Conservation planning in a changing world
229
of which is the widespread distribution of Norway rats
( Rattus norvegicus ). This rodent species was regularly,
and inadvertently, transported with human colonists
as they expanded across the globe. They serve as the
reservoir and vector for a variety of particularly troublesome human diseases, the most well know being
bubonic plague.
In general, scientists reserve the term ‘ invasive ’ for
these few non - native species that cause ecological or
economic harm. It is an open question as to whether
these few invasive species have characteristics that
make them unique amongst the world ’ s species, but
there is a clear need to be able to identify them as
potentially harmful long before they have the chance
to become invasive.
9.2 BIOTIC HOMOGENIZATION
The regional connectivity of the world is stronger and
more varied than ever before and, consequently, there
are very few places where non - native species have not
become established. Looking back over human history,
it is apparent that changes in species diversity are
frequently the result of the widespread invasion of
ubiquitous non - native species into areas containing
rare, and often unique, native species (Elton, 1958 ,
Ricciardi, 2007 ).
If the same non - native species are being introduced
to multiple locations, then there is potential for disparate regions to become more similar in their species
composition through time, a process known as biotic
homogenization. There are certainly well - known
invaders that can be found nearly everywhere. These
days, for example, you can land at nearly any airport
in the world and, while waiting for your next fl ight,
watch house sparrows ( Passer domesticus ) cavorting on
the tarmac. This species is native to Eurasia, but it has
realized a very broad geographical distribution via
human - mediated introductions.
For many years, the biodiversity crisis has been
focused on the loss of species through global extinction. Although this is clearly of prime importance, at
sub - global scales the loss of populations through local
extirpation, combined with the invasion of already
common non - native species, may be the more dramatic reconfi guration of modern biodiversity. In fact,
changes in diversity patterns at fi ne and coarse scales
of analysis can be either concordant or, alternatively,
can be decoupled and even confl icting.
• Second, as the number of non - native populations
increases, scientists fi nd it increasingly hard to ignore
them. It is important to recognize that many of these
species present unique opportunities to test various
ecological, evolutionary and biogeographical concepts
and theories. Certainly the basic insights gained from
the study of modern invasion events are substantial
(Sax et al. , 2007 ).
• Third, some of the non - native populations that have
established have gone on to impart substantial economic and ecological cost (Simberloff et al. , 2005 ;
Pimentel et al. , 2006 ).
As detailed above and shown in Figure 9.1 , not all
species that are dispersed via human actions have
negative impacts within their new environment. The
defi nition of what constituents ‘ impact ’ is somewhat
problematic for at least two reasons:
1 There are scientifi c and societal infl uences on the
perception of impact (not to mention that the effects of
invasive species are often subtle and diffi cult to observe).
2 After impact is perceived, there is a variety of ecological factors that determine the level of impact produced (Lockwood et al. , 2007 ).
Let us move past this issue by simply conceding that
human perception and valuation are an integral part
of the integration stage of the invasion process (Figure
9.1 ). It is important to recognize that the proportion of
species that do cause harm as compared to those that
are simply moved out of their native range is quite low.
Nevertheless, these few species will eat, parasitize and
compete with native species, often driving the latter
extinct or into very low population numbers (Elton,
1958 ; Clavero & Garc í a - Berthou, 2005 ; Strayer et al. ,
2006 ). Some non - native populations invade natural
areas such as parks or wildlife reserves and disrupt
native species communities (Simberloff et al. , 2005 ). In
these instances, the value of the natural area in terms
of its ability to conserve biodiversity may be reduced if
the non - native is not controlled or eradicated.
Many species threaten human economic interests,
notable examples including the zebra mussels ( Dreissena
polymorpha ) that clog utility companies ’ water intake
valves (MacIsaac, 1996 ); emerald ash borers ( Agrilus
planipennis ) that devastate urban and commercial
forests (Poland & McCollough, 2006 ); and monk parakeets ( Myiopsitta monachus ), whose bulky nests can
cause electric power line failures (Avery et al. , 2002 ).
A substantial number of non - native species have
adverse impacts on human health by transmitting diseases (Lounibos, 2002 ; Tatem, 2009 ), the most obvious
229
of which is the widespread distribution of Norway rats
( Rattus norvegicus ). This rodent species was regularly,
and inadvertently, transported with human colonists
as they expanded across the globe. They serve as the
reservoir and vector for a variety of particularly troublesome human diseases, the most well know being
bubonic plague.
In general, scientists reserve the term ‘ invasive ’ for
these few non - native species that cause ecological or
economic harm. It is an open question as to whether
these few invasive species have characteristics that
make them unique amongst the world ’ s species, but
there is a clear need to be able to identify them as
potentially harmful long before they have the chance
to become invasive.
9.2 BIOTIC HOMOGENIZATION
The regional connectivity of the world is stronger and
more varied than ever before and, consequently, there
are very few places where non - native species have not
become established. Looking back over human history,
it is apparent that changes in species diversity are
frequently the result of the widespread invasion of
ubiquitous non - native species into areas containing
rare, and often unique, native species (Elton, 1958 ,
Ricciardi, 2007 ).
If the same non - native species are being introduced
to multiple locations, then there is potential for disparate regions to become more similar in their species
composition through time, a process known as biotic
homogenization. There are certainly well - known
invaders that can be found nearly everywhere. These
days, for example, you can land at nearly any airport
in the world and, while waiting for your next fl ight,
watch house sparrows ( Passer domesticus ) cavorting on
the tarmac. This species is native to Eurasia, but it has
realized a very broad geographical distribution via
human - mediated introductions.
For many years, the biodiversity crisis has been
focused on the loss of species through global extinction. Although this is clearly of prime importance, at
sub - global scales the loss of populations through local
extirpation, combined with the invasion of already
common non - native species, may be the more dramatic reconfi guration of modern biodiversity. In fact,
changes in diversity patterns at fi ne and coarse scales
of analysis can be either concordant or, alternatively,
can be decoupled and even confl icting.
• Second, as the number of non - native populations
increases, scientists fi nd it increasingly hard to ignore
them. It is important to recognize that many of these
species present unique opportunities to test various
ecological, evolutionary and biogeographical concepts
and theories. Certainly the basic insights gained from
the study of modern invasion events are substantial
(Sax et al. , 2007 ).
• Third, some of the non - native populations that have
established have gone on to impart substantial economic and ecological cost (Simberloff et al. , 2005 ;
Pimentel et al. , 2006 ).
As detailed above and shown in Figure 9.1 , not all
species that are dispersed via human actions have
negative impacts within their new environment. The
defi nition of what constituents ‘ impact ’ is somewhat
problematic for at least two reasons:
1 There are scientifi c and societal infl uences on the
perception of impact (not to mention that the effects of
invasive species are often subtle and diffi cult to observe).
2 After impact is perceived, there is a variety of ecological factors that determine the level of impact produced (Lockwood et al. , 2007 ).
Let us move past this issue by simply conceding that
human perception and valuation are an integral part
of the integration stage of the invasion process (Figure
9.1 ). It is important to recognize that the proportion of
species that do cause harm as compared to those that
are simply moved out of their native range is quite low.
Nevertheless, these few species will eat, parasitize and
compete with native species, often driving the latter
extinct or into very low population numbers (Elton,
1958 ; Clavero & Garc í a - Berthou, 2005 ; Strayer et al. ,
2006 ). Some non - native populations invade natural
areas such as parks or wildlife reserves and disrupt
native species communities (Simberloff et al. , 2005 ). In
these instances, the value of the natural area in terms
of its ability to conserve biodiversity may be reduced if
the non - native is not controlled or eradicated.
Many species threaten human economic interests,
notable examples including the zebra mussels ( Dreissena
polymorpha ) that clog utility companies ’ water intake
valves (MacIsaac, 1996 ); emerald ash borers ( Agrilus
planipennis ) that devastate urban and commercial
forests (Poland & McCollough, 2006 ); and monk parakeets ( Myiopsitta monachus ), whose bulky nests can
cause electric power line failures (Avery et al. , 2002 ).
A substantial number of non - native species have
adverse impacts on human health by transmitting diseases (Lounibos, 2002 ; Tatem, 2009 ), the most obvious