232
Biological invasions and the homogenization of faunas and fl oras
Although the jury is still out on this, it is clear that the
study of biotic homogenization represents a unique
challenge because it is a multifaceted process, encompassing both species invasions and extirpations, which
requires the explicit consideration of how the identities
of species (not just species richness) change over both
space and time.
A simple perusal of the literature shows that the
majority of research to date has focused on quantifying
patterns of taxonomic homogenization, whereas the
processes of genetic and functional homogenization
have received considerably less attention. Moreover,
even estimates of taxonomic homogenization are
sparse and highly variable within and between taxonomic groups. Despite this trend, tremendous progress
has been made in recent years to better understand
and quantify patterns of taxonomic homogenization
(Table 9.2 ). We review the taxonomic groups (fi shes,
birds, plants and mammals) that have received the
most attention next.
9.3.1 Fishes
The homogenization of freshwater fi sh faunas has
received the greatest attention thus far. In a landmark
study, Rahel (2000) compared the species similarity of
US states between present - day and pre - European settlement time frames and found that pairs of states averaged 15.4 more species in common now than they did
in the past. On average, fi sh faunas became more
similar by 7.2 per cent, with the highest increases in
similarity observed in western and north - eastern states
(Figure 9.3 a). The high degree of biotic homogenization is best illustrated by the fact that the 89 pairs of
states that historically had zero similarity (no species
in common) now share an average of 25.2 species,
resulting in an average present - day similarity of 12.2
per cent. Patterns of fi sh homogenization were primarily the result of non - native species establishment associated with fi sh stocking for recreational purposes (e.g.
brown trout ( Salmo trutta ), rainbow trout ( Oncorhynchus
mykiss ) and smallmouth bass ( Micropterus dolomieu ) or
aquaculture (e.g. common carp, Cyprinus carpio ), and
to a smaller degree the extirpation of endemic species
(harelip sucker, Lagochila lacera ).
Taylor (2004) found a similar pattern among
Canadian provinces and territories, where average
faunal similarity increased from 27.8 per cent to 29.1
per cent – a trend driven in large part by the differential
• Second, introductions of species outside of their
original range(s) increases the likelihood of a founder
effect and reduced levels of genetic variability, as well
as setting the stage for interspecifi c hybridization (i.e.
hybridization between different species within the
same genus) (Rhymer & Simberloff, 1996 ).
• Third, if extirpations were a cause for faunal homogenization, then one consequence might be bottleneck(s)
in local populations of the impacted species, along with
lowered effective population size(s) (Lee, 2002 ).
Functional homogenization refers to an increase in
the functional similarity of biotas over time resulting
from the replacement of ecological specialists by the
same widespread generalists. It occurs primarily
because patterns of species invasions and extirpations
are not random, but instead are related to particular
biological traits that commonly predispose native
species to extirpation and non - native species to successful establishment. The end result is an increase in
the functional convergence of biotas over time associated with the establishment of species with similar
‘ roles ’ in the ecosystem (e.g. high redundancy of functional forms or traits) and the loss of species possessing
unique functional ‘ roles ’ (McKinney & Lockwood,
1999 ; Olden et al. , 2004 ).
For example, Winter et al . (2008) examined how the
presence of non - native plant species in Germany
affected the distribution of a genetic trait, namely
ploidy level (referring to the number of homologous
sets of chromosomes in a biological cell), at two spatial
scales. It is commonly accepted that polyploidy species
should have a greater ability to colonize or invade new
habitats due to greater genetic variability. Interestingly,
this study found evidence for functional differentiation
at fi ne spatial scales ( < 130 km
2
) due to more heterogeneous ploidy levels of non - native plants compared to
native plants, whereas, at a coarser spatial scale, more
homogeneous ploidy levels of non - native species lead
to functional homogenization.
9.3 PATTERNS OF BIOTIC
HOMOGENIZATION
Many scientists, including ourselves, have argued that
we are entering a period characterized by widespread
faunal and fl oral homogenization, fi ttingly dubbed the
‘ Homogecene ’ , in a place appropriately called the ‘ New
Pangaea ’ (the original Pangaea being the global supercontinent of approximately 250 million years ago).
Biological invasions and the homogenization of faunas and fl oras
Although the jury is still out on this, it is clear that the
study of biotic homogenization represents a unique
challenge because it is a multifaceted process, encompassing both species invasions and extirpations, which
requires the explicit consideration of how the identities
of species (not just species richness) change over both
space and time.
A simple perusal of the literature shows that the
majority of research to date has focused on quantifying
patterns of taxonomic homogenization, whereas the
processes of genetic and functional homogenization
have received considerably less attention. Moreover,
even estimates of taxonomic homogenization are
sparse and highly variable within and between taxonomic groups. Despite this trend, tremendous progress
has been made in recent years to better understand
and quantify patterns of taxonomic homogenization
(Table 9.2 ). We review the taxonomic groups (fi shes,
birds, plants and mammals) that have received the
most attention next.
9.3.1 Fishes
The homogenization of freshwater fi sh faunas has
received the greatest attention thus far. In a landmark
study, Rahel (2000) compared the species similarity of
US states between present - day and pre - European settlement time frames and found that pairs of states averaged 15.4 more species in common now than they did
in the past. On average, fi sh faunas became more
similar by 7.2 per cent, with the highest increases in
similarity observed in western and north - eastern states
(Figure 9.3 a). The high degree of biotic homogenization is best illustrated by the fact that the 89 pairs of
states that historically had zero similarity (no species
in common) now share an average of 25.2 species,
resulting in an average present - day similarity of 12.2
per cent. Patterns of fi sh homogenization were primarily the result of non - native species establishment associated with fi sh stocking for recreational purposes (e.g.
brown trout ( Salmo trutta ), rainbow trout ( Oncorhynchus
mykiss ) and smallmouth bass ( Micropterus dolomieu ) or
aquaculture (e.g. common carp, Cyprinus carpio ), and
to a smaller degree the extirpation of endemic species
(harelip sucker, Lagochila lacera ).
Taylor (2004) found a similar pattern among
Canadian provinces and territories, where average
faunal similarity increased from 27.8 per cent to 29.1
per cent – a trend driven in large part by the differential
• Second, introductions of species outside of their
original range(s) increases the likelihood of a founder
effect and reduced levels of genetic variability, as well
as setting the stage for interspecifi c hybridization (i.e.
hybridization between different species within the
same genus) (Rhymer & Simberloff, 1996 ).
• Third, if extirpations were a cause for faunal homogenization, then one consequence might be bottleneck(s)
in local populations of the impacted species, along with
lowered effective population size(s) (Lee, 2002 ).
Functional homogenization refers to an increase in
the functional similarity of biotas over time resulting
from the replacement of ecological specialists by the
same widespread generalists. It occurs primarily
because patterns of species invasions and extirpations
are not random, but instead are related to particular
biological traits that commonly predispose native
species to extirpation and non - native species to successful establishment. The end result is an increase in
the functional convergence of biotas over time associated with the establishment of species with similar
‘ roles ’ in the ecosystem (e.g. high redundancy of functional forms or traits) and the loss of species possessing
unique functional ‘ roles ’ (McKinney & Lockwood,
1999 ; Olden et al. , 2004 ).
For example, Winter et al . (2008) examined how the
presence of non - native plant species in Germany
affected the distribution of a genetic trait, namely
ploidy level (referring to the number of homologous
sets of chromosomes in a biological cell), at two spatial
scales. It is commonly accepted that polyploidy species
should have a greater ability to colonize or invade new
habitats due to greater genetic variability. Interestingly,
this study found evidence for functional differentiation
at fi ne spatial scales ( < 130 km
2
) due to more heterogeneous ploidy levels of non - native plants compared to
native plants, whereas, at a coarser spatial scale, more
homogeneous ploidy levels of non - native species lead
to functional homogenization.
9.3 PATTERNS OF BIOTIC
HOMOGENIZATION
Many scientists, including ourselves, have argued that
we are entering a period characterized by widespread
faunal and fl oral homogenization, fi ttingly dubbed the
‘ Homogecene ’ , in a place appropriately called the ‘ New
Pangaea ’ (the original Pangaea being the global supercontinent of approximately 250 million years ago).