Biology Reference
In-Depth Information
coast of France, and elsewhere. The parental genomes of
S. anglica
have
remained remarkably stable since formation of the hybrid and doubling
of its chromosome number (Baumel et al. 2002b). It has displaced the
native cordgrass in many areas and now occupies areas of formerly open
mudflat that were important as wintering habitat for shorebirds such as
the dunlin (
Calidris alpina
). It has also stimulated increased sediment dep-
osition, with accumulation reaching 4 cm/yr. The genetic uniformity of
S. anglica
apparently also makes it highly susceptible to infection by an
ergot fungus (
Claviceps purpurea
). Extensive diebacks of this cordgrass
occur, at which times massive amounts of silt are flushed into salt marsh
channels (Gray et al. 1991). The spread of this new species has been
accompanied by profound ecological changes in coastal salt marshes.
Hybridization and Origin of Invasive Aliens
Spartina anglica
was one of the first examples of the evolutionary potential
that is created by the introduction of alien species to new regions, where
they meet native species or other aliens that are close relatives. Hybridiza-
tion can occur between alien species, between aliens and natives, or
between hybrid forms and other aliens or natives. Hybrid forms involved
in these further hybridizations can be of spontaneous origin or can be arti-
ficial hybrids created and released by humans. Hybridization, in the broad-
est sense, can also occur between members of populations of a single
species that come from different source areas. The potential for origin of
novel genetic forms is therefore quite complex (fig. 5.1).
Hybridization is likely to be one of the important stimuli for the
development of an invasive exotic after a significant time lag (Ewel et al.
1999; Ellstrand and Schierenbeck 2000). Hybridization may result in
genetic recombination that produces a genotype better adapted to the
new environment than its parents (Abbott 1992). It may also lead to
greater genetic variability among hybrid offspring than that shown by
parental populations, so offspring may occupy a wider range of microhab-
itats than can parents. Genetic mechanisms such as allopolyploidy, the
doubling of chromosome number following hybridization, can stabilize
the genetic structure of hybrids. Such individuals may possess substantial
hybrid vigor, giving them an adaptive advantage over their parents.
Finally, hybridization may eliminate effects of fixed detrimental alleles that
have accumulated in parental populations.
The reproductive capability and vigor of hybrid species vary greatly. If
the hybridizing forms are similar enough for chromosomes to pair and
