Roots, relevance, aims and values
27
islands and other areas that experience large infrequent disturbances recover when species recolonize
from surrounding areas or the mainland.
A long - term perspective raises the issue that defi nitions of ‘ native ’ and ‘ non - native ’ are ‘ based on value
judgements associated with a selective time frame, and
a selective categorization of which types of humans
can legitimately act as modes of dispersal ’ (Kendle &
Rose, 2000 , p. 22).
Moreover, as Gurevitch & Padilla (2004) point out,
the correlation between extinctions and invasions does
not necessarily imply causation, and care needs to be
taken to distinguish whether invasive species directly
displace native ones, causing local extinction, or
whether both invasion and biodiversity loss are a consequence of habitat degradation. Clearly then, it is not
the process of invasion itself that is problematic, but
the invasion by a species that would not ‘ naturally ’ be
in the area and, furthermore, whose arrival to an area
reduces biodiversity and threatens ecological processes, and/or economic resources or livelihoods.
Returning to the concept of naturalness, Brown &
Sax (2005) caution against what they term the ‘ naturalist fallacy ’ , meaning the assumption that ecosystems prior to invasion must be natural and pristine,
and that this is the ecosystem state that should be preserved. As Kendle and Rose ( 2000 , p. 20) point out,
this is problematic because ‘ it commits us to supporting a fl ora that refl ected a particular environmental
and climatic state that cannot continue forever and
has probably already changed ’ . Stability of species
composition is especially unlikely if the projections of
rapid 21st century climate change made by the global
climate science community are borne out, as these
changes will generate reshuffl ing and redistribution of
species as they track (or in some cases fail to track)
suitable climate space (Chapter 7 ; Ara ú jo et al ., 2004a ;
Williams & Jackson, 2007 ).
As mentioned above, although the narratives of
invasion scientists are not meant to apply to people,
they may nevertheless have negative overtones for
immigrant communities (Kendle & Rose, 2000 ). The
potential for confusion is exacerbated by the use of the
same words in both a societal and a biological context.
While biologists may have an unambiguous understanding of the term ‘ alien ’ as referring to an animal,
plant or pathogen, the same terminology is also used
with regard to human immigration – specifi cally the
use of the term ‘ illegal aliens ’ in the USA and other
countries. To many people, this issue may seem like
foreigners differently from natives: with distrust, dislike
even with loathing ’ (Brown & Sax, 2004 , p. 530).
While this direct transposition of biological terms with
social critique/ideology may surprise many invasion
biologists, the confl uence of language is evident.
In order to circumvent issues of confusing, ambiguous or emotive terminology, Colautti and MacIssac
(2004) suggested that the terminology surrounding
‘ invasive ’ species could be made more neutral. They
proposed a fi ve - stage model in which each stage,
labelled numerically, describes an operational fi lter
that allows some species to persist, reproduce and
spread while others do not. However, this scheme has
not been widely adopted, perhaps because the reader
needs anyway to return to a description of each fi lter
and therefore the parsimony and neutrality of a series
of numbered stages is lost.
Richardson et al . (2000) prefer to distinguish the
introduction (transport by humans) and naturalization (survival and reproduction) from invasion, which
requires that the introduced species has spread to and
reproduced in areas distant from the sites of introduction (Richardson et al ., 2000 ). This distinction is
helpful because the process of naturalization is not
necessarily seen as ‘ bad ’ . Many favoured exotic species
in gardens and farms are naturalized, but do not spread
independently from the locations in which they are
desired (Kendle & Rose, 2000 ; Sagoff, 2005 ). In short,
not all exotic species are invasive.
Conversely, not all invasive species are exotic, as
exemplifi ed by bracken, Pteridium aquilinum which,
although native in the UK, is an often unwelcome
invader of heathlands and moorlands. Furthermore,
many non - native species hold particular utilitarian,
cultural or aesthetic roles. For example, the bird of
paradise fl ower ( Strelitzia ) is native to South Africa, but
is the offi cial fl ower of the city of Los Angeles,
California, USA.
Nonetheless, the negative connotation of the word
‘ invasion ’ still remains and is problematic when one
considers that ongoing climatic change, ecosystem
interactions and anthropogenic management all cause
variations in the distribution and abundance of species
over time, and in this sense all species ’ distributions
may be considered the result of a past invasion (Keitt
et al ., 2004 ). For example, ice sheets covered large
areas of the northern hemisphere land mass during
the last glacial maximum, c. 21,000 years ago, and
these areas were recolonized or invaded by species that
persisted in more southerly refugia. Similarly, volcanic
27
islands and other areas that experience large infrequent disturbances recover when species recolonize
from surrounding areas or the mainland.
A long - term perspective raises the issue that defi nitions of ‘ native ’ and ‘ non - native ’ are ‘ based on value
judgements associated with a selective time frame, and
a selective categorization of which types of humans
can legitimately act as modes of dispersal ’ (Kendle &
Rose, 2000 , p. 22).
Moreover, as Gurevitch & Padilla (2004) point out,
the correlation between extinctions and invasions does
not necessarily imply causation, and care needs to be
taken to distinguish whether invasive species directly
displace native ones, causing local extinction, or
whether both invasion and biodiversity loss are a consequence of habitat degradation. Clearly then, it is not
the process of invasion itself that is problematic, but
the invasion by a species that would not ‘ naturally ’ be
in the area and, furthermore, whose arrival to an area
reduces biodiversity and threatens ecological processes, and/or economic resources or livelihoods.
Returning to the concept of naturalness, Brown &
Sax (2005) caution against what they term the ‘ naturalist fallacy ’ , meaning the assumption that ecosystems prior to invasion must be natural and pristine,
and that this is the ecosystem state that should be preserved. As Kendle and Rose ( 2000 , p. 20) point out,
this is problematic because ‘ it commits us to supporting a fl ora that refl ected a particular environmental
and climatic state that cannot continue forever and
has probably already changed ’ . Stability of species
composition is especially unlikely if the projections of
rapid 21st century climate change made by the global
climate science community are borne out, as these
changes will generate reshuffl ing and redistribution of
species as they track (or in some cases fail to track)
suitable climate space (Chapter 7 ; Ara ú jo et al ., 2004a ;
Williams & Jackson, 2007 ).
As mentioned above, although the narratives of
invasion scientists are not meant to apply to people,
they may nevertheless have negative overtones for
immigrant communities (Kendle & Rose, 2000 ). The
potential for confusion is exacerbated by the use of the
same words in both a societal and a biological context.
While biologists may have an unambiguous understanding of the term ‘ alien ’ as referring to an animal,
plant or pathogen, the same terminology is also used
with regard to human immigration – specifi cally the
use of the term ‘ illegal aliens ’ in the USA and other
countries. To many people, this issue may seem like
foreigners differently from natives: with distrust, dislike
even with loathing ’ (Brown & Sax, 2004 , p. 530).
While this direct transposition of biological terms with
social critique/ideology may surprise many invasion
biologists, the confl uence of language is evident.
In order to circumvent issues of confusing, ambiguous or emotive terminology, Colautti and MacIssac
(2004) suggested that the terminology surrounding
‘ invasive ’ species could be made more neutral. They
proposed a fi ve - stage model in which each stage,
labelled numerically, describes an operational fi lter
that allows some species to persist, reproduce and
spread while others do not. However, this scheme has
not been widely adopted, perhaps because the reader
needs anyway to return to a description of each fi lter
and therefore the parsimony and neutrality of a series
of numbered stages is lost.
Richardson et al . (2000) prefer to distinguish the
introduction (transport by humans) and naturalization (survival and reproduction) from invasion, which
requires that the introduced species has spread to and
reproduced in areas distant from the sites of introduction (Richardson et al ., 2000 ). This distinction is
helpful because the process of naturalization is not
necessarily seen as ‘ bad ’ . Many favoured exotic species
in gardens and farms are naturalized, but do not spread
independently from the locations in which they are
desired (Kendle & Rose, 2000 ; Sagoff, 2005 ). In short,
not all exotic species are invasive.
Conversely, not all invasive species are exotic, as
exemplifi ed by bracken, Pteridium aquilinum which,
although native in the UK, is an often unwelcome
invader of heathlands and moorlands. Furthermore,
many non - native species hold particular utilitarian,
cultural or aesthetic roles. For example, the bird of
paradise fl ower ( Strelitzia ) is native to South Africa, but
is the offi cial fl ower of the city of Los Angeles,
California, USA.
Nonetheless, the negative connotation of the word
‘ invasion ’ still remains and is problematic when one
considers that ongoing climatic change, ecosystem
interactions and anthropogenic management all cause
variations in the distribution and abundance of species
over time, and in this sense all species ’ distributions
may be considered the result of a past invasion (Keitt
et al ., 2004 ). For example, ice sheets covered large
areas of the northern hemisphere land mass during
the last glacial maximum, c. 21,000 years ago, and
these areas were recolonized or invaded by species that
persisted in more southerly refugia. Similarly, volcanic