116
(recessive) alleles caused by the reduction of population size,
leading to an actual increase in additive genetic diversity
(Hänfling 2007, and references therein; Facon et al. 2008).
If, by chance events, advantageous genotypes develop under
these conditions, they can rapidly become fixed in a small
founder population due to the strong selective forces.
Multiple introductions of the same species can further mitigate possibly negative effects of small founder populations.
They will be often not recognized as long as no genetic studies are performed (‘cryptic invasions’), but are likely to occur
in many introduced species. If repeated introductions originate from different source populations, this leads to an
admixture of genotypes, holding the potential to strongly
increase the adaptive abilities of the species by novel combination of alleles (Hänfling 2007; Herborg et al. 2007; Chan
and Briski 2017). This is believed to considerably contribute
to the invasiveness of global invaders like the European shore
crab Carcinus maenas (Geller et al. 1997; Roman 2006).
Hybridization between native species and NIS regularly
occurs in animals with external fertilization like mollusks
and fish, and especially in plants. From the invaders perspective, it increases the chances to successfully establish despite
small founder population sizes either by introgression of
native alleles which enhance adaptive evolution, or by the
development of new hybrid lineages combining beneficial
traits from both parental lineages (Sakai et al. 2001; Hänfling
2007). The latter can occasionally lead to hybrid superiority
and eventually result in the displacement of native species by
newly evolved hybrids. This has been for example observed
for cordgrass, Spartina sp., in Great Britain and North
America, where hybrids between native and invasive species
disperse more successfully than their parent species (Huxel
1999; Williams and Grosholz 2008, and references therein).
Why Does It Matter?
Ecological Impacts
The evidence is overwhelming that NIS invasions are a significant stressor to marine communities and has been
observed in invasions by plants, fish, crabs, snails, clams,
mussels, bryozoans, and nudibranchs (Ruiz et al. 1999).
Furthermore, anthropogenic derived disturbances and the
introduction of new species are skewing food webs towards
a loss of higher trophic groups and a gain in lower order consumers (Byrnes et al. 2007). On the other hand, the invasion
of marine NIS may increase local biodiversity, as marine
invaders often appear to accommodate besides native species
rather than replacing them (Briggs 2007; an example in
Reise et al. 2017). As a higher biodiversity stabilizes
communities, invaders may also have overall positive effects,
especially in otherwise disturbed habitats. This can also
include the resistance against further invasions (Stachowicz
et al. 2002; Marraffini and Geller 2015). Species most likely
to have wide-reaching ecosystem impacts are those that alter
the biotic and abiotic factors of the environment, namely
ecosystem engineers (Vitousek et al. 1996).
An ecosystem engineer is an organism that alters the
availability of resources to other species. Jones et al. (1994)
described ecosystem engineers as falling into two categories,
autogenic and allogenic. Autogenic engineers change the
environment through their own physical structure. Corals for
example, provide habitats for many reef dwelling species.
Allogenic engineers alter the environment by transforming
living or non-living materials between physical states, as is
the case for sea urchins which alter the environment by eating the kelp that would otherwise be providing a habitat for
organisms as autogenic engineers (Jones et al. 1994). Broadly
speaking, NIS as ecosystem engineers can provide both positive and negative impacts on their environments. As a prominent example, Pacific oysters (Magallana gigas) have been
introduced globally for aquaculture purposes and have in
some cases established wild oyster beds among its introduced ranges (Lejart and Hily 2011). The impact of Pacific
oysters has varied from displacement of Sabellaria reefs, a
species of conservation importance, to increases in sessile
invertebrate diversity via secondary settlement on oyster
shells (Olyarnik et al. 2009; Herbert et al. 2016).
Apart from the introduction of ecosystem engineers themselves, the introduction of pathogens can indirectly cause a
significant alteration to the physical environment by infecting ecosystem engineers. The introduced protistan pathogens
Haplosporidium nelsoni and Perkinsus marinus were partly
responsible for the decline of the Virginia oyster (Crassostrea
virginica) (Crooks 2002), a historically important ecosystem
engineer in the Chesapeake Bay. The Chesapeake Bay has
seen a decrease in over 90% of its oyster population in the
last century and the pathogen introduction has been recorded
as a dominant factor of mortality. Additionally, results of the
pathogen introduction have limited the physical structure of
oysters as a habitat and as a filter feeder, altering the benthic
and planktonic food webs (Ruiz et al. 1999).
Positive ecological impacts of NIS also occur outside the
group of ecosystem engineers. For example, the mitten crab
E. sinensis is able to transfer native and non-native invertebrates to new habitats (Ojaveer et al. 2007). The large carapace acts as a substrate for flora and fauna (e.g., algae and
barnacles) to inhabit. Furthermore, the ‘hairy’ patches on the
crabs’ claws could also provide a habitat for nematodes,
bivalves, crustaceans, oligocheates, and gastropods (Normant
et al. 2007). Other ecological advantages include new food
sources for fish, novel habitats, and increased biofiltration. In
a recent meta-analysis, Katsanevakis et al. (2014) found that
among the assessed NIS, 35% had been reported to have a
positive impact on other species.
J. C. Geburzi and M. L. McCarthy
(recessive) alleles caused by the reduction of population size,
leading to an actual increase in additive genetic diversity
(Hänfling 2007, and references therein; Facon et al. 2008).
If, by chance events, advantageous genotypes develop under
these conditions, they can rapidly become fixed in a small
founder population due to the strong selective forces.
Multiple introductions of the same species can further mitigate possibly negative effects of small founder populations.
They will be often not recognized as long as no genetic studies are performed (‘cryptic invasions’), but are likely to occur
in many introduced species. If repeated introductions originate from different source populations, this leads to an
admixture of genotypes, holding the potential to strongly
increase the adaptive abilities of the species by novel combination of alleles (Hänfling 2007; Herborg et al. 2007; Chan
and Briski 2017). This is believed to considerably contribute
to the invasiveness of global invaders like the European shore
crab Carcinus maenas (Geller et al. 1997; Roman 2006).
Hybridization between native species and NIS regularly
occurs in animals with external fertilization like mollusks
and fish, and especially in plants. From the invaders perspective, it increases the chances to successfully establish despite
small founder population sizes either by introgression of
native alleles which enhance adaptive evolution, or by the
development of new hybrid lineages combining beneficial
traits from both parental lineages (Sakai et al. 2001; Hänfling
2007). The latter can occasionally lead to hybrid superiority
and eventually result in the displacement of native species by
newly evolved hybrids. This has been for example observed
for cordgrass, Spartina sp., in Great Britain and North
America, where hybrids between native and invasive species
disperse more successfully than their parent species (Huxel
1999; Williams and Grosholz 2008, and references therein).
Why Does It Matter?
Ecological Impacts
The evidence is overwhelming that NIS invasions are a significant stressor to marine communities and has been
observed in invasions by plants, fish, crabs, snails, clams,
mussels, bryozoans, and nudibranchs (Ruiz et al. 1999).
Furthermore, anthropogenic derived disturbances and the
introduction of new species are skewing food webs towards
a loss of higher trophic groups and a gain in lower order consumers (Byrnes et al. 2007). On the other hand, the invasion
of marine NIS may increase local biodiversity, as marine
invaders often appear to accommodate besides native species
rather than replacing them (Briggs 2007; an example in
Reise et al. 2017). As a higher biodiversity stabilizes
communities, invaders may also have overall positive effects,
especially in otherwise disturbed habitats. This can also
include the resistance against further invasions (Stachowicz
et al. 2002; Marraffini and Geller 2015). Species most likely
to have wide-reaching ecosystem impacts are those that alter
the biotic and abiotic factors of the environment, namely
ecosystem engineers (Vitousek et al. 1996).
An ecosystem engineer is an organism that alters the
availability of resources to other species. Jones et al. (1994)
described ecosystem engineers as falling into two categories,
autogenic and allogenic. Autogenic engineers change the
environment through their own physical structure. Corals for
example, provide habitats for many reef dwelling species.
Allogenic engineers alter the environment by transforming
living or non-living materials between physical states, as is
the case for sea urchins which alter the environment by eating the kelp that would otherwise be providing a habitat for
organisms as autogenic engineers (Jones et al. 1994). Broadly
speaking, NIS as ecosystem engineers can provide both positive and negative impacts on their environments. As a prominent example, Pacific oysters (Magallana gigas) have been
introduced globally for aquaculture purposes and have in
some cases established wild oyster beds among its introduced ranges (Lejart and Hily 2011). The impact of Pacific
oysters has varied from displacement of Sabellaria reefs, a
species of conservation importance, to increases in sessile
invertebrate diversity via secondary settlement on oyster
shells (Olyarnik et al. 2009; Herbert et al. 2016).
Apart from the introduction of ecosystem engineers themselves, the introduction of pathogens can indirectly cause a
significant alteration to the physical environment by infecting ecosystem engineers. The introduced protistan pathogens
Haplosporidium nelsoni and Perkinsus marinus were partly
responsible for the decline of the Virginia oyster (Crassostrea
virginica) (Crooks 2002), a historically important ecosystem
engineer in the Chesapeake Bay. The Chesapeake Bay has
seen a decrease in over 90% of its oyster population in the
last century and the pathogen introduction has been recorded
as a dominant factor of mortality. Additionally, results of the
pathogen introduction have limited the physical structure of
oysters as a habitat and as a filter feeder, altering the benthic
and planktonic food webs (Ruiz et al. 1999).
Positive ecological impacts of NIS also occur outside the
group of ecosystem engineers. For example, the mitten crab
E. sinensis is able to transfer native and non-native invertebrates to new habitats (Ojaveer et al. 2007). The large carapace acts as a substrate for flora and fauna (e.g., algae and
barnacles) to inhabit. Furthermore, the ‘hairy’ patches on the
crabs’ claws could also provide a habitat for nematodes,
bivalves, crustaceans, oligocheates, and gastropods (Normant
et al. 2007). Other ecological advantages include new food
sources for fish, novel habitats, and increased biofiltration. In
a recent meta-analysis, Katsanevakis et al. (2014) found that
among the assessed NIS, 35% had been reported to have a
positive impact on other species.
J. C. Geburzi and M. L. McCarthy