reshuffling: How alien species help feed the global economy
Costs and benefits of alien species
It is probably fair to say that most people who seek to introduce an exotic or alien species into a new habitat are doing so for an economic reason. They may wish to increase their profits from agriculture, they may believe that the public will like a new flower from a distant part of the globe, or they may think that exotic species will be able to carry out functions that native species cannot carry out as effectively (examples of these will be given below).
But it may also be fair to say that most of those introducing exotic species have not carried out a thorough cost-benefit analysis before initiating the introduction, at least partly because they may not have been aware of the advantages of such analyses. On the other hand, it is also possible that at least some people would prefer to ignore the negative impacts that may follow from species introductions, because they might be expected to compensate those who are negatively affected.
Similarly, those who have been responsible for inadvertently introducing species into new habitats may not have been willing to make the investment to prevent such accidents from occurring. They may not have realized the dangers, and in any case the dangers would be unlikely to have much economic impact on their own welfare. Rather, the costs of such accidents are borne by people other than those who are permitting the accidents to happen. Thus the costs of introducing alien species into new habitats are "externalized" in considerations of the costs of global trade. The line of responsibility is insufficiently clear to bring about the necessary changes in behaviour, so the general public -- or future generations -- ends up paying most of the costs.
This paper will introduce, in a preliminary way, some of the economic factors affecting the issue of alien species. It will quickly become clear that this field is still relatively immature, but that considerable benefits for biodiversity will come from a more inclusive consideration of economic factors, and the application of economic tools to deal with them.
Good Intentions: Somebody Is Going To Make Money
Many exotic species were introduced for economic purposes. Introduced fish can produce excellent sport fishing, introduced plants can provide food, fodder, and energy, and introduced insects can provide biological controls. A few examples (from among hundreds that could be quoted):
• Brush-tailed possums from Australia were introduced to New Zealand between 1858 and 1900 to establish a fur trade, but in New Zealand they have fewer competitors, fewer predators, and fewer parasites than in their native Australia, so they have successfully spread and have sometimes reached densities ten times greater than in their native Australia. They have been a bonanza for the fur industry.
• A number of woody plants from various parts of the world, such as acacias from Australia, were introduced into South Africa in the middle of the 19th century for purposes of dune stabilization, tannin extraction, and firewood. This appears to have been an economically successful invasion, with the greater Capetown region alone supporting a 30 million Rand charcoal and firewood industry.
• In the Thar Desert of India, the African tree Prasopsis juliflora was introduced 70 years ago and has become the dominant flora around human habitats. With its dense green vegetation, this tree is very useful in checking soil erosion, reducing the dryness of the desert air, giving shelter to several species of wild animals, and providing legumes which are relished by wild as well as domesticated animals. It meets 85 percent of firewood demands of rural people.
• The Triclad flatworm Platydemus manokwari, first described from New Guinea in 1963, is a successful predator of the giant African snail Achatina fulica, so it was transported as a biological control agent to areas where the African snail had become established in the Pacific.
• Water hyacinth Eichhornia crassipes was introduced into China from South America in the 1930s and was spread through mass campaigns in the 1950s to the 1970s as an ornamental plant, to provide livestock food, and to control pollution through absorbing heavy metals.
But Something Went Wrong: Somebody Had To Pay The Costs
But we all know that there is no free lunch. Introduced species can carry a heavy pricetag, in terms of reduced crop and livestock production, loss of native biodiversity, increased production costs, and so forth. As just one example, OTA (1993) estimates that the total direct costs to the US of invasive species of weeds is $3.6-$5.4 billion per year. This figure does not include environmental, human health, regulatory, and other indirect costs of using herbicides on these weeds, though these costs are estimated to be at least $1 billion per year; if herbicides were not available, the crop losses would jump to nearly $20 billion per year.
All of the introductions listed above carried with them some hidden -- or, in retrospect, obvious -- costs:
• The Australian brush-tailed possums introduced into New Zealand have caused considerable damage to native forests, changing forest composition and structure through the defoliation and progressive elimination of favoured food plants. Note that none of these costs are particularly relevant to those interested primarily in the benefits from furs. In an effort to control these possums, New Zealand is working on bio-control agents, including the possibility of a genetically-engineered immunocontraceptive virus. This innovative approach could have profound implications elsewhere in the world, showing that some problems may lead to solutions which have considerable global value.
• As a result of the introduced species, South Africa's highly-endemic Cape Flora is under serious threat and the watersheds are becoming less productive, potentially causing a considerable increase in the price of water (Wilgen et al., 1996). (See below for more details).
• While Prasopsis juliflora has been a boon to people in the Thar Desert who need firewood and fodder, it overwhelms other flora in the area, thereby reducing the range of products available to local people and reducing biodiversity.
• The Triclad flatworm now poses a serious threat to the native gastropod fauna of the Pacific region. This is especially troubling because the Pacific has seen a remarkable radiation of the snail family Partulidae, and some 24 of these are on the 1994 IUCN Red List of Threatened Animals. The Triclad flatworm has become established on Guam, Saipan, Tinian, Rotar, and Palau.
• In China, the water hyacinth has become the worst weed in many aquatic habitats, leading the loss of species of both plants and animals. In Dianchi Lake, just outside of Kunming, Yunnan, the total number of fish species has declined from 68 to about 30 and Chinese scientists attribute this to water hyacinth. Reduction of the lake area as a result of the water hyacinth infestation has also caused notable climatic changes in Kunming (Jianqing et al., 1995).
Few of these examples have explicit costs attached to them, but qualitative costs are often available. For example, in the early 1900s, the most economically important hardwood species in eastern American forests was the chestnut (Castanea dentata), but the chestnut blight brought in on diseased horticultural stock from China killed nearly a billion trees and all but eliminated this species, leading to profound ecosystem changes in the eastern hardwood forests (USDA, 1991).
More quantitative cost estimates have also been made. For example, OTA (1993) estimated the cumulative losses to the USA from 79 alien species from 1905 to 1991 at more that US$96 billion, a conservative estimate since data were available for only a few years (Figure 3).
OTA (1993) also projected potential future losses under a "worst-case scenario," estimating that just 15 species of plants, insects, plant pathogens, and aquatic invertebrates could eventually cause economic losses of over US$134 billion. Since the USA has over 4,500 alien species, the potential for considerable damages is very great.
Despite such figures, the issue of costs and benefits is not always clear, at least partly because different people have different perceptions of what these are. Luken and Thieret (1996), for example, report that within less than a century after its deliberate introduction into North America to improve habitat for birds, serve ornamental functions in landscape plantings, and stabilize and reclaim soil, the Amur honeysuckle had become established in at least 24 states in the eastern USA. While many resource managers perceive the plant as an undesirable element, gardeners and horticulturalists consider it an extremely useful plant. Thus the "noxious invasive" of one group is the "desirable addition" of other groups. How can costs and benefits be determined in such a case?
Diamond et al. (1991) have estimated the costs and benefits of controlling the invasive tree Melaleuca quinquenervia in Florida. Total annual benefits, based largely on tourism, of preventing infestation from the tree would be $168.6 million, while the costs to honey producers to whom the tree provides nectar would amount to just $15 million. Again, the costs and benefits in this case are differentially distributed: those who suffer losses are unlikely to be compensated, while those who benefit pay few of the costs.
Considerable work has now been done in the USA, at least, on the cost-benefit ratios for various forms of managing invasive species (though the distribution of these continue to be ignored). OTA (1993) presents a summary of these, with Figure 3 highlighting a number of cases. Note the very wide range of cost-benefit ratios, though in nearly all cases, the benefits of control far outweigh the costs involved. This strongly suggests that significantly increased investments in managing invasive species is justified in economic terms, though again those paying the costs -- usually the taxpaying public -- may not always be the primary beneficiaries; and those who earned the benefits from the invasives in the first place are paying virtually none of the costs.
As an indication of one interesting approach measuring costs and benefits, Wilgen et al. (1996) presented a case study showing how invasion by alien plants has affected water resources in the mountain catchment areas of the Western Cape Province, South Africa. They found that the sustained supply of high-quality water depends on maintaining the cover of fynbos (shrubland) vegetation. The fynbos binds the soil, preventing erosion, while its relatively low biomass ensures conservative water use and low-intensity fires, which in turn ensure high water yields and low impacts on the soil from periodic fires. Fynbos-clad mountain catchments fulfil approximately two-thirds of the Western Cape's water requirements, an ecosystem service that plays a crucial role in the region's economy and contributed to a gross domestic product of US$15.3 billion in 1992. The fynbos flora is widely harvested for cut flowers, dried flowers, and thatching grass, producing a combined value in 1993 of $18-19.5 million and providing a livelihood for 20-30,000 people.
Source: OTA (1993).
However, catchment management is complicated by the invasion of the fynbos vegetation by non-indigenous woody trees and shrubs, which increase biomass and reduce runoff. Fynbos ecosystems are remarkably prone to invasion by alien woody species which displace the native fynbos and increase biomass by between 50% and 1000%. These invasive plants were introduced to South Africa to provide a source of fast-growing timber in the relatively treeless landscape, as hedge plants, as agents for binding the shifting dunes along the coast, and as ornamental plants. The most important invasive species originated in Australia and the Mediterranean-climate areas of Europe and North America. On the slopes of Table Mountain, above Cape Town, invasion by alien species has increased fire intensities, leading to severe soil erosion.
Wilgen et al. (1996) developed a computerized model that indicated that alien plants would invade approximately 40% of the area within 50 years and 80% after 100 years, with a corresponding increase in biomass of 150% or more. This invasion would result in an average decrease of 347 cubic metres per ha per year of water at the end of 100 years, resulting in average losses of more than 30% of the water supply to the city of Cape Town. In some years, when large areas would be covered by mature trees, losses would be much greater, exceeding 50% of the runoff from similar uninvaded areas. They concluded that investments in managing alien plants at a level that would ensure that they are no longer part of the ecosystem would lead to a net unit cost of water of $.12 per cubic metre, as compared to $.14 without the management of alien plants.
While it is important to identify the costs and benefits, such determination does not automatically determine a decision because value judgements and distributional questions are nearly always involved. Further, the magnitude of the costs may sometimes be so high as to render an action politically unacceptable, even when the benefits are likely to be even greater; part of the problem is that the benefits may be widely spread throughout the public over a period of many years, while the costs of control may need to be paid rather quickly by tax payers.