230
Biological invasions and the homogenization of faunas and fl oras
diversity across a study region, a phenomenon termed
‘ biotic differentiation ’ by Olden and Poff (2003) .
9.2.1 The p rocess of b iotic h omogenization
In the simplest sense, human activities that increase
rates of species invasions and extirpations are the ultimate cause of biotic homogenization. However, biotic
homogenization can arise when only invasions occur
without the concurrent loss of species, or conversely
where only species extirpations occur. In other words,
species additions or replacements need not occur for
regions to become homogenized or even differentiated
over time (Olden & Poff, 2003 ).
To illustrate this point, we provide a simple graphical
example showing how the number and manner in
which non - native species establishment and native
species extirpations occur may lead to very different
levels of homogenization or differentiation (Figure
9.2 ). In the absence of any extirpation, the establishment of the same non - native species at two separate
localities will lead to increases in the similarity of the
invaded communities. Conversely, the establishment of
a different non - native species at each locality will
decrease community similarity. Although this example
is useful to illustrate the simplest way biotic homogenization can occur, both empirical data and theoretical
modelling suggests that the process is both complex
and sensitive to the spatial and temporal scale of investigation (Olden, 2006 ).
9.2.2 Different m anifestations of b iotic
h omogenization
Biotic homogenization is considered an overarching
process that encompasses either the loss of taxonomic,
genetic or functional distinctiveness over time (Olden
et al. , 2004 ). Taxonomic homogenization, which we
used to introduce the concept of homogenization
above, has been the primary focus of previous research
and is commonly referred to as biotic homogenization.
However, imposing a narrow defi nition of biotic homogenization does not truly refl ect the multidimensional
nature of this process. Consequently, it is useful to
think of biotic homogenization as a broader ecological
process by which formerly disparate biotas lose biological distinctiveness at any level of organization, including in their genetic and functional characteristics.
For example, Pautasso (2007) conducted a meta -
analysis of the relationship between human population size and change in the plant and animal species
richness of study areas. The study reported negative
changes in richness at small spatial scales of analysis
(or small extent) but positive changes at larger spatial
scales. The introduction of non - native species by
humans is typically integral to such changes. In
essence, anthropogenic changes driving habitat loss,
fragmentation, species invasions and ecosystem transformation may result in declining local richness but,
across larger landscapes and regions, relatively few
native species may become entirely extinct, while non -
natives boost the richness above natural baseline levels.
Changes such as these, in the inventory richness of
smaller areas nested within larger regions, may also be
accompanied by changing patterns in differentiation
diversity, i.e. in the degree of compositional turnover
between localities – also known as ‘ beta diversity ’ . A
change in beta diversity can, in fact, occur either
through a reduction in the total number of species in
the region (regional species richness or sometimes
‘ epsilon diversity ’ ) or through a change in the species
similarity between areas. Basically, if a similar suite of
species is shared across the areas in a region, beta
diversity will be quite low. If very different species
occur in different areas, beta diversity will be high.
Biotic homogenization is thus a term describing the
process of reducing differentiation diversity between
regions, but it may be accompanied by varying
patterns of change in inventory richness at different
scales of analysis. See Box 1.2 for an explanation of
terminology.
Put another way, biotic homogenization is described
as the process by which regionally distinct native communities are gradually replaced by locally expanding,
cosmopolitan, non - native communities (McKinney &
Lockwood, 1999 ). Some have likened the process of
biotic homogenization to the now global distribution of
fast - food restaurants, coffee houses and big - box retailers (Olden et al. , 2005 ). The more connected we are as
a society, the more likely we are to see the trans - global
distribution of both species and businesses. In circumstances where invasive species impact negatively on
locally co - occurring native species, rare and endemic
native species may be lost, resulting in rapid loss of
differentiation diversity. However, it is also important
to recognize that the reverse can also occur and that,
in cases, the combined effects of invasions and extirpations can be to increase the mean differentiation
Biological invasions and the homogenization of faunas and fl oras
diversity across a study region, a phenomenon termed
‘ biotic differentiation ’ by Olden and Poff (2003) .
9.2.1 The p rocess of b iotic h omogenization
In the simplest sense, human activities that increase
rates of species invasions and extirpations are the ultimate cause of biotic homogenization. However, biotic
homogenization can arise when only invasions occur
without the concurrent loss of species, or conversely
where only species extirpations occur. In other words,
species additions or replacements need not occur for
regions to become homogenized or even differentiated
over time (Olden & Poff, 2003 ).
To illustrate this point, we provide a simple graphical
example showing how the number and manner in
which non - native species establishment and native
species extirpations occur may lead to very different
levels of homogenization or differentiation (Figure
9.2 ). In the absence of any extirpation, the establishment of the same non - native species at two separate
localities will lead to increases in the similarity of the
invaded communities. Conversely, the establishment of
a different non - native species at each locality will
decrease community similarity. Although this example
is useful to illustrate the simplest way biotic homogenization can occur, both empirical data and theoretical
modelling suggests that the process is both complex
and sensitive to the spatial and temporal scale of investigation (Olden, 2006 ).
9.2.2 Different m anifestations of b iotic
h omogenization
Biotic homogenization is considered an overarching
process that encompasses either the loss of taxonomic,
genetic or functional distinctiveness over time (Olden
et al. , 2004 ). Taxonomic homogenization, which we
used to introduce the concept of homogenization
above, has been the primary focus of previous research
and is commonly referred to as biotic homogenization.
However, imposing a narrow defi nition of biotic homogenization does not truly refl ect the multidimensional
nature of this process. Consequently, it is useful to
think of biotic homogenization as a broader ecological
process by which formerly disparate biotas lose biological distinctiveness at any level of organization, including in their genetic and functional characteristics.
For example, Pautasso (2007) conducted a meta -
analysis of the relationship between human population size and change in the plant and animal species
richness of study areas. The study reported negative
changes in richness at small spatial scales of analysis
(or small extent) but positive changes at larger spatial
scales. The introduction of non - native species by
humans is typically integral to such changes. In
essence, anthropogenic changes driving habitat loss,
fragmentation, species invasions and ecosystem transformation may result in declining local richness but,
across larger landscapes and regions, relatively few
native species may become entirely extinct, while non -
natives boost the richness above natural baseline levels.
Changes such as these, in the inventory richness of
smaller areas nested within larger regions, may also be
accompanied by changing patterns in differentiation
diversity, i.e. in the degree of compositional turnover
between localities – also known as ‘ beta diversity ’ . A
change in beta diversity can, in fact, occur either
through a reduction in the total number of species in
the region (regional species richness or sometimes
‘ epsilon diversity ’ ) or through a change in the species
similarity between areas. Basically, if a similar suite of
species is shared across the areas in a region, beta
diversity will be quite low. If very different species
occur in different areas, beta diversity will be high.
Biotic homogenization is thus a term describing the
process of reducing differentiation diversity between
regions, but it may be accompanied by varying
patterns of change in inventory richness at different
scales of analysis. See Box 1.2 for an explanation of
terminology.
Put another way, biotic homogenization is described
as the process by which regionally distinct native communities are gradually replaced by locally expanding,
cosmopolitan, non - native communities (McKinney &
Lockwood, 1999 ). Some have likened the process of
biotic homogenization to the now global distribution of
fast - food restaurants, coffee houses and big - box retailers (Olden et al. , 2005 ). The more connected we are as
a society, the more likely we are to see the trans - global
distribution of both species and businesses. In circumstances where invasive species impact negatively on
locally co - occurring native species, rare and endemic
native species may be lost, resulting in rapid loss of
differentiation diversity. However, it is also important
to recognize that the reverse can also occur and that,
in cases, the combined effects of invasions and extirpations can be to increase the mean differentiation