Conservation planning in a changing world
227
Box 9.1 The h uman i mprint on m odern d ay s pecies d ispersal p atterns
The Earth is now better connected via human transport than ever before. In recent decades, human
activities have greatly increased the frequency and spatial extent of species introductions across
the globe through both intentional and unintentional actions. These include ballast - water discharge
from international shipping; bait - bucket releases associated with recreational fi shing; the global pet
trade; intentional translocations of wildlife for recreation purposes; biological control; and inadvertent
releases from aquaculture and horticulture activities. The following two case studies illustrate how
modern biotas are connected via social and economic networks and by sea and air.
Ship t raffi c
In marine and estuarine systems, the dominant invasion pathway worldwide is the ballast water of
commercial ships (Carlton & Geller, 1993 ; Drake & Lodge, 2004 ). Ocean - going vessels must achieve
proper stability to minimize drag (and thus maximize speed) and to reduce the likelihood of capsizing
in rough seas. To achieve this, early ships strategically fi lled ballast compartments within the hull
with soil, rocks or scrap metal – essentially, anything with some weight that could be easily loaded
into a ship at dock. Today, ships pump water into ballast tanks, and a typical commercial bulk vessel
might carry over 30,000 metric tonnes of ballast water during an inter - oceanic voyage. Ballast water
is usually taken from the harbour in one port and subsequently may be discharged in a recipient
port through openings in the ship ’ s hull.
The number of non - native species that are transported via ship ballast has increased with the
rise in global commerce and the consequent upsurge in the number of ships travelling the world ’ s
oceans and major waterways (Figure B9.1a ). Current estimates suggest that a global fl eet of approximately 35,000 commercial vessels transports an annual volume of about 3.5 × 10
9 metric tonnes of
ballast water, containing some 7,000 – 10,000 species (mostly marine) at any one time (Wonham
et al. , 2005 ). Even if only a small fraction of these species establish non - native populations, it is
easy to see that ballast water is a primary mechanism by which aquatic invasions are occurring.
By tracking the number of ships that visit ports worldwide, Drake and Lodge (2004) were able to
map ‘ hotspots ’ of marine invasions and, via network modelling, to determine which ports are likely
to have increased rates of invasions in the coming years (Figure B9.1a ). These hotspots are clearly
the product of economic and social infl uences on global trade and are in marked contrast to what
we might expect given natural dispersal patterns of marine species via oceanic currents.
by the release of hundreds to thousands of individuals
of a species into one novel locale, although there is
much variation around this number.
Finally, human - assisted invasions serve to connect
two or more locations that are geographically very
distant from one another, whereas natural dispersal
events tend to link sites that are comparatively close
together or otherwise linked naturally. Quite simply,
patterns of modern dispersal unite parts of the world
solely by social and economic ties, as opposed to biophysical pathways such as prevailing wind directions,
jet streams or ocean currents, as would happen for
natural dispersal events (Box 9.1 ).
9.1.3 Economic and e cological i mpacts
of i nvasion
The interest in human - assisted invasions has grown
rapidly over the past two decades, which is attributable
to three factors (Lockwood et al. , 2007 ):
• First, as the world economy globalizes, there are
increased trade and social connections between geographical localities, and along with these connections
come the introduction of non - native species (Perrings
et al. , 2005 ; Hulme, 2009 ). Thus, the sheer number of
non - native populations establishing worldwide has
increased substantially in recent times.
227
Box 9.1 The h uman i mprint on m odern d ay s pecies d ispersal p atterns
The Earth is now better connected via human transport than ever before. In recent decades, human
activities have greatly increased the frequency and spatial extent of species introductions across
the globe through both intentional and unintentional actions. These include ballast - water discharge
from international shipping; bait - bucket releases associated with recreational fi shing; the global pet
trade; intentional translocations of wildlife for recreation purposes; biological control; and inadvertent
releases from aquaculture and horticulture activities. The following two case studies illustrate how
modern biotas are connected via social and economic networks and by sea and air.
Ship t raffi c
In marine and estuarine systems, the dominant invasion pathway worldwide is the ballast water of
commercial ships (Carlton & Geller, 1993 ; Drake & Lodge, 2004 ). Ocean - going vessels must achieve
proper stability to minimize drag (and thus maximize speed) and to reduce the likelihood of capsizing
in rough seas. To achieve this, early ships strategically fi lled ballast compartments within the hull
with soil, rocks or scrap metal – essentially, anything with some weight that could be easily loaded
into a ship at dock. Today, ships pump water into ballast tanks, and a typical commercial bulk vessel
might carry over 30,000 metric tonnes of ballast water during an inter - oceanic voyage. Ballast water
is usually taken from the harbour in one port and subsequently may be discharged in a recipient
port through openings in the ship ’ s hull.
The number of non - native species that are transported via ship ballast has increased with the
rise in global commerce and the consequent upsurge in the number of ships travelling the world ’ s
oceans and major waterways (Figure B9.1a ). Current estimates suggest that a global fl eet of approximately 35,000 commercial vessels transports an annual volume of about 3.5 × 10
9 metric tonnes of
ballast water, containing some 7,000 – 10,000 species (mostly marine) at any one time (Wonham
et al. , 2005 ). Even if only a small fraction of these species establish non - native populations, it is
easy to see that ballast water is a primary mechanism by which aquatic invasions are occurring.
By tracking the number of ships that visit ports worldwide, Drake and Lodge (2004) were able to
map ‘ hotspots ’ of marine invasions and, via network modelling, to determine which ports are likely
to have increased rates of invasions in the coming years (Figure B9.1a ). These hotspots are clearly
the product of economic and social infl uences on global trade and are in marked contrast to what
we might expect given natural dispersal patterns of marine species via oceanic currents.
by the release of hundreds to thousands of individuals
of a species into one novel locale, although there is
much variation around this number.
Finally, human - assisted invasions serve to connect
two or more locations that are geographically very
distant from one another, whereas natural dispersal
events tend to link sites that are comparatively close
together or otherwise linked naturally. Quite simply,
patterns of modern dispersal unite parts of the world
solely by social and economic ties, as opposed to biophysical pathways such as prevailing wind directions,
jet streams or ocean currents, as would happen for
natural dispersal events (Box 9.1 ).
9.1.3 Economic and e cological i mpacts
of i nvasion
The interest in human - assisted invasions has grown
rapidly over the past two decades, which is attributable
to three factors (Lockwood et al. , 2007 ):
• First, as the world economy globalizes, there are
increased trade and social connections between geographical localities, and along with these connections
come the introduction of non - native species (Perrings
et al. , 2005 ; Hulme, 2009 ). Thus, the sheer number of
non - native populations establishing worldwide has
increased substantially in recent times.