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Biological Invasion as a Global Change

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Session Description: 

Overview & Relevance:
Humans move many species from their native ranges to places all over Earth, both deliberately and inadvertently.
Where those species establish new, self-maintaining populations, this movement leads to human-caused biological invasion. The thesis of this workshop is that human-caused biological invasions have become so widespread as to blur the regional distinctiveness of Earth's biota. That distinctiveness is itself an important component of Earth's biological diversity, and it contributes substantially to the maintenance of species-level and even human cultural diversity. Biological invasions thus represent an important component of anthropogenic global change.

Growth in population and resource use by the human population has entrained vast industrial and agricultural enterprises, involving extraordinary rates of energy use and an unprecedented mobility of people and goods. These in turn have caused a set of global changes in the Earth system that are relatively well documented and known to be human-caused. Some of these changes result directly from human resource use, while others are caused by the accumulation of waste products; some are inherent to the scale of human activity, while others are avoidable. Global changes in this set interact with each other; they also cause changes in regional and global climate, and drive ongoing losses of biological diversity.

How important are invasions in comparison to other components of global change? That question is not easily answered, because the timing, scale, and consequences of each component of change are so different, and because these changes interact so strongly with each other. We believe that it is reasonable to suggest that other than the ongoing epidemic of extinction, biological invasions are the least reversible of the major anthropogenic global changes. We also believe that biological invasions are second only to land use/land cover change in driving global losses of biological diversity.

An estimate of the potential impact of biological invasion on biological diversity can be calculated using species-area curves. Preston (1960) used this approach in an analysis of the spatial distribution of breeding bird populations; he demonstrated that the slope of a regression of species number against the log of area would underestimate the total number of bird species on Earth substantially. Using an approach suggested at the meeting by Randy Westbrooks, we carried out a similar analysis of mammals, regressing the number of species on the log area of continents (using a database at the Center for Conservation Biology, Stanford University). The resultant plot is strongly log-linear, with an r-squared of .94. Extrapolating this relationship to the land area of Earth, if all the continents were combined into one land mass, that continent should support ~2000 species of mammals. In fact, the separated continents support 4200 species. This result suggests that if biological invasions were so widespread as to cause a complete breakdown of biogeographic barriers, the extinction of more than half the continental mammals (and most likely a larger fraction of those on islands) would be entrained. We believe that this analysis is at least as soundly based as calculations of entrained species loss due to land use change.

Biological invasions often are considered to be primarily a phenomenon of island ecosystems, whether of terrestrial islands in the ocean or the island-like aquatic and coastal marine ecosystems of continents. Indeed, there is no doubt that the consequences of invasions are particularly severe on islands (Loope and Mueller- Dombois 1989, Simberloff 1995, D'Antonio and Dudley in press). However, invasions of continental systems also are widespread -- and damaging in many cases. For example, the eastern deciduous forests of North America were cleared extensively in the last century, but they have rebounded substantially in this century; they are discussed widely as an example of the resilience of many natural ecosystems. A great deal of effort has gone into determining current and probable future effects of climate change, increased carbon dioxide concentrations, acid rain, and oxidant air pollution on these forests. However, the big change in this century has been the invasion of wave after wave of introduced pests and diseases (Castello et al. 1995). Some of these pests, such as the gypsy moth, consume a variety of species. Other, more specialized pathogens have eliminated the American chestnut (once the most abundant tree in the eastern United States) (McCormick and Platt 1980) and American elm (Huenneke 1983), and have greatly altered the dynamics of American beech, flowering dogwood, and other species. Several other widespread species are threatened. We suspect that invasions will continue to represent the most important environmental and economic threat to eastern deciduous forests for the foreseeable future.

We acknowledge that while all biological invasions represent an alteration of natural processes, we are not equally concerned with all of them. Many invasions reflect other components of environmental change, rather than being themselves drivers of change. For example, invading plants that stably occupy roadside areas cannot be regarded as serious threats to native biological diversity; they are mainly a consequence of land use change (which may itself threaten diversity). Moreover, some introduced species clearly are beneficial to humanity; for example, it would be quite a trick to support the population of the United States entirely on native foods. However, some invading species are damaging, either by degrading human health and/or wealth directly, or by affecting the structure and functioning of ecosystems, and/or the maintenance of native biological diversity. Harmful invasions might best be prevented by excluding all non-indigenous species, except for some that are specifically designated as "safe" -- but once invasions occur, control and management will have to be focused on those species with the capability to act as agents of change.

The goal of this AGCI session was to educate each other about what is involved and how to effectively take information from these fields into political processes. We wanted to take the opportunity to resolve some issues such as the use of biological control of invasive species in natural systems and proposals to use exotic species for biological control of native "pest" species. We need more action politically; what strategy should we be pursuing? Should we fight exotics aggressively, do triage (differentiate between those we can't fight, those we can fight and win, and those that don't matter), blockade, or get used to it cause its going to happen anyway? How should we conceptualize and activate our approach to invasions? Thinking longer term, 200 years in the future, if the biological invasion problem was solved: How could this happen? What is the solution? In thinking about how to get from here to there, it helps to know what "there" looks like.

Case Studies
The following case studies were addressed at this workshop:

There are numerous examples of invading organisms that threaten human health; most infectious diseases fit in this category, over most of their range. Several centuries ago, the indigenous people of North America could cite smallpox as a devastating Old World invader (Crosby 1986) -- just as modern Americans can point to HIV. Another recent invader is the Asian tiger mosquito Aedes albopictus, which entered the United States in imported used automobile and truck tires, and which is an aggressive and effective vector for dengue fever and eastern equine encephalitis (Craven et al. 1988, Mitchell et al. 1992, OTA 1993).

These are numerous examples of invading species that impose substantial costs on society in the invaded areas. Fouling of water intakes in North America by zebra mussels (Dreissena polymorpha) is a recent example, one that could cost water- using utilities $3 billion over a 10 year period (OTA 1993). Another example is the golden snail in Asian rice ecosystems. The golden snail was introduced deliberately to provide a source of export income for small farmers; it has been spread throughout Asian irrigated rice systems. However, the snail cannot be exported to Europe due to health regulations, it is not considered palatable locally, it spreads rapidly through irrigation systems, and it voraciously consumes young rice plants. Naylor (submitted) calculates that in the Philippines alone, the golden snail costs farmers around $55 million per year -- one third of that for molluscicides, one third for labor replanting damaged areas, and one third in lost yields. That represents 14% of the gross income from Philippine rice -- and this in a developing agricultural system that is neither plagued by overproduction nor dominated by a few wealthy producers.

Effects on ecosystems
Invasions that alter the structure and/or function of invaded ecosystems are particularly threatening; they do not merely compete with or consume particular native species, they change the nature of the ecosystem in which all natives survive. In other words, they do not just add players to the game, they change its rules -- often to the benefit of that and other invaders. Many species of grasses provide clear examples of invaders that alter ecosystems, in many regions of Earth. Pasture grasses have been introduced deliberately, particularly from the old world into the new; other grass invasions have occurred through contaminated seed lots (Mack 1991), while still others have been transported as ornamentals. In the Intermountain West of the United States, cheatgrass (Bromus tectorum) apparently entered in contaminated seed. Once established, it spread in the interstices of sagebrush shrublands -- making a continuous layer of fine fuel that can carry fire effectively. After fire, cheatgrass (an annual) regrows more rapidly than woody plants; after several fires in succession, cheatgrass is the dominant plant, and the probability and extent of fires increases enormously (Whisenant 1990, Billings 1990). Cheatgrass now dominates more than 40 million hectares in Western North America (Stewart and Hull 1949, Whisenant 1990); in pastures of its range, the fire return interval has decreased from 60-110 years to 3-5 years, and the average size of fires has increased by orders of magnitude (Whisenant 1990).

Similar dynamics are set in motion by grass invasions elsewhere, including African C4 grasses in humid regions of Australia, tropical America, and Oceania, and other African and Mediterranean grasses in semiarid areas of Australia and the Southwestern U.S. (American woody plants -- but rarely grasses -- return the favor to the Old World.) These grass invasions interact with land use change in a positive feedback cycle that reinforces forest clearing, and prevents the re-establishment of forests following abandonment of agricultural and pastoral land (D'Antonio and Vitousek 1992).

Loss of biological diversity
In North America, the most immediate threats to biological diversity caused by biological invasions occur in aquatic ecosystems. Individual drainage systems and lakes may function as islands, and be more susceptible to alteration by invasions than are surrounding terrestrial areas. At the same time, invasion pressure on aquatic systems is substantial -- from aquarium fish dumped into culverts to multimillion dollar efforts to establish sportsfisheries using introduced species, some from other continents (especially the German brown trout), and others transported well beyond their native ranges within North America. Moreover, the most popular introduced sports fish are top carnivores -- precisely the group likely to have the greatest effect on food webs in recipient lakes and streams, and to exert top-down pressure on all other organisms in the system.

The consequences of this conjunction of factors are predictable, and depressing. From one-third to two-thirds of all amphibian, fish, crayfish, and mussel species in North America are rare, declining, or endangered, or already extinct. Of the 86 native fish officially listed as threatened or endangered in 1991, 44 were affected by introduced fish -- 29 of those by introductions for sportsfishing, 11 by trout alone (Wilcove and Bean 1994).

Even invasions that do not change ecosystem structure and function appreciably can nevertheless alter biological diversity; for example, Medeiros et al. (1992, 1993) report that an Australian tree fern which invades Hawaiian forests appears very similar to native tree ferns -- except that the natives serve as important substrates for epiphytic plants, and the invader does not.

These particular cases are not atypical examples of biological invasion and its consequences -- except in that they have been documented far better than have most invasions. Similar examples can be found almost anywhere. What isn't always clear, even to the many careful observers documenting invasions in many local areas, is that such examples occur almost everywhere; that all of the local examples are manifestations of a global phenomenon.
This AGCI session was designed to go beyond listing invasions and their effects to analyzing what can be done to prevent, control, and manage invasions.

To that end, the group included biologists and land managers, people from federal agencies and non-governmental organizations, and an economist and a lawyer.

The discussions and presentations led to a number of points of agreement, and some solid progress towards developing means for coping with invasion. Five major results were:

1. Pathways of invasion
We need to recognize the diversity of pathways by which organisms are introduced to new habitats, and develop strategies that are appropriate for each of those pathways. Many species are introduced deliberately, in hopes of economic gain -- pasture grasses and golden snails fall into this category, as do African bees and the European gypsy moth. Others are deliberately introduced for recreational or aesthetic reasons -- brown trout, aquarium fish, and fountain grass fall into this category (although the scale of the plant nursery and pet trades blur the distinction between economic and aesthetic motivations). Others are accidental (if often predictable) contaminants of other goods -- such as the contaminated seed that brought cheatgrass to North America, the Asian tiger mosquito, or the brown tree snake that has devastated Guam's native birds (Savidge 1987) -- and threatens Hawaii's. Each of these pathways requires different means of control, will run into different sorts of opposition, and offers different opportunities for collaboration with other interests.

2. Variation in consequences of invasion.
We must also recognize that there is substantial variation in the consequences of invasions, both in the likelihood of success of invasions by different species and (as noted above) in the consequences of successful invasions. Sarah Reichard presented a model that predicts the likelihood that a potential woody plant invader would become established in a new area; such a model could be used to screen proposed introductions. It is interesting to note that the most important single predictor of successful invasion is whether or not a species has invaded successfully elsewhere. Clearly a database containing information on invasions worldwide would be a valuable tool for the prevention of future invasions.

3. Techniques for countering invasions.
Once a species has invaded a new area (and appears likely to affect health, wealth, ecosystems, or biological diversity) -- how can it be controlled or eliminated? One answer is "early" -- the chances for successful control are greatest and the costs least early in the invasion process. Many invasions display a lag (apparent or real) between introduction and obvious explosive growth. However, the political and/or bureaucratic machinery for focusing on an invasion often is slow, and the best opportunities slip by. The development of quick-response decision pathways and control teams could help substantially (Hobbs and Humphries in press). A second conclusion was that the development and evaluation of a wider variety of techniques for controlling invasions would be worthwhile. For example, Australia and New Zealand make use of toxicants for controlling introduced mammals to a much greater extent than does the United States. Toxicants have environmental costs, but it is not obvious that these are always greater than those of biological control (generally through the use of additional introduced species) or other approaches.

4. Education about invasions.
Most of us who work with invasions have had the experience of dealing with people who consider any effort at control pointless ("invasions are inevitable"), wrong-headed ("oh, so now there are politically correct species?"), or reactionary ("and I bet you believe in shooting illegal aliens on sight, too"). Education about invasions, the threats they pose to native organisms and ecosystems, and the values of those native organisms and ecosystems themselves, are the only effective way to counter those sentiments. There are some notable successes -- many Floridians now understand the threat posed by plant invasions there -- but this awareness took considerable time and effort to develop.

5. Invasions and the law.
After substantial discussion of legal and political aspects of biological invasion, we decided that it would be interesting and rewarding to draft a model law outlining how we believe any society should deal with exotic species. The current draft of the law is included later in this report. It is in the format of a U.S. law, but it draws on elements of proposed and actual laws and regulations in Australia and New Zealand, and it is meant to be sufficiently generic to be applicable anywhere.

Workshop Topic (s): 
  • Ecosystems
  • Human Contributions & Responses
  • Land-Use/Land-Cover Change