226
Biological invasions and the homogenization of faunas and fl oras
show a much wider array of biological traits than
those species that are likely to experience natural long -
distance dispersal.
The rate at which non - native populations are establishing around the world is consistently several orders
of magnitude larger than fossil - derived estimates for
natural dispersal events at the same locations. For
example, the invasion rate of terrestrial species for the
Hawaiian Islands was approximately 30 species per
million years (0.00003 per year) prior to human settlement, but increased to 20,000 species per million
years (0.02 per year) after the arrival of the Polynesians
and to approximately 20 per year during the past two
centuries (Ricciardi, 2007 ). In other words, contemporary rates of biological invasions are nearly one million
times higher than the prehistoric rate for Hawaii before
human infl uence.
The number of individuals of each species being
transported is also vastly different between natural and
human - assisted invasion events. Natural dispersal
events typically involve a few individuals of a species
fi nding their way out of the native range and attempting to establish a self - sustaining population in the
novel locale. Occasionally the number of individuals in
these natural events can be quite high – as for instance,
during biotic interchanges involving episodic events of
mass dispersal. For example, the opening of the transpolar corridor between the Pacifi c and Atlantic oceans
and the formation of the Panamanian land bridge
between North and South America during the Great
American Interchange permitted a massive fl ux of
species between formerly isolated regions (Vermeij,
2005 ; Lomolino et al. , 2006 ). By contrast, human -
assisted dispersal events are commonly characterized
therefore focused on attempts to understand which
factors differentiate between those species that successfully progress through all invasion stages and those
that do not (Lockwood et al. , 2007 ).
9.1.2 Human - a ssisted v ersus
p rehistoric i nvasions
A valid and persistent question is the extent to which
modern trends in species invasions differ from those
that occur naturally. This question is especially relevant to students of biogeography because range expansions are a very clear component of palaeoecological
and historical biodiversity patterns (Vermeij, 2005 ).
Do modern invasions warrant the attention currently
given to them by scientists? How different are the
mechanisms, spatial patterns and rates of modern
versus prehistoric invasions? Can we use prehistoric
trends to help predict the consequences of modern biological invasions?
Human - assisted dispersal of non - native species
differs from natural dispersal events in several important aspects (J.R.U. Wilson et al. , 2009 ). Ricciardi
(2007) detailed the differences between prehistoric
and human - assisted invasions, which we summarize
below and in Table 9.1 .
The most obvious differences are in the number
and frequency of ‘ dispersal ’ events. Natural dispersal
events are characteristically rare, both in the number
of species being transported and in the temporal frequency with which species disperse. By contrast,
modern human - assisted dispersal events happen constantly and involve a wide variety of species, which
Table 9.1 A comparison of key characteristics of prehistoric versus human - assisted invasions. Modifi ed from Table 1
of Ricciardi (2007) .
Characteristics
Prehistoric invasions
Human - assisted invasions
Frequency of long - distance dispersal event
Very low
Very high
Number of species transported per event
Low *
High
Propagule size per event
Small *
Potentially large
Number of mechanisms and routes of dispersal
Low
High
Temporal and spatial scales of mass transport events Episodic (short - distance) Continuous (long - distance)
Degree of homogenizing effect
Regional
Global
Potential for interactions with other stressors
Low
Very high
* Except during biotic interchange events.
Biological invasions and the homogenization of faunas and fl oras
show a much wider array of biological traits than
those species that are likely to experience natural long -
distance dispersal.
The rate at which non - native populations are establishing around the world is consistently several orders
of magnitude larger than fossil - derived estimates for
natural dispersal events at the same locations. For
example, the invasion rate of terrestrial species for the
Hawaiian Islands was approximately 30 species per
million years (0.00003 per year) prior to human settlement, but increased to 20,000 species per million
years (0.02 per year) after the arrival of the Polynesians
and to approximately 20 per year during the past two
centuries (Ricciardi, 2007 ). In other words, contemporary rates of biological invasions are nearly one million
times higher than the prehistoric rate for Hawaii before
human infl uence.
The number of individuals of each species being
transported is also vastly different between natural and
human - assisted invasion events. Natural dispersal
events typically involve a few individuals of a species
fi nding their way out of the native range and attempting to establish a self - sustaining population in the
novel locale. Occasionally the number of individuals in
these natural events can be quite high – as for instance,
during biotic interchanges involving episodic events of
mass dispersal. For example, the opening of the transpolar corridor between the Pacifi c and Atlantic oceans
and the formation of the Panamanian land bridge
between North and South America during the Great
American Interchange permitted a massive fl ux of
species between formerly isolated regions (Vermeij,
2005 ; Lomolino et al. , 2006 ). By contrast, human -
assisted dispersal events are commonly characterized
therefore focused on attempts to understand which
factors differentiate between those species that successfully progress through all invasion stages and those
that do not (Lockwood et al. , 2007 ).
9.1.2 Human - a ssisted v ersus
p rehistoric i nvasions
A valid and persistent question is the extent to which
modern trends in species invasions differ from those
that occur naturally. This question is especially relevant to students of biogeography because range expansions are a very clear component of palaeoecological
and historical biodiversity patterns (Vermeij, 2005 ).
Do modern invasions warrant the attention currently
given to them by scientists? How different are the
mechanisms, spatial patterns and rates of modern
versus prehistoric invasions? Can we use prehistoric
trends to help predict the consequences of modern biological invasions?
Human - assisted dispersal of non - native species
differs from natural dispersal events in several important aspects (J.R.U. Wilson et al. , 2009 ). Ricciardi
(2007) detailed the differences between prehistoric
and human - assisted invasions, which we summarize
below and in Table 9.1 .
The most obvious differences are in the number
and frequency of ‘ dispersal ’ events. Natural dispersal
events are characteristically rare, both in the number
of species being transported and in the temporal frequency with which species disperse. By contrast,
modern human - assisted dispersal events happen constantly and involve a wide variety of species, which
Table 9.1 A comparison of key characteristics of prehistoric versus human - assisted invasions. Modifi ed from Table 1
of Ricciardi (2007) .
Characteristics
Prehistoric invasions
Human - assisted invasions
Frequency of long - distance dispersal event
Very low
Very high
Number of species transported per event
Low *
High
Propagule size per event
Small *
Potentially large
Number of mechanisms and routes of dispersal
Low
High
Temporal and spatial scales of mass transport events Episodic (short - distance) Continuous (long - distance)
Degree of homogenizing effect
Regional
Global
Potential for interactions with other stressors
Low
Very high
* Except during biotic interchange events.