1.2 Susceptibility of ecosystems
2 Mechanisms of Dispersal
In the first part, this essay aims at providing a rough overview about the most important topics in the literature dealing with alien invasive species. This overview tries to answer questions about the general biology of alien invasive species and the existence of a perfect invader. However, some ecosystems seem to be more susceptible to invasion than others do. General assumptions are that the tropics are less invasible than temperate regions and that islands are more vulnerable than continental areas. An alien species must complete three stages of human-mediated spreading which are transportation, introduction and establishment in order to become invasive. An alien invasive species might then have positive or negative impacts. Either it adds to the species richness or it has detrimental effects on the ecological, economical or human health sector. However, sophisticated treatments are available to prevent several means of introduction. The second part of this essay will provide a critical evaluation about the literature reviewed. There still exists a great deal of uncertainty and variety of opinions within publications. The need of increased prevention of introductions and collaboration are conclusions that can be drawn. Furthermore, public awareness has to be raised and a clear definition of terms is a must. Finally, it is important to recall that many alien species are responsible for human well-being by providing food and aesthetics.
To begin this essay it is important to assess the terminology currently used for this topic. The term of “Alien Invasive Species” (AIS) remains in constant change within the literature. Aliens or alien species are also known as “introductions, introduced, exotic, foreign, non-native, naturalized, immigrant and non-indigenous species” that are brought unintentionally or deliberately into other countries. Invasive, however, is more conspicuous and implies that the respective species causes or may have a certain impact.
It is difficult to distinguish between species that might be potential invaders and others that are not. McNeely et al. (2001) states for example that species from all taxonomic groups might invade new locations; hence, it is a difficult task to determine in advance, which species may have an impact on a new environment. However, other authors (Courtenay and Meffe, 1989; Eldredge, 1999) provide eight ecological, genetic and physiological characteristics that foster alien species to exploit a new habitat. Those are (1) high abundance in original range; (2) polyphagous feeding habit; (3) short generation time; (4) high genetic variability; (5) single females can colonize alone; (6) large size; (7) close association with humans and; (8) ability to tolerate a wide range of physical conditions. Courtenay and Meffe (1989) further apply those traits to the western mosquitofish, Gambusia affinis, one of the “100 Worst Invasive Species of the World” (Lowe et al., 2000). Many mosquitofishes have been introduced to several tropical countries on behalf of feeding on mosquitos and thus reducing mosquito-borne diseases. The introductions, however, had no deleterious effects on mosquitos but on the rest of the native biota. Many other fish species and invertebrates significantly decreased in abundance due to predation by G. affinis on eggs, larvae and juveniles. The damselfly Megalagrion xanthomelas, endemic to the islands of Hawaii, has been extirpated in many locations by predation of G. affinis on their freshwater larvae (Englund, 1999). Additionally, the wide tolerance of salinity (0 to 40 ppt), its aggressive and mouth brooding behaviour that reduces mortality of offspring and the ability to spawn several times a year have detrimental effects on native species. Interestingly, G. affinis has a relative small size of about 50 cm (Courtenay and Meffe, 1989; Englund, 1999). This example clearly shows that the more of the above characteristics are combined within one species the more successful it is in colonizing a new habitat. In this sense the perfect invader would be a species with all of the traits mentioned above. Ricciardi and Rasmussen (1998) distinguish two ways to identify probable invasive species. They mention an inductive and a deductive approach. The inductive methods try to assess the invasive species beforehand. One example might be to look at the characteristics of a certain organism as stated above. Deductive methods focus on species that have already an invasion history and show the ability to spread with human-mediated means.
1.2 Susceptibility of ecosystems
There is another factor that influences the success of an alien species to become established in a new environment. Apart from species-specific abilities to colonize successfully (s.a.), the resistance of an ecosystem itself also plays an important role. In this case, resistance means the ability to stay essentially unchanged during disturbances. Some ecosystems are more susceptible to be disturbed and altered by an alien species than others.
There have been many attempts to develop different theories due to the complexity of this topic and the different aspects are still often subject to discussion within the literature (Hobbs, 1989; Loope and Mueller- Dombois, 1989; Mooney and Drake, 1989; Ramakrishnan and Vitousek, 1989; Cohen and Carlton, 1998; Stachowicz et al., 1999; Hewitt, 2002; Hutchings et al., 2002).
Some scientists agree to the hypothesis that the invasibility of tropical ecosystems is lower than of temperate ecosystems, although there exist only few studies that show this pattern (Stachowicz et al., 1999). These authors further propose that the different degrees of invasibility strongly correlate with the different degrees of biodiversity found in tropical and temperate systems. This idea is also confirmed by Hutchings et al. (2002). They assume that higher diversity results in a more complete utilization of resources like space or food due to a more complex food web. The higher use of resources leads on the other hand to a smaller amount of vacant niches, in which alien invasive species may specialize in. It is important to point out that the same authors express strong uncertainty by mentioning several possible human-based biases. Deficits in this hypothesis about the lower invasion success in tropical ecosystems may be a function of the smaller amount of surveys in tropical areas. That causes less comprehensive and incomplete baseline data, so that, eventually, invasions might not be recognized as such. Furthermore, Hewitt (2002) even clearly objects to the theory of Stachowicz et al. (1999) and Hutchings et al. (2002) and emphasizes the need for further studies.
There is another theory which is more widely accepted and states that islands are more vulnerable to biological invasions than continental areas (DeFelice et al., 1998; Eldredge, 1999). Loope and Mueller-Dombois (1989) explain this by the higher rate of endemism of islands. Original biota started to evolve millions of years ago in almost total isolation. Native species are thus highly specialized and adapted to local disturbances. On the other hand, this implies a high sensitivity to global disturbances (e.g. human settlements) and/or invasions by generalist invaders.
Macroalgae introduced to the Hawaiian Islands are good examples of such generalist invaders. Many studies have been carried out on Hawaii, one of the most remote islands in the world (Loope and Mueller-Dombois, 1989; Coles and Eldredge, 2002). Smith et al. (2002) conducted a survey of the most abundant non-indigenous macroalgae found on the island. They found five different alien algae, of which Acanthophora spicifera and Gracilaria salicornia were the most common. The red alga G. salicornia has been originally introduced for experimental aquaculture in the agar industry (Smith et al., 2004). However, Smith et al. (2004) further argue that this idea has been abandoned and the alga escaped into the wild, from where it spread quickly via fragmentation. Another advantage for this species is its remarkable resistance to temperature, salinity and chemical extremes. One might assume that native herbivores feed on the extra amount of food this alga represents, but a native Gracilaria species is consumed far more frequently. The invasive G. salicornia poses a double threat: First, it occupies space that is liberated from native Gracilaria by herbivores and secondly, it can cover reef structures. Similar observations have also been made by Stimson et al. (2001) about the alien invasive green alga Dictyosphaeria cavernosa on Hawaii. By later introductions of A. spicifera herbivore feeding patterns shifted from D. cavernosa to A. spicifera. Without grazing pressure D. cavernosa could thrive and cover larger areas of corals. Stimson et al. (2001) predicted a phase shift from the coral-dominated state to the macroalgae-dominated state.
2 Mechanisms of Dispersal
The unintentional and deliberate human-mediated spreading of organisms is a process of extreme intricacy. In order to provide a clearer understanding of this issue, it is useful to subdivide the anthropogenic dispersal of organisms into three distinguishable component phases (Wonham et al., 2000). These three phases essentially include: (1) transport of organisms along a pathway, (2) introduction into a new environment and (3) establishment and survival of the organism in the new ecosystem.
The human-mediated spread of organisms began at least in the fifteenth century, when European exploration started to establish trade routes with several Asian countries (Coles et al., 1998). The bulk of species has been transported via fouling and boring organisms attached to the hull of ships. Another mode of dissemination, which has been substituted by ballast water, was dry ballast such as rocks, sand or soil that organisms were attached to (Carlton, 2001). The author further points out that the spread of organisms and thus the probability of an alien invasive species to become established was steadily increasing since the early twentieth century. Faster trade routes (pathways) and more sophisticated transport vehicles (vectors) increased the chance of survival.
Minchin and Gollasch (2002) delivered three possible vectors that are primarily responsible for global transportation of organisms. Those are shipping (ballast water and hull fouling), aquaculture and trade (import and export of aquaria and ornamentals). Carlton (2001) has supplemented this list by man-made landscape alteration to channels and floating marine debris such as drifting nets or wood. Only ballast water and hull fouling shall be dealt with in this part of phase 1.
Ballast water is commonly regarded as the vector that transports most aquatic organisms (Griffiths et al., 1991; Carlton and Geller, 1993; Wonham et al., 2000; Zardus and Hadfield, 2005). Ballast is used to stabilize and trim a vessel and to submerge the propeller, when the ship is not fully loaded (Fedder, pers. comm.). There exists great uncertainty about the amount of taxa being transported in international shipping each day. Numbers vary from 3000 (Gollasch et al., 2002) up to 7000 species (Carlton, 2001; Taylor et al., 2002).
Reasons for this high variance might be the heterogeneous structure of ballast tanks. Organisms might not only be found within the water column but also within the sediment. Hamer (2002) discovered many different species of microorganisms and various resting stages in the sediment. A special threat are cysts of potentially harmful dinoflagellates that are freed when the tank is occasionally cleaned from sediment.
Studies to assess the main species present in the water column of ballast tanks have been conducted by Carlton and Geller (1993). They recognized that the predominating taxa are planktonic crustaceans, polychaetes, turbellarian flatworms, cnidarians and mollusks.
In addition, Gollasch et al. (2002) reported diatoms and dinoflagellates as the most common representatives of algae.