(Wallentinus and Nyberg, 2003). Changing the substrate
makes the habitat unsuitable for many native species and
more suitable for a new group of species; reducing light
penetration harms rooted sea grasses that need to photosynthesize. When the marsh surface becomes more elevated, the amount of time that it is underwater during the
tidal cycle will be reduced. Organisms that dig burrows
at the marsh edge can contribute to erosion and loss of
marsh area.
Many invaders become larger in their new location than
in their native range (Grosholz and Ruiz, 2003), perhaps
due to the presence of fewer predators at the new location.
This is referred to as the “enemy release hypothesis.”
Parasites are often lost during the transition to a new
location. For example, if a species arrives as planktonic
larva, it will not have brought along the parasites that are
present in the adult animal. Parasites often have complicated life cycles that require more than one host, and if
all the hosts are not present in the new location, those
parasites cannot survive. Reduced predators and parasites
can contribute to the increased size and success in the
new location.
Examples of some major troublemakers
The common reed: Phragmites australis
East Coast brackish marshes have been invaded by a new
European strain of the common reed, Phragmites australis
(Saltonstall, 2002). This species has been in North America for thousands of years, but until recent decades it
remained a minor part of the high marsh and freshwater
marsh communities until the arrival of the new strain. This
particular strain, which may have arrived from Europe in
dry ballast in the hold of a ship during the nineteenth century, produces more shoots and grows faster than the
native variety, outcompeting and reducing the diversity
of native marsh plants. It expands rapidly using rhizomes
which can grow more than 10 m in a growing season, and
it is more tolerant of salt and low oxygen than the native
variety. It also spreads easily in low-salinity areas where
tidal flow is restricted, and it is more dense and taller than
native plants, crowding them and blocking their exposure
to the sun. It produces dense litter layers that sit on the
marsh surface and inhibit germination and growth of other
species. It thrives on high levels of nitrogen found in estuaries receiving sewage plant effluent or farm runoff, giving it yet another advantage over native plants (Figure 1).
Managers have established removal policies costing
millions of dollars. In many areas where it was removed,
the reed has reinvaded, and at the same time research has
found that its effects on the marsh are not all negative.
While it does reduce plant diversity, many animals do
not seem to be affected by its takeover of the marsh. On
the negative side, it has reduced value as a nursery habitat
for killifish; larval and juvenile killifish are much less
abundant in Phragmites marshes because these smooth
out the marsh surface, removing the small puddles and
depressions that the larvae and juveniles live in at low tide
(Able and Hagen, 2000). On the positive side, invertebrates such as fiddler crabs, ribbed mussels, grass shrimp,
and benthic infauna seem to thrive regardless of which
species of marsh plant is dominant (Weis and Weis,
2003). Birds are mixed – some prefer Spartina, but some
prefer Phragmites (Benoit and Askins, 1999). The detritus
produced by decaying leaves of the reed provides nutrition
comparable to that of cordgrass, and it enters estuarine
food webs the same way as cordgrass detritus.
Another way that the common reed changes the marsh
is by increasing the rate at which sediment is trapped, filling in creeks and tidal channels, increasing the elevation
of the marsh (Rooth et al., 2003). The overall smoothing
and filling of the marsh surface reduces its variability
and reduces habitat available for animals. Ditches and
creeks can become filled, reducing tidal exchange with
the high marsh and limiting access for aquatic organisms
to go up on the marsh surface. In some areas, the marsh
may eventually become so elevated that it will be higher
than the upper tide level and will dry out. Ironically, given
the forecasted rate of sea level rise, it may be that only
Phragmites marshes will be able to keep pace and survive.
The reed can sequester pollutants more effectively
than native cordgrass. It absorbs more nitrogen, which
reduces effects of the nitrogen on the rest of the marsh/
estuary community. It retains more metal pollutants
belowground in its roots, while Spartina puts more into
its leaves and excretes them back into the water. Keeping
the contaminants belowground, as Phragmites does, is
beneficial to the ecosystem since the contaminants are
not available to other species, while contaminants that
are excreted back into the water can then be taken up
by other species (Windham et al., 2001). Phragmites also
appears to be more tolerant of contaminants than other
marsh grass species (Valega et al., 2008).
Because of the controversy over this species,
a conference entitled “Phragmites australis: A sheep in
wolf’s clothing?” was held to focus on and review all the
aspects of the invasion and effects of this plant. The
plant’s ecology was examined with regard to how it affects
habitat quality in marshes. The conclusion was that it is
neither a villain nor a hero. Given the scientific findings
about this plant, managers should reconsider their automatic and expensive extermination programs and replace
them with well-thought-out goals based on site-specific
findings.
Cordgrass: Spartina alterniflora
It is ironic that while Spartina alterniflora is valued in
East Coast marshes, in West Coast marshes it is considered a noxious invasive species because it converts valued
mudflat areas into marsh. Mudflats, rather than marshes,
are considered the most biologically productive estuarine
habitats because they are teeming with invertebrates of
all kinds. Marine worms, crustaceans, and mollusks support the hundreds of thousands of shorebirds in the estuary. When Eastern cordgrass dominates marshes above
the mudflats, it displaces native cordgrass and other native
364
INVASIVE SPECIES
makes the habitat unsuitable for many native species and
more suitable for a new group of species; reducing light
penetration harms rooted sea grasses that need to photosynthesize. When the marsh surface becomes more elevated, the amount of time that it is underwater during the
tidal cycle will be reduced. Organisms that dig burrows
at the marsh edge can contribute to erosion and loss of
marsh area.
Many invaders become larger in their new location than
in their native range (Grosholz and Ruiz, 2003), perhaps
due to the presence of fewer predators at the new location.
This is referred to as the “enemy release hypothesis.”
Parasites are often lost during the transition to a new
location. For example, if a species arrives as planktonic
larva, it will not have brought along the parasites that are
present in the adult animal. Parasites often have complicated life cycles that require more than one host, and if
all the hosts are not present in the new location, those
parasites cannot survive. Reduced predators and parasites
can contribute to the increased size and success in the
new location.
Examples of some major troublemakers
The common reed: Phragmites australis
East Coast brackish marshes have been invaded by a new
European strain of the common reed, Phragmites australis
(Saltonstall, 2002). This species has been in North America for thousands of years, but until recent decades it
remained a minor part of the high marsh and freshwater
marsh communities until the arrival of the new strain. This
particular strain, which may have arrived from Europe in
dry ballast in the hold of a ship during the nineteenth century, produces more shoots and grows faster than the
native variety, outcompeting and reducing the diversity
of native marsh plants. It expands rapidly using rhizomes
which can grow more than 10 m in a growing season, and
it is more tolerant of salt and low oxygen than the native
variety. It also spreads easily in low-salinity areas where
tidal flow is restricted, and it is more dense and taller than
native plants, crowding them and blocking their exposure
to the sun. It produces dense litter layers that sit on the
marsh surface and inhibit germination and growth of other
species. It thrives on high levels of nitrogen found in estuaries receiving sewage plant effluent or farm runoff, giving it yet another advantage over native plants (Figure 1).
Managers have established removal policies costing
millions of dollars. In many areas where it was removed,
the reed has reinvaded, and at the same time research has
found that its effects on the marsh are not all negative.
While it does reduce plant diversity, many animals do
not seem to be affected by its takeover of the marsh. On
the negative side, it has reduced value as a nursery habitat
for killifish; larval and juvenile killifish are much less
abundant in Phragmites marshes because these smooth
out the marsh surface, removing the small puddles and
depressions that the larvae and juveniles live in at low tide
(Able and Hagen, 2000). On the positive side, invertebrates such as fiddler crabs, ribbed mussels, grass shrimp,
and benthic infauna seem to thrive regardless of which
species of marsh plant is dominant (Weis and Weis,
2003). Birds are mixed – some prefer Spartina, but some
prefer Phragmites (Benoit and Askins, 1999). The detritus
produced by decaying leaves of the reed provides nutrition
comparable to that of cordgrass, and it enters estuarine
food webs the same way as cordgrass detritus.
Another way that the common reed changes the marsh
is by increasing the rate at which sediment is trapped, filling in creeks and tidal channels, increasing the elevation
of the marsh (Rooth et al., 2003). The overall smoothing
and filling of the marsh surface reduces its variability
and reduces habitat available for animals. Ditches and
creeks can become filled, reducing tidal exchange with
the high marsh and limiting access for aquatic organisms
to go up on the marsh surface. In some areas, the marsh
may eventually become so elevated that it will be higher
than the upper tide level and will dry out. Ironically, given
the forecasted rate of sea level rise, it may be that only
Phragmites marshes will be able to keep pace and survive.
The reed can sequester pollutants more effectively
than native cordgrass. It absorbs more nitrogen, which
reduces effects of the nitrogen on the rest of the marsh/
estuary community. It retains more metal pollutants
belowground in its roots, while Spartina puts more into
its leaves and excretes them back into the water. Keeping
the contaminants belowground, as Phragmites does, is
beneficial to the ecosystem since the contaminants are
not available to other species, while contaminants that
are excreted back into the water can then be taken up
by other species (Windham et al., 2001). Phragmites also
appears to be more tolerant of contaminants than other
marsh grass species (Valega et al., 2008).
Because of the controversy over this species,
a conference entitled “Phragmites australis: A sheep in
wolf’s clothing?” was held to focus on and review all the
aspects of the invasion and effects of this plant. The
plant’s ecology was examined with regard to how it affects
habitat quality in marshes. The conclusion was that it is
neither a villain nor a hero. Given the scientific findings
about this plant, managers should reconsider their automatic and expensive extermination programs and replace
them with well-thought-out goals based on site-specific
findings.
Cordgrass: Spartina alterniflora
It is ironic that while Spartina alterniflora is valued in
East Coast marshes, in West Coast marshes it is considered a noxious invasive species because it converts valued
mudflat areas into marsh. Mudflats, rather than marshes,
are considered the most biologically productive estuarine
habitats because they are teeming with invertebrates of
all kinds. Marine worms, crustaceans, and mollusks support the hundreds of thousands of shorebirds in the estuary. When Eastern cordgrass dominates marshes above
the mudflats, it displaces native cordgrass and other native
364
INVASIVE SPECIES