taken up at one port and released in another. Millions of
planktonic organisms including larvae are contained in
the ballast water, and when the water is taken up into the
ship, some sediments are sucked into the ballast tanks as
well. In this way whole benthic communities can be
moved around the world.
In addition to ballast water, sea chests, the intake areas
for ballast water, can house diverse organisms. They have
grates or screens to prevent large organisms from coming
in with the water, but small ones can survive and grow
inside the chests. Sponges, sea anemones, worms, mussels, oysters, scallops, barnacles, crabs, sea urchins, and
fish have been found in sea chests. Although antifouling
paints have decreased hull fouling, many nooks and
crannies including rudders, gratings, holes, and sea chests
are difficult to paint and are subject to high levels of fouling. Fouling organisms can settle on anchors or chains as
juveniles and grow into adults that are transported to
a new region where they reproduce or drop off. Mud accumulates on anchors while on the bottom, and organisms in
the mud remain on the anchor as it is hauled aboard ship. If
it is not washed thoroughly, the organisms may survive in
the damp mud until the anchor is dropped at the next port,
and then they may invade the new habitat.
There are other important vectors for moving marine
and estuarine species around the planet. Aquariums and
trade in aquarium and ornamental species are another
important source for species likely to invade aquatic habitats (Padilla and Williams, 2004). In addition, aquaculture
is a major source for marine invasive species. Other mechanisms include seafood companies, bait shops, research
and educational organizations, public aquariums, and
coastal restoration projects (Weigle et al., 2005). Recently
marine debris has also been found to be a way to transport
species around the world. Almost 2 years after the major
Japanese earthquake and tsunami, large pieces of docks,
dislodged by the earthquake, started arriving on the US
West Coast complete with large populations of attached
Japanese organisms.
After arrival
There is usually a lag period of several years between the
arrival and establishment of an exotic species and its rapid
population growth. Scientists think that stressed or disturbed environments are more susceptible to invasion, so
that natural communities that have been changed by human
impact may be more vulnerable. When native species are
reduced in numbers or impaired, it is easier for new species
to succeed, and the relative scarcity of certain functional
groups (e.g., filter feeders) will facilitate invasions by other
species of those groups. If there are unused resources
such as nutrients or unused habitats (e.g., hard surfaces),
an area will be more susceptible to invasion by species that
use those resources. For example, European brackish water
seas (Baltic Sea, Black Sea, Caspian Sea) are subject to
intense invasion. In these seas, natives have rather low species richness because salinity is the most important limiting
factor. However, this salinity is very suitable to many
invaders, which are abundant in these brackish waters
(Paavola et al., 2005). Nehring (2006) studied why estuaries seem to be invaded more than other ecosystems and
concluded that the combination of brackish water with
unsaturated ecological niches plus intensive international
ship traffic produces the highest potential invasion rate for
aquatic systems. Estuaries are also subjected to invasions
both from the ocean (mainly shipping) and from inland
waters (e.g., canals).
We cannot predict when or where a species will become
a problem, a phenomenon that has been called “ecological
roulette” (Carlton and Geller, 1993). Why does a species
arrive at a particular time? Why not last year or 10 years
ago? Sometimes the explanation is a change in the donor
region such as an increase in population or a change in
the recipient region such as diminished water quality due
to a new source of pollution. The possibility of invasion
is increased by more and faster transport between regions
that are climatically similar and the use of bigger ships
carrying more ballast water. The number of individuals
introduced at one time and the frequency with which they
arrive, called “propagule pressure,” play an important role.
Sometimes sheer numbers can make the difference
between success and failure of an introduced species.
Degree of the problem
In the waters of the United States, it was estimated a few
years ago that there are over 500 species of introduced
organisms, including over 140 in the Great Lakes, over
200 in Chesapeake Bay, and over 240 in San Francisco
Bay. In San Francisco Bay alone, a new species is estimated to arrive every 14 weeks (Cohen and Carlton,
1998). More than half of the area’s fish and most of its
benthic organisms are not native. Hawaii is another region
with high levels of invasion, with 73 marine invasive species, 42 % of which are considered harmful.
Effects
Invasive species can cause major economic problems. The
invasion of zebra mussels in the Great Lakes in the late
1980s had a major economic impact because they clogged
up intake pipes for the water systems in the Great Lakes. It
is estimated that invasive species cost the world’s economy hundreds of billions of dollars annually; the cost to
the United States is estimated at 128 billion dollars each
year. Much of this cost is due to impacts of invasive species on fisheries, boating, and coastal recreation, as well
as to the expense of controlling and/or attempting to eradicate them (Pimentel et al., 2000).
Invaders can be major sources of stress in estuaries
(Ruiz et al., 1999), although many nonindigenous species
do not appear to have major impacts. One type of invader
that can have major impacts is termed an “ecological
engineer.” These species modify their habitat, for
example, by changing the substrate, changing light penetration, digging burrows, or elevating the marsh surface
INVASIVE SPECIES
363
planktonic organisms including larvae are contained in
the ballast water, and when the water is taken up into the
ship, some sediments are sucked into the ballast tanks as
well. In this way whole benthic communities can be
moved around the world.
In addition to ballast water, sea chests, the intake areas
for ballast water, can house diverse organisms. They have
grates or screens to prevent large organisms from coming
in with the water, but small ones can survive and grow
inside the chests. Sponges, sea anemones, worms, mussels, oysters, scallops, barnacles, crabs, sea urchins, and
fish have been found in sea chests. Although antifouling
paints have decreased hull fouling, many nooks and
crannies including rudders, gratings, holes, and sea chests
are difficult to paint and are subject to high levels of fouling. Fouling organisms can settle on anchors or chains as
juveniles and grow into adults that are transported to
a new region where they reproduce or drop off. Mud accumulates on anchors while on the bottom, and organisms in
the mud remain on the anchor as it is hauled aboard ship. If
it is not washed thoroughly, the organisms may survive in
the damp mud until the anchor is dropped at the next port,
and then they may invade the new habitat.
There are other important vectors for moving marine
and estuarine species around the planet. Aquariums and
trade in aquarium and ornamental species are another
important source for species likely to invade aquatic habitats (Padilla and Williams, 2004). In addition, aquaculture
is a major source for marine invasive species. Other mechanisms include seafood companies, bait shops, research
and educational organizations, public aquariums, and
coastal restoration projects (Weigle et al., 2005). Recently
marine debris has also been found to be a way to transport
species around the world. Almost 2 years after the major
Japanese earthquake and tsunami, large pieces of docks,
dislodged by the earthquake, started arriving on the US
West Coast complete with large populations of attached
Japanese organisms.
After arrival
There is usually a lag period of several years between the
arrival and establishment of an exotic species and its rapid
population growth. Scientists think that stressed or disturbed environments are more susceptible to invasion, so
that natural communities that have been changed by human
impact may be more vulnerable. When native species are
reduced in numbers or impaired, it is easier for new species
to succeed, and the relative scarcity of certain functional
groups (e.g., filter feeders) will facilitate invasions by other
species of those groups. If there are unused resources
such as nutrients or unused habitats (e.g., hard surfaces),
an area will be more susceptible to invasion by species that
use those resources. For example, European brackish water
seas (Baltic Sea, Black Sea, Caspian Sea) are subject to
intense invasion. In these seas, natives have rather low species richness because salinity is the most important limiting
factor. However, this salinity is very suitable to many
invaders, which are abundant in these brackish waters
(Paavola et al., 2005). Nehring (2006) studied why estuaries seem to be invaded more than other ecosystems and
concluded that the combination of brackish water with
unsaturated ecological niches plus intensive international
ship traffic produces the highest potential invasion rate for
aquatic systems. Estuaries are also subjected to invasions
both from the ocean (mainly shipping) and from inland
waters (e.g., canals).
We cannot predict when or where a species will become
a problem, a phenomenon that has been called “ecological
roulette” (Carlton and Geller, 1993). Why does a species
arrive at a particular time? Why not last year or 10 years
ago? Sometimes the explanation is a change in the donor
region such as an increase in population or a change in
the recipient region such as diminished water quality due
to a new source of pollution. The possibility of invasion
is increased by more and faster transport between regions
that are climatically similar and the use of bigger ships
carrying more ballast water. The number of individuals
introduced at one time and the frequency with which they
arrive, called “propagule pressure,” play an important role.
Sometimes sheer numbers can make the difference
between success and failure of an introduced species.
Degree of the problem
In the waters of the United States, it was estimated a few
years ago that there are over 500 species of introduced
organisms, including over 140 in the Great Lakes, over
200 in Chesapeake Bay, and over 240 in San Francisco
Bay. In San Francisco Bay alone, a new species is estimated to arrive every 14 weeks (Cohen and Carlton,
1998). More than half of the area’s fish and most of its
benthic organisms are not native. Hawaii is another region
with high levels of invasion, with 73 marine invasive species, 42 % of which are considered harmful.
Effects
Invasive species can cause major economic problems. The
invasion of zebra mussels in the Great Lakes in the late
1980s had a major economic impact because they clogged
up intake pipes for the water systems in the Great Lakes. It
is estimated that invasive species cost the world’s economy hundreds of billions of dollars annually; the cost to
the United States is estimated at 128 billion dollars each
year. Much of this cost is due to impacts of invasive species on fisheries, boating, and coastal recreation, as well
as to the expense of controlling and/or attempting to eradicate them (Pimentel et al., 2000).
Invaders can be major sources of stress in estuaries
(Ruiz et al., 1999), although many nonindigenous species
do not appear to have major impacts. One type of invader
that can have major impacts is termed an “ecological
engineer.” These species modify their habitat, for
example, by changing the substrate, changing light penetration, digging burrows, or elevating the marsh surface
INVASIVE SPECIES
363