120 



pox than from any other cause, this surely must have been the most devastating 

 exotic from the human perspective. 



Disease organisms (and parasites) are among the most completely devastating 

 exotics when one considers their potential to completely eliminate a susceptable spe- 

 cies. Thus, important tree species such as American chestnut (Castanea dentata) 

 and American elm iUlmus americana) of the eastern deciduous forest biome have 

 been essentially eliminated, and may well be faced with extinction. Additionally, 

 flowering and Pacific dogwoods {Cornus florida and Cornus nuttalli respectively) 

 and butternut {Juglans cinerea) are currently in precipitous decline due to exotic 

 diseases of uncertain origin. 



Of even greater current concern are those tree species that provide the majority 

 of lumber for construction. Although numerous diseases and parasitic organisms (in- 

 cluding insects) have already become established, few at present seem destined to 

 eliminate any commercially valuable conifer species. However, that soon may 

 change. Ill-advised and ecologically irresponsible plans to import raw logs for milling 

 in this country carry only a single certainty with them, that they will surely intro- 

 duce insect or disease pests that will have a net negative economic and biological 

 effect. 



The United States is currently the world's largest producer of industrial 

 roundwood by a wide margin (U.N.F.A.O. 1993. Forest Products Yearbook, 1991. 

 Rome). We produce 26 percent of the world's supply in a Nation of well less than 

 5 percent of the global population. To propose importation of additional logs that 

 have the potential to jeopardise the viability of the world's largest wood p/oducts 

 industry is at best naive, short-sighted, and perhaps unthinkably mercenary. In 

 common language it is exceedingly stupid. There are other, more insidious ways 

 that exotic organisms impact species and communities. In general, there is only so 

 much space, food, or other resources in any environment. Thus, in a given forest 

 there is only so much space where frees can grow. They are limited by the nutrients 

 in the soil, and the availability of sunlight in the canopy. Any space on the ground 

 or in the canopy that is usurped by an exotic species equates to a commensurate 

 loss of biomass of one or more members of the native flora. 



Although in general a natural forest community will not be invaded by an exotic 

 tree, there are ways in which it can happen. Most of the exotic trees of concern (ex. 

 Chinese Tree-of-Heaven, Ailanthus altissima) exploit disturbed sites. They colonize 

 areas by spreading along road corridors, and generally produce large numbers of 

 easily dispersed seeds which settle throughout undisturbed habitat. As long as there 

 is no disturbance the exotic is not likely to be a concern: however, forest commu- 

 nities have a background level of predictable disturbance from windthrow, lightning 

 strike, ice-storms, and other natural causes such as old-age mortality of individual 

 trees. When there is a large seed source "waiting" for any such disturbance, dis- 

 turbed sites are often quickly colonized by aggressive invaders. Although such an 

 invasion is rapid in ecological time, it may appear imperceptably slow to a human 

 observer who generally has a lifespan considerably less than the period required for 

 significant forest community conversion. The end product of such a scenario may 

 well be a forest dominated by a valueless invasive exotic tree species, and if that 

 exotic is of little value to native fauna, an impoverished animal community. 



Aquatic communities are affected in ways similsu" to forest communities in that 

 a body of water essentially has a relatively finite capacity to produce, for example, 

 fish. If an invasive fish species is introduced into an aquatic community, and it in- 

 creases in number and biomass, there will be a roughly commensurate decrease in 

 the native fish. With the introduction of carp {Cyprinnus carpio) in North America, 

 their rapid continental invasion resulted in a permanent foregone production of 

 more desirable native species roughly equal to carp biomass, and that amount of 

 fish is a substantial portion of the total North American fish biomass. Perhaps the 

 best North American example of this phenomenon is the introduction of the ruff"e 

 (Gymnocephalus cernus) into Duluth Harbor in Lake Superior in the 1980'8. Pre- 

 sumably carried from Europe in ballast water, it was first collected in 1986. By Sep- 

 tember 1992 it had replaced 50 percent of the fish biomass in the area sampled in 

 Duluth Harbor, and only a year later the biomass replacement level reached 99 per- 

 cent. We might speculate that the 1 percent of remaining native fish biomass does 

 not represent populations with a secure future. If the ruffe spreads throughout the 

 Great Lakes, the loss of native fish biomass will be extraordinary. 



There is little hope for controlling invasive exotics like the ruffe; eradication is 

 a mere pipe-dream. Like it or not, such exotic species, once introduced, are part of 

 a new North American biota, and usually a new biota a significantly less value, use, 

 or esthetic appeal. Such biota are degraded and in many ways we are poorer for 

 it; often native species are eliminated or severely reduced. 



