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a larger depth range than native species; canopy-forming Sargassum muticum
reduces native biodiversity in invaded areas)
4. High levels of sexual reproduction and high fecundity (e.g., in the invasive
lineage of Asparagopsis taxiformis )
5. Parthenogenetic reproduction and broad environmental
tolerances ( Codium
fragile ssp. tomentosoides )
A quantitative ranking of European introduced and native seaweed species
(using comparisons of categories of biological traits such as dispersal capabilities,
environmental tolerances, reproductive mode, and size) indicated clearly that
introduced species with a number of these biological traits have an increased
likelihood of being a successful invader (Nyberg and Wallentinus 2005 ). In some
cases, however, the biological traits identified in important invasive seaweeds are
also present in noninvasive co-specific or co-generic relatives (Paula and Eston
1987 ; Trowbridge 1996 ; Chapman 1999 ; Vroom and Smith 2001 ).
Rapid micro-evolutionary changes are common in invasive species because
introduced populations are often subject to founder effects and population
bottlenecks, have higher incidence of hybridization (suggested to provide innova-
tive genetic variants), and are exposed to a range of novel selective pressures
encountered in the introduction range (Brown and Marshall 1981 ). Acclimatization,
adaptation, and thereafter phenotypic modification may arise in response to new
biotic (e.g., competitors, consumers, or parasites) and environmental conditions and
the drivers of the functional changes are likely to be genetic (reviewed in Whitney
and Gabler 2008 ). However, the question remains whether well-established gene
regulation mechanisms, already present in the genome's background, are simply
activated by the local selective pressure or the aforementioned changes are the
result of a de novo genomic response.
Published accounts suggest that only a limited number of biologically distinct
species within algal orders or just one ESU (evolutionary significant taxonomic
unit); among a number of cryptic ESUs within the same morpho-species become
suddenly invasive. In plants, the switch to invasiveness has been recently related
with differences in ploidy levels suggesting that genetic attributes such as poly-
ploidy and high chromosome counts may be the drivers for this transformation
(endangered plants exhibit disproportionally low levels of ploidy and chromosome
numbers compared to invasive plant species; Pandit et al. 2011 ). Similar to
hybridization, polyploidy may lead to the production of novel and greater numbers
of genetic variants, which increases the probability of a successful invasion. An
association between polyploidy and invasiveness has been reported in the red alga
Asparagopsis taxiformis (Andreakis et al. 2007b , 2009 ) and should be further
explored in other invasive marine algae, given the general propensity for polyploidy
in seaweeds (based on nuclear genome size estimates; Kapraun 2005 ).
It has been debated whether ecosystems exhibiting high species richness are less
vulnerable to biological invasions because in theory only a limited, highly
specialized, number of empty niches are available for the invader. Furthermore, it
has been assumed that polluted “recipient” environments are likely to promote
invasive traits in NIMS (Davis et al. 2000 ; Davis and Pelsor 2001 ; Dunstan and
Johnson 2007 ; Valentine et al. 2007 ; Whitney and Gabler 2008 ). In addition,
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