An impact assessment of species introductions require information from a number of sources on a number of areas such as the biology, ecology, and genetics of the alien species; the information will need to be readily available and understandable to those performing the risk assessment and to policy makers. Risk assessment must also include benefit assessment; an accurate accounting of the benefits derived from exotic species is essential.
Impacts of introduced species will fall into two broad categories: ecological, which includes biological and genetic effects and socio-economical (Table 1). However, these two categories are not independent and socio-economic changes brought about by alien species can in turn cause more ecological changes. Thus, a reduction in native species may be from direct interaction with an exotic species, or it may result from increased fishing pressure or changes in land use brought about by the presence of a newly established species.
| Table 1. Some potential adverse effects of alien species |
| Effect || Mechanism - Biological || Mechanism - Social |
| Reduction or elimination of aquatic species || Competition, hybridization, predation/herbivory, disease transmission || Change in fishing pressure and land use (accessibility); treatment measures |
| Change in terrestrial fauna || Change in abundance of preferred prey of waterfowl || Fish farms providing more food or killing predatory birds |
| Change in fishery management || Change in stock composition || Success breeds interest, failure breeds experimentation |
| Alteration in habitat || Burrowing, sediment mobilizataion, removal of vegetation || Change in land use, e.g. creation of fish farms |
| Socio-economic impacts || Change in species abundance or distribution to change fishing or consumption practices || Change in access rights, land tenure; financial liability for damages through national and international legislation |
Impacts may depend on the objective of the introduction. Analyses of the FAO database on introductions of aquatic species (DIAS) reveals that aquaculture development was the most often cited reason for fish introductions, and that government organizations were responsible for more introductions than any other group. Table 2 presents information that most of the ecological effects of introduced species reported were negative, however the socio-economic impacts were reported to be more often beneficial and there were more positive socio-economic benefits reported than negative ecological impacts.
| Table 2. Effects of introduced fish on Ecological and (Socio-economic) environments by reason for the introduction. Data represents number of records from FishBase |
| Impact || Fishing || Aquaculture || Ornamental || Bio-control || Unknown || Other1 |
| Adverse || 36 (2) || 78 (8) || 17 (5) || 23 (9) || 13 (0) || 40 (12) |
| Beneficial || 16 (87) || 52 (283) || 11 (42) || 11 (19) || 3 (10) || 6 (15) |
| Unknown || 28 (16) || 76 (49) || 9 (9) || 8 (2) || 21 (3) || |
| Blank || 196 (299) || 949 (815) || 169 (150) || 106 (122) || 459 || 283 |
| 1 Other includes: accidents, bait, forage, to fill niche, research, diffusion |
Table 3 presents some popular conceptions regarding impacts of alien species and how the databases can provide information to support or refute the generalizations. The purpose of challenging the broad generalizations is not to replace one generalization with another, but to provide some estimate of their validity.
| Table 3. Some popular conceptions regarding alien species |
| Statement || Information from databases || Possible biases in data |
| Most introductions fail || 65% of the introductions lead to established population, where establishment was assessed || Data from questionnaires, i.e. biased reporting |
| Top carnivores are the most dangerous || Herbivores and carnivores were reported to cause negative impacts in > 60% of the cases where impact was assessed, whereas the figure for omnivores was 81% || Small sample size of carnivore introductions |
| r-selected species most likely to establish || Establishment success negatively correlated with max. size || Larger fish subsequently removed by fishing or other factors after establishment; larger fish take longer to establish noticeable populations |
| Diverse environment hinders alien establishment || Data-set cannot address the issue || |
| Disturbed environment helps alien establishment || Data-set cannot address the issue || |
| Genome size inversely related to invasive ability || DNA content and chromosome number were not related to establishment success || |