115
niche, which, in turn, facilitates their own or their offspring’s
establishment. Common mechanisms are superiority in the
competition for food and shelter or for optimal settlement
space in the case of sessile animals and plants, respectively
(Ruiz et al. 1999; Jensen et al. 2002; Levin et al. 2002; van
den Brink et al. 2012; Katsanevakis et al. 2013). Several
studies also reported direct predation pressure by invaders on
native species within the same guild (Ruiz et al. 1999, and
references therein). Some authors (Briggs 2010) relate the
strong competitiveness of many NIS to a regularly observed
biogeographical pattern of marine invasions: They often
originate from regions with high biodiversity (e.g., the western and central Indo-Pacific and the NW-Pacific for NIS in
Europe, see Tsiamis et al. 2018) and are, therefore, well
adapted to strong competition. This makes them superior
over native species of their recipient regions, which are often
characterized by lower biodiversity. This pattern is also
incorporated in the ‘enemy-release-hypothesis’ (Bax et al.
2001; Brockerhoff and McLay 2011), and assumed to significantly contribute to the observation that successful invaders often ‘perform better’ (grow bigger, reproduce more) in
their invaded compared to their native ranges (Parker et al.
2013).
The success of marine NIS may also be enhanced by positive interactions which benefit the invader. They have been
described to occur among species invading the same region,
where the establishment of a first species (often an ecosystemengineer, see section “Vectors, Pathways and Altered
Habitats – Human Impacts”) facilitates subsequent invasions
of further species (Fridley et al. 2007; Altieri and Irving
2017). The initial invader might either directly provide beneficial effects for subsequent invaders (e.g., habitat or food)
or exert detrimental effects for native competitors of subsequent invaders (e.g., predation, pathogens, structural habitat
changes, Fig. 2). Such cascading effects have led to the
assumption that increasingly invaded systems become more
susceptible to further introductions, cumulating in ‘invasional meltdown’ scenarios (Simberloff and Von Holle 1999;
Grosholz 2005). Empirical evidence for ‘invasional meltdown’ is however scarce (Simberloff 2006; Briggs 2012). At
the same time, an increasing number of studies report both
negative interactions between NIS (Lohrer and Whitlatch
2002; Griffen et al. 2008; Griffen 2016) and positive effects
of NIS on native species (Rodriguez 2006, and references
therein). In summary, these studies underline the complexity
of species interactions in the context of NIS establishment,
making predictions on general interaction patterns and longterm invasion success extremely difficult.
Parasitism is another type of species’ interaction with the
potential to strongly affect invasion success. Just like being
released from enemies, a release from parasites often occurs
during the translocation process of many species, resulting in
a much lower parasite load of introduced compared to native
populations (Snyder and Evans 2006; McDermott 2011;
Fowler et al. 2013). Direct positive effects of reduced parasite load include, for example, increased survival and fecundity (especially, when released from sterilizing parasites).
Even more important are the indirect effects by the reduced
need to invest in parasite defense, allowing organisms to
reallocate those resources to traits like growth or reproduction (Goedknegt et al. 2016). Reduced investment in parasite
defense, however, results in higher susceptibility to parasite
infections, which may in turn negatively impact establishment success (Keogh et al. 2016). Introduced non-native
parasites, on the other hand, can reach extreme invasion success when they are able to infect native species which are
closely related to their original host, but have only weak
defensive traits due to the lack of coevolution (examples in
Ruiz et al. 1999; Feis et al. 2016). This could theoretically
even promote the invasion success of the original host, which
may gain competitive advantages over its native relative by
being better adapted to infections.
Selection, Multiple Introductions
and Hybridization – Invasion Genetics
Species introductions have the potential to trigger rapid evolutionary changes and adaptation processes acting on the
genetic level. Invasion genetics, therefore, play an important
role in determining long-term success of species introductions and their evolutionary consequences for the respective
species (Holland 2000; Geller et al. 2010). Furthermore,
invasion genetics is a tool to determine the origin of invasive
species and potential pathways of introduction. The Veined
rapa whelk (Rapana venosa), for example, is genetically
highly diverse in native Chinese populations (Yang et al.
2008), but genetically monomorphic in all introduced populations in Europe and the Americas. This implies that all
introduced populations originate from one single introduction, which has been localized in the Black Sea (Chandler
et al. 2008). Similarly, all invasive populations of the seaweed Caulerpa taxifolia in the Mediterranean, Australia and
North America could be traced back genetically to a strain
that was released or escaped from a European aquarium
(Wiedenmann et al. 2001; Padilla and Williams 2004).
Usually, introduction and colonization processes of species into new habitats are associated with a considerable
reduction of genetic diversity by strong genetic drift or bottleneck effects. One would, therefore, expect to regularly
observe negative effects of genetic depletion in newly establishing populations, especially a reduced ability to adapt to
changing environmental conditions. This seems, however,
often not to be the case (a terrestrial example in Tsutsui et al.
2000; Hänfling 2007). Possible reasons are for example cosegregation of fixed loci or changes in frequencies of rare
How Do They Do It? – Understanding the Success of Marine Invasive Species
niche, which, in turn, facilitates their own or their offspring’s
establishment. Common mechanisms are superiority in the
competition for food and shelter or for optimal settlement
space in the case of sessile animals and plants, respectively
(Ruiz et al. 1999; Jensen et al. 2002; Levin et al. 2002; van
den Brink et al. 2012; Katsanevakis et al. 2013). Several
studies also reported direct predation pressure by invaders on
native species within the same guild (Ruiz et al. 1999, and
references therein). Some authors (Briggs 2010) relate the
strong competitiveness of many NIS to a regularly observed
biogeographical pattern of marine invasions: They often
originate from regions with high biodiversity (e.g., the western and central Indo-Pacific and the NW-Pacific for NIS in
Europe, see Tsiamis et al. 2018) and are, therefore, well
adapted to strong competition. This makes them superior
over native species of their recipient regions, which are often
characterized by lower biodiversity. This pattern is also
incorporated in the ‘enemy-release-hypothesis’ (Bax et al.
2001; Brockerhoff and McLay 2011), and assumed to significantly contribute to the observation that successful invaders often ‘perform better’ (grow bigger, reproduce more) in
their invaded compared to their native ranges (Parker et al.
2013).
The success of marine NIS may also be enhanced by positive interactions which benefit the invader. They have been
described to occur among species invading the same region,
where the establishment of a first species (often an ecosystemengineer, see section “Vectors, Pathways and Altered
Habitats – Human Impacts”) facilitates subsequent invasions
of further species (Fridley et al. 2007; Altieri and Irving
2017). The initial invader might either directly provide beneficial effects for subsequent invaders (e.g., habitat or food)
or exert detrimental effects for native competitors of subsequent invaders (e.g., predation, pathogens, structural habitat
changes, Fig. 2). Such cascading effects have led to the
assumption that increasingly invaded systems become more
susceptible to further introductions, cumulating in ‘invasional meltdown’ scenarios (Simberloff and Von Holle 1999;
Grosholz 2005). Empirical evidence for ‘invasional meltdown’ is however scarce (Simberloff 2006; Briggs 2012). At
the same time, an increasing number of studies report both
negative interactions between NIS (Lohrer and Whitlatch
2002; Griffen et al. 2008; Griffen 2016) and positive effects
of NIS on native species (Rodriguez 2006, and references
therein). In summary, these studies underline the complexity
of species interactions in the context of NIS establishment,
making predictions on general interaction patterns and longterm invasion success extremely difficult.
Parasitism is another type of species’ interaction with the
potential to strongly affect invasion success. Just like being
released from enemies, a release from parasites often occurs
during the translocation process of many species, resulting in
a much lower parasite load of introduced compared to native
populations (Snyder and Evans 2006; McDermott 2011;
Fowler et al. 2013). Direct positive effects of reduced parasite load include, for example, increased survival and fecundity (especially, when released from sterilizing parasites).
Even more important are the indirect effects by the reduced
need to invest in parasite defense, allowing organisms to
reallocate those resources to traits like growth or reproduction (Goedknegt et al. 2016). Reduced investment in parasite
defense, however, results in higher susceptibility to parasite
infections, which may in turn negatively impact establishment success (Keogh et al. 2016). Introduced non-native
parasites, on the other hand, can reach extreme invasion success when they are able to infect native species which are
closely related to their original host, but have only weak
defensive traits due to the lack of coevolution (examples in
Ruiz et al. 1999; Feis et al. 2016). This could theoretically
even promote the invasion success of the original host, which
may gain competitive advantages over its native relative by
being better adapted to infections.
Selection, Multiple Introductions
and Hybridization – Invasion Genetics
Species introductions have the potential to trigger rapid evolutionary changes and adaptation processes acting on the
genetic level. Invasion genetics, therefore, play an important
role in determining long-term success of species introductions and their evolutionary consequences for the respective
species (Holland 2000; Geller et al. 2010). Furthermore,
invasion genetics is a tool to determine the origin of invasive
species and potential pathways of introduction. The Veined
rapa whelk (Rapana venosa), for example, is genetically
highly diverse in native Chinese populations (Yang et al.
2008), but genetically monomorphic in all introduced populations in Europe and the Americas. This implies that all
introduced populations originate from one single introduction, which has been localized in the Black Sea (Chandler
et al. 2008). Similarly, all invasive populations of the seaweed Caulerpa taxifolia in the Mediterranean, Australia and
North America could be traced back genetically to a strain
that was released or escaped from a European aquarium
(Wiedenmann et al. 2001; Padilla and Williams 2004).
Usually, introduction and colonization processes of species into new habitats are associated with a considerable
reduction of genetic diversity by strong genetic drift or bottleneck effects. One would, therefore, expect to regularly
observe negative effects of genetic depletion in newly establishing populations, especially a reduced ability to adapt to
changing environmental conditions. This seems, however,
often not to be the case (a terrestrial example in Tsutsui et al.
2000; Hänfling 2007). Possible reasons are for example cosegregation of fixed loci or changes in frequencies of rare
How Do They Do It? – Understanding the Success of Marine Invasive Species