Environmental Engineering Reference
In-Depth Information
result of hybridization? Ellstrand (1992) believes that
interspecific hybridization and subsequent introgres-
sion or hybrid swarm formation is harmful to an endan-
gered species, because local adaptive differentiation can
be disrupted or prevented. Others promote the view
that plant hybridization and the formation of hybrid
swarms allows for the maintenance of genes from the
rare species and may well contribute to the formation
of new species (Grootjans et al . 1987, Rieseberg
1991). Interspecific hybrids and their derivatives are
predominantly observed in disturbed or intermediate
habitats (Stace 1975). Although the notion of hybrid
habitat has been specified only occasionally after it
was launched by Anderson (1948), this concept could
be of great value when identifying suitable habitats.
Again, cascade effects should be taken into consid-
eration. For example, a number of studies addressed
the distribution of herbivores and parasites in hybrid
zones of their hosts. Strauss (1994) pointed to the
susceptibility of herbivores to parasites, as related to
behavioural, ecological and genetic differences be-
tween the herbivores, and genetically based resistance
to parasites could become disrupted as a result of
hybridization. This issue is still calling for original
research. In Chapter 7 the risk of hybridization in re-
introduction programmes will be dealt with.
6.7.2 Hybridization
An interesting phenomenon, which so far has not been
dealt with in the context of migration and invasion
in metapopulation research, is the potential of inter-
specific hybridization. Stace's (1991) New Flora of the
British Isles lists 715 hybrids, and of those known to
have been produced in the British Isles 70 are deemed
to be products of hybridization between native and
alien species, and four between an introduced hybrid
and a native species. In total, approximately 7% of
introduced species have been involved in the production
of interspecific hybrids now recognized to be part of
the flora of the British Isles, while approximately 4%
of the native plants have hybridized with alien spe-
cies. These data have been provided by Abbott (1992),
who also suggested that interspecific hybridization be-
tween a native and an invading plant species, or two
invading species, sometimes results in a new, sexually
reproducing taxon. Several examples of such taxa have
been confirmed by molecular and isozyme analyses.
Hybridization is rarely considered among the biotic
factors that promote species extinction (Levin et al .
1996), but it may have a profound effect on the per-
sistence of a species. Is there a risk of maladaptive
genes being introduced into a new population as a
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