July 15, 1999
§ 2. The continuing history of invasions:
What they are, the damage they do, and where they come from
Not all exotics are detrimental. Only about 10% of the exotics introduced into the Great Lakes have been, but these "have had a significant impact on ecosystem health."(5)
What I am referring to generally as "exotics" are also commonly called "aquatic nuisance species" or "ANS." That term, although cumbersome, has the virtue of making clear that we are mostly concerned with undesirable aquatic invaders of the Great Lakes ecosystem. It is slightly misleading. The detrimental exotics are much more than just a "nuisance," and we sometimes have reason to be concerned about strains within species. They are also referred to in various places as "nonindigenous species" or "NIS," "non-native species," "harmful exotic species," "ecologically harmful species," or "unwanted aquatic organisms and pathogens," among other things. The most recent addition to the collection, deliberately chosen for the title of recent legislation in order to better mobilize political support, is "invasive species." Also, the term "biological pollution" has been floated lately. It has not yet caught on, but it has already been strongly objected to by some representatives of the shipping industry sensitive to the policy implications of such a term. This is exactly why it is used, and the debate about how ballast water should be regulated may well result in this term becoming more popular. I will speak of detrimental exotics as a form of "pollution," because that is exactly what they are in terms of both the technicalities of law and the theory of economics,(6) as well as common sense.
The sea lamprey. One of the first invaders of the Great Lakes was the sea lamprey (Peromyzon marinus), a primitive but highly predacious parasitic fish, resembling an eel, anadromous, which was apparently introduced through the canal systems in the 1830s. In the 1940s, it devastated native populations of whitefish and lake trout which were worth $14 million a year back then in commercial takes,(7) and which probably would be at least a billion dollars per year in commercial and recreational value to the Great Lakes today.(8) Almost all of the other large fish species in the upper Great Lakes were also significantly affected by lamprey predation.(9) As bad as the sea lamprey has been, it has actually been much more susceptible to control than most other exotics. But control of the sea lamprey costs over $15 million per year,(10) and requires the introduction of tens of thousands of pounds of biocide into the lakes each year, as well as other control measures which are being developed to reduce that.(11)
The zebra mussel. The best known invader, detected in 1988, and the one that finally prompted the first political action to prevent new invasions, is the zebra mussel (Dreissena polymorpha). It was originally from the Black and Caspian seas, and was apparently introduced from Europe through the ballast water of oceanic shipping sometime before 1988.(12) It fouls industrial water systems, smothers and starves out other benthic organisms, dramatically alters nutrient balances and lower food chains, and may play a significant role in making toxics more bioavailable. The zebra mussel is the "poster child" which led directly to both the Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 (NANPCA 90)(13) as it impacted the Great Lakes, and to the National Invasive Species Act of 1996 (NISA 96)(14) as it spread through the Mississippi Basin and threatened the rest of the United States. It has been notable for the amount of direct economic cost it causes by clogging industrial water piping and almost anything else that touches the water. Estimates of the direct costs vary, but they range from hundreds of millions to a few billion per year. One of the most recent estimates puts the total cost of the zebra mussel to the United States (mainly the US Great Lakes and the Mississippi Basin) at $3 billion per year.(15) (One can assume comparable costs, relative to population and industry, to the Canadian side of the Great Lakes Basin.) These are "nuisance" costs. Although they are substantial (and have been responsible for the political attention), the damage in disruption of the native ecosystems of the Great Lakes may be much worse.
It is difficult to be precise. No invader, except perhaps the sea lamprey, has been studied as well as the zebra mussel. But the exact nature of its ecosystem effects is still being sorted out. It has certainly had a dramatic effect on other mollusks, and could well be leading to their extirpation or extinction. "The Lake St. Clair-Western Lake Erie corridor once had the richest and most diverse assemblages of large freshwater clams in North America. Within six years of the discovery of the zebra mussel in this region, freshwater clam populations in the region had declined to almost zero. Biodiversity has declined sharply as the functional community has shifted from a stable, slow-growing, multi-species clam community to a single-species population of zebra mussels with a relatively high turnover rate of energy that strongly affects ecosystem dynamics."(16) Other benthic organisms (things on the bottom) are impacted by the amazing ability of the zebra mussel to physically cover the bottom of shallow areas - some parts of Lake Erie appear to be carpeted by the things - which squeezes out habitats for a variety of other native organisms, including fish eggs. Limnetic organisms (things floating or swimming in the water column) are impacted by their consumption of nutrients.
More generally, the fact that zebra mussels are highly active filter feeders, who then spit out the end product of this process on the bottom as "pseudofeces," changes the whole "energy balance" or food distribution in the system. In the course of doing so they clarify the water column. They may have some desirable effects. Among other things, they may help increase the production of native mayflies, which are important to desirable fish in the Great Lakes,(17) and they can make the water look cleaner. However, there are some distinctly undesirable side-effects of this "cleaning." The effect may be to concentrate toxic contaminates and make them more bioavailable. Sudden changes in light available to the bottom can cause blooms of submerged vegetation. And there is some indication that the mussels select out the more desirable algae, giving a competitive advantage to toxic algae. Overall - although research continues - the general consensus in the scientific community seems to be that, whatever benefit might be expected from zebra mussels cleaning up the water column is more than counteracted by the starvation of beneficial fish, concentration of contaminates in the food chain, and possible preferential selection of toxic algae.(18)
The ruffe. The ruffe (Gymnocephalus cernuus), referred to as either "European" or "Eurasian" ruffe (and pronounced "rough"), is another invader detected in 1988, just before the zebra mussel. It is a garbage fish from Europe and Asia, introduced through the ballast water of oceanic ships sometime before 1986, which has a high potential to displace native species. It illustrates the great difficulty of determining the precise nature of the economic and ecosystem effects. It has been largely confined for the time being to waters on the western end of Lake Superior, after being deposited there by the ballast water of international shipping calling on the Port of Duluth-Superior. (This is the largest Great Lakes port for shipping grain overseas, and therefore is also the largest recipient of overseas ballast water, which is pumped off when the vessel takes on cargo.) In 1993, the population of ruffe plateaued, and it has not yet broken out of Western Superior except for a small number discovered in Thunder Bay, Michigan, in 1995. The good news is that the ruffe, which does better in warmer water, has had difficulty migrating past the natural barrier of the cold waters in the central part of Lake Superior. The bad news, however, is that it is likely to eventually work its way along the shoreline, and make significantly faster progress once it breaks through the Kewennaw Peninsula. It is unclear whether the few ruffe found in Thunder Bay, probably brought there by internal Great Lakes shipping, are representatives of an established population.
The ruffe threatens to displace native fish because it has high rates of reproduction, is an efficient bottom feeder (eating a wide variety of foods, including the eggs of other fish), is tolerant of poor water conditions, and has few natural predators. Protective spiny fins make it an unsavory meal for the higher level-predators in the Great Lakes. Because the ruffe is still confined to a small area of cold water (despite its complete success in establishing itself there) it is difficult to predict its effect when it eventually breaks out to the rest of the Great Lakes. "Experience in Scotland and Russia, however, points to serious problems for North American fisheries if ruffe escapes from its limited range in western Lake Superior. In Loch Lomond, Scotland, native perch populations declined dramatically when the ruffe was introduced. In some Russian waters, the ruffe has harmed whitefish populations by preying heavily on whitefish eggs. Movement of the ruffe to the lower Great Lakes, or to inland lakes and rivers, could have devastating consequences."(19) Valuable warm water fisheries in the lower lakes, such as the Ohio Lake Erie fishery worth about $600 million per year, could be significantly affected.(20)
Attempts to conduct an objective threat evaluation, a difficult exercise in prediction when dealing with any biological problem, have been complicated by a vehement policy dispute among fellow environmentalists about whether or not to try blocking the slow progress of the ruffe along the shoreline with biocides. When the Ruffe Control Committee of the US Aquatic Nuisance Species Task Force met to approve a cost-benefit analysis of the overall Ruffe Control Program, some committee members thought that "the projections were so unrealistic as to be useless." One participant was of the opinion that "arguments over economic impacts seemed to reflect policy interests; those opposed to aggressive ruffe control also opposed projecting impacts of ruffe."(21) Ironically, the shipping industry was able to play an entirely constructive and non-controversial role in the control program. The Canadian Shipowners Association and the US Lake Carriers' Association put together a voluntary program for the exchange of ballast water in the cold depths of Western Superior, by vessels leaving the ports of Duluth-Superior and Thunder Bay, in order to help slow down the spread. At this point, however, "prospects for controlling their spread in the Great Lakes are poor…."(22)
Other invaders. Other invaders of the Great Lakes which biologists believe to have had "significant impacts" include purple loosestrife,(23) the alewife,(24) furunculosis,(25) Eurasian watermilfoil,(26) and Glugea hertwigi.(27) This is the short list of the worst, out of more than 140 documented introductions to the Great Lakes since the 1800s.(28) (Others may make the short list after we get to know them a little better.) Note that this list includes more than just the larger organisms such as the sea lamprey and the zebra mussel. Those are the easiest to identify and study. But exotic species which may be of real threat include every form of flora and fauna up and down the taxonomic scale.(29)
A 1993 study by the Congressional Office of Technology Assessment (OTA) attempted a comprehensive survey of all invasions of the United States - over 4,000 species, including 2,000 plants, 2,000 insects, 239 plant pathogens, 142 terrestrial vertebrates, 91 mollusks, and 70 species of fish. OTA reported in 1993 that the damage from those which are harmful runs anywhere from hundreds of millions to billions of dollars per year, but declined to offer precise estimates of the economic damage, or to put a value on what the OTA called "profound environmental consequences."(30) A more recent study at Cornell University in 1999 counted more than 30,000 exotics in the United States and totaled up the economic costs of the harmful ones at about $123 billion per year.(31) (This does not mean that the number increased so much during that time.) The accounting for specific categories of aquatic species is less impressive, but still amounts to real money. In addition to the zebra mussel's cost of $3 billion per year, the study found that the Asiatic clam costs the US about $1 billion per year, fish cost another $1 billion per year, and aquatic plants cost $110 thousand per year.(32)
More importantly, exotic species are a threat to the survival of native species in the areas they invade and the long-term biological heritage of the human species. The Convention on Biodiversity identifies exotics and genetically modified organisms as major threats to global biodiversity.(33) Exotics "cause fundamental, irreversible alterations in the structure of communities through predation, competition, disturbance and the introduction of disease and parasites. No introduced marine organism, once established, has ever been successfully removed or contained…."(34) A recent study by the Environmental Defense Fund was one of the few attempts at a quantitative assessment of environmental threats. It found that invasions by exotics in general are second only to destruction of habitats as a cause of extinctions - although aquatic species are third, behind destruction of habitats and pollution, as a cause of aquatic extinctions.(35) (The study includes changes in aquatic habitat characteristics as a form of pollution. That, in fact, is one of the effects of some exotics.) A study by the American Fisheries Society found that 40 North American freshwater fishes have become extinct over the past century, and that exotics were a contributing factor in 68% of these extinctions. They also found that exotics are implicated in the decline of 42% of those species listed as threatened or endangered by the US Fish and Wildlife Service.(36) Mention of endangered species helps to put the problem into perspective, because what we are really talking about here is protecting endangered ecosystems. The damage may not be as immediate and dangerous to human health as toxic contaminants, but it is forever.
It might help to put the problem in perspective by comparing the damage done by exotics (and our political response to it) with oil spills. More than 10.8 million gallons of crude oil spilled into the waters of Prince William Sound when the tanker Exxon Valdez ran aground in 1989. This was the worst oil spill in US waters, and led directly to major new programs for prevention and response under the Oil Pollution Act of 1990 (OPA 90),(37) which are far more comprehensive (and expensive) than any of the analogous measures for the control of ballast water under NISA 96. Although the impact on life in the sound was devastating, it was temporary. It does not appear that any species was driven to extinction by the spill. Many of the affected populations have not fully recovered. Some have, including the salmon that is so important to the economy of the area. The Exxon Valdez Oil Spill Trustee Council, which oversees ongoing monitoring and restoration, predicts that "Complete recovery from the Exxon Valdez oil spill will not occur for decades," but also observes that "Based on results gathered to date, through restoration monitoring studies, it appears that affected systems and their constituent populations may regain normal species composition, diversity, and functional organization through natural successional processes."(38) Without minimizing the scope of that disaster, or discounting the possibility of unpredictable long-term impacts on some of the affected species, I would suggest that the long-term damage to aquatic ecosystems is far less than what is being done by the zebra mussel as it continues to spread through the Great Lakes and the Mississippi Basin. There will never be any recovery of "normal species composition, diversity, and functional organization" in this huge area of natural aquatic resources in the heartland of North America.
The primary vectors for exotic invasions of the Great Lakes have changed over time.(39) Whereas it was once canals and intentional releases which were the predominant vectors (or pathways), it is now ballast water and "unintentional releases." Ballast water is very likely to remain the predominant vector in the near future. (Other shipborne vectors, such as hull fouling, may also be a problem.) "Unintentional releases" from aquaculture, bait, and aquaria may be a significant problem in the future, but this is more debatable.
Canals. The sea lamprey and the alewife are characteristic of an older type of introduction. They were native to the Atlantic seaboard, perhaps even native to Lake Ontario, but spread upward into the whole Great Lakes ecosystem via the canals developed around the turn of the century. Purple loosestrife may have originally invaded the Atlantic seaboard via solid ballast in shipping, and then also followed the canals and railroads into the lakes around the turn of the century. That vector has declined in importance over recent decades, perhaps simply because the railroads and canals are already built and most of those organisms which could make use of the opportunity to spread around the continent on human transportation systems have already done so.
Intentional and "Unintentional" Releases. Furunculosis, a fish pathogen, and Eurasian watermilfoil are representative of another vector, or group of vectors. Furunculosis came along with trout and salmon which were deliberately introduced around the turn of the century. Eurasian watermilfoil was probably introduced in aquariums, if not transported by recreational boats or shipping, and became established in the lakes around 1960. This sort of introduction is sometimes called an "unintentional release," defined to include "escape from cultivation, aquaculture and aquaria, and accidental releases due to fish stocking and from unused bait."(40) But the distinction between an "intentional" and "unintentional" introduction is fuzzy. For example, the US Aquatic Nuisance Species Task Force defines "escapes from aquaculture or aquarium facilities and activities such as dumping of baitfish and home aquarium species," along with deliberate fish stocking, as "intentional" introductions.(41) The difference is whether there is a focus on the initial introduction of the organism in a controlled setting, which is intentional, or the consequent escape of the organism into the natural environment, which is usually unintentional (but often easily foreseeable and negligent(42)).
Also, the Great Lakes are filled with exotic salmonids deliberately stocked by public authorities. This began in the 1870s and became accelerated with advances in culturing technology in the 1960s. Although they were selected for their beneficial characteristics, and are valuable to many people, they have detrimental long-term effects on native species and the integrity of the Great Lakes.(43) Some biologists hope that "the era of widespread, intentional introduction of salmonids and other fish species as a fishery management activity justifiably is drawing to a close."(44)
Ballast water. The zebra mussel and the European ruffe, most probably carried by the ballast water of transoceanic shipping and introduced sometime before 1988, are characteristic of most current introductions. Shipping was always a major vector, but it became more so, and has now significantly overtaken all other vectors, since the opening of the Saint Lawrence Seaway in 1959.(45) Before then, transoceanic ships could not enter the Great Lakes. Now they do, in numbers varying between four and six hundred or more per year, and a large number of them travel all the way in to Duluth or Chicago. Ships have become faster over the years, thus increasing the probability that organisms will survive on board during the voyage. They come to Great Lakes ports from all over the world, including many tropical ports, but about 70% participate in a "triangle trade" between the Great Lakes, the Mediterranean, and Northern European ports. (See § 3.3 below.) Those areas, both ports of commerce in the times of the ancient Greeks, may be the most badly infected, "cosmopolitan"(46) waters in the world. (The zebra mussel was originally native to the Black and Caspian Seas. It first invaded the trade ports at the mouth of the Rhine in 1820-1830 before eventually being carried across the Atlantic to the Great Lakes in the 1980s.(47) The Mediterranean is currently being impacted by a "mutant seaweed," Caulerpa taxifolis, which seems to be as aggressive and as deadly to other benthic life as the zebra mussel.(48) Also, both North Atlantic and Mediterranean waters have been infected by exotics from North America.) The structure of this triangle trade, moreover, could not be much better designed for the transport of exotic pathogens. They are most likely to be present in large concentrations in warm waters, such as the Mediterranean. But they are more likely to survive the transoceanic voyage if refrigerated and made inactive by cool water, picked up in the North Atlantic. Exotics may be thought of as a form of plague. Unfortunately, shipping has always been an efficient means for spreading plague.
In sum, the great preponderance of introductions, throughout the whole history of the lakes, has been through two vectors, "unintentional releases" and shipping.(49) Along with a steady flow of introductions via unintentional releases, the increase in shipping introductions is responsible for a dramatic increase in overall introductions in recent decades. Biologists count 141 invasions of the Great Lakes during the 188-year span of 1810-1997. Of those, 43 (30% of the total number) have occurred in the 38-year period of 1960-1997 (20% of the time span) since the opening of the St. Lawrence Seaway, almost entirely from shipping and unintentional releases.(50) Introductions of exotics are on the rise.
In both ecology and economics, it is a mistake to assume that a trend will continue at current rates of increase (that, in other words, the curve will continue to bend in the same direction(51)). It is perhaps dismal, but relevant, to ask whether or not the Great Lakes are already so significantly impacted that the worst has already happened and there is little to be accomplished by future preventative efforts? Probably not. What little science there is to provide a threat assessment of future invasions seems to tell us that there is still a lot out there that could be harmful. And it is particularly relevant that even some of the most heavily infested waters around the world, such as the Mediterranean and the Northern European ports, continue to discover new invaders which could well be on their way to the Great Lakes.(52) For example, a review of the literature in 1998 identified 17 freshwater aquatic animals from the basin of the Black, Caspian, and Azov Seas which are prime candidates for invasion of the Great Lakes because they have recent invasion histories, are likely to be transported overseas in ship ballast water, and have broad salinity tolerance which could allow them to survive an incomplete ballast water exchange.(53) I trust the biologists will not be offended if we assume that there is much more out there that has yet to be identified. Moreover, the future potential for global warming and climate change in the Great Lakes may have totally unpredictable effects on the vulnerability of the Great Lakes to new invasions.(54)
The overwhelming importance of ballast water as the primary vector for current invasions is fairly well accepted. Aside from the obvious association of shipping with identified invasions in time (increases corresponding to the opening of the St. Lawrence Seaway) and space (Duluth-Superior Harbor), and the lack of other plausible vectors to explain many introductions, a number of actual studies of ballast water arriving in the Great Lakes have confirmed the presence of live organisms able to colonize a fresh water system.(55)
Things are less clear when we look at other vectors. Some of the environmental agencies are particularly concerned about potential introductions from commercial aquaculture, baitfish transportation across watersheds, and imports of aquarium fish and plants. These are all potential vectors for the sort of "unintentional releases" mentioned above, although they do not exhaust the general category. They can be lumped together (and sometimes overlap a bit in actual practice) as "commercial uses" of live aquatic organisms. The commercial nature of the activities has two important aspects (aside from simply making them an attractive target for government regulation, in the opinion of some of the people in these businesses).
First, because they are commercial activities, they have a potential to transport a large number of aquatic organisms across wide ecological boundaries. Some purely private or recreational activities, such as private collection of baitfish or simple movement of recreational boats around the country, also move exotics around. On the whole, however, the scale of that transport, in terms of both quality and distance, is less by orders of magnitude. Private recreational fishers and boaters, moreover, do not introduce genetically modified organisms.
Second, because they are legitimate and useful activities, they will tend to expand. (In fact, there is a strong government policy of promoting the development of aquaculture, in both the US and Canada, at both federal and state and provincial levels.) The legitimate special interests who engage in this commerce will naturally exert constant pressure to widen the use of exotics - including the use of genetically modified organisms.(56) On the other hand, some of these businesses also have a strong interest, within certain limits, in preventing unintentional introduction of exotics and diseases associated with them. They should also have an enlightened self-interest in maintaining some diversity in the genetic stock.(57) More generally, there are naturally some control measures used in these businesses as a matter of quality control for their own purposes, and there are sometimes natural concentrations of storage or transport facilities in large-scale commerce. This offers some natural targets of opportunity, "critical control points," at which government regulators might interdict exotics in cooperation with the businesses concerned.(58)
In the following sections, I present a closer look at the major technological and economic issues specific to these major vectors of concern.