Biology Reference
In-Depth Information
invasion attempts are more likely to be successful when the environmental
conditions in recipient ecosystems are similar to conditions in the ecosystem the
invader has evolved. In the latter case, the chances of survival and establishment of
invasive seaweeds following introduction are often improved because their natural
enemies such as herbivores or parasites are not introduced with them (i.e., the
enemy release hypothesis; Cacabelos et al.
2010
; Engelen and Henriques
2011
).
This means that the energy invested into defense in their native environment (e.g.,
grazing resistance; see Chap.
8
by Iken) can now support growth and biomass
production, increasing the competitiveness of the introduced species.
For a successful invasion, biological traits such as broad ecophysiological
tolerance and the consequences of differential gene expression are likely to act in
synergy under sufficient inoculation pressure and favorable characteristics of the
“recipient” ecosystem. The relative importance of each of these traits, however, is
poorly understood because they are likely to be species and location-specific and it
is impossible to perform controlled experiments at large biogeographic and evolu-
tionary timescales (Schaffelke and Hewitt
2007
).
12.2.1 Seaweed Invasions Are Mostly Human-Mediated
The introduction of a seaweed species often comprises a plethora of immigrants of
several genetic variants and ecotypes entering the “recipient” environment. Sim-
plistically, settlement and establishment will be feasible if the species' ecophysio-
logical limits for survival match the environmental conditions in the “recipient”
environment. While not fully understood for many individual species, the stages of
a successful seaweed invasion have been described as: (1) uptake and transport of
propagules, (2) survival and release of propagules and establishment of low-density
populations, and (3) increased competition with native biota, spread, expansion,
and impact of populations of introduced seaweeds (Schaffelke et al.
2006
).
The accidental uptake, transport, and release of propagules into the recipient
environment are the only stages of the process when human intervention can
prevent biological invasions. Marine seaweeds make use of many vectors for
dispersal such as specialized floating structures (see Chap.
17
by Roth
ausler
et al.), gametes or spores for sexual reproduction, fragments of thalli for vegetative
propagation, or attachment to long-distance moving organisms (e.g., sea turtles).
The dispersal of most invasive seaweeds, however, is associated with human
activities, which could be managed (Hewitt et al.
2009a
). Biofouling (i.e., the
attachment of the seaweed to any part of a vessel or marine equipment) is one of
the most common and also oldest mechanisms of human-mediated relocation of
seaweeds. The other main vectors are ballast waters, used to stabilize large vessels
for long-distance voyages, that transport algal propagules and intentional
relocations of aquaculture species (e.g., as fouling on oyster shells or as packing
material) consequently responsible for seaweed introductions (Hewitt et al.
2006
).
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