area, states its location and explains when the work was done. If specimens were collected for study, where and when that material was collected are stated. The general rule to remember is that the Materials and Methods section should be detailed and clear enough so that any reader knowledgeable in basic scientific techniques could duplicate the study if she/he wished to do so. For examples, see the Appendix.
DO NOT write this section as though it were directions in a laboratory exercise book. Instead of writing:
Simply describe how the experiment was done:
Also, DO NOT LIST the equipment used in the experiment. The materials that were used in the research are simply mentioned in the narrative as the experimental procedure is described in detail. If well-known methods were used without changes, simply name the methods (e.g., standard microscopic techniques; standard spectrophotometric techniques). If modified standard techniques were used, describe the changes.
Here the researcher presents summarized data for inspection using narrative text and, where appropriate, tables and figures to display summarized data. Only the results are presented. No interpretation of the data or conclusions about what the data might mean are given in this section. Data assembled in tables and/or figures should supplement the text and present the data in an easily understandable form. Do not present raw data! If tables and/or figures are used, they must be accompanied by narrative text. Do not repeat extensively in the text the data you have presented in tables and figures. But, do not restrict yourself to passing comments either. (For example, only stating that "Results are shown in Table 1." is not appropriate.) The text describes the data presented in the tables and figures and calls attention to the important data that the researcher will discuss in the Discussion section and will use to support Conclusions. (Rules to follow when constructing and presenting figures and tables are presented in a later section of this guide.)
Here, the researcher interprets the data in terms of any patterns that were observed, any relationships among experimental variables that are important and any correlations between variables that are discernible. The author should include any explanations of how the results differed from those hypothesized, or how the results were either different from or similar to those of any related experiments performed by other researchers. Remember that experiments do not always need to show major differences or trends to be important. "Negative" results also need to be explained and may represent something important--perhaps a new or changed focus for your research.
A useful strategy in discussing your experiment is to relate your specific results back to the broad theoretical context presented in the Introduction. Since your Introduction went from the general to a specific question, going from the specific back to the general will help to tie your ideas and arguments together.
This section simply states what the researcher thinks the data mean, and, as such, should relate directly back to the problem/question stated in the introduction. This section should not offer any reasons for those particular conclusions--these should have been presented in the Discussion section. By looking at only the Introduction and Conclusions sections, a reader should have a good idea of what the researcher has investigated and discovered even though the specific details of how the work was done would not be known.
In this section you should give credit to people who have helped you with the research or with writing the paper. If your work has been supported by a grant, you would also give credit for that in this section.
This section lists, in alphabetical order by author, all published information that was referred to anywhere in the text of the paper. It provides the readers with the information needed should they want to refer to the original literature on the general problem. Note that the Literature Cited section includes only those references that were actually mentioned (cited) in the paper. Any other information that the researcher may have read about the problem but did not mention in the paper is not included in this section. This is why the section is called "Literature Cited" instead of "References" or "Bibliography".
The system of citing reference material in scientific journals varies with the particular journal. The method that you will follow is the "author-date" system. Listed below are several examples of how citations should be presented in the text of your paper. The name(s) of the author(s) and year of publication are included in the body of the text. Sentence structure determines the placement of the parentheses.