240
Biological invasions and the homogenization of faunas and fl oras
9.5 BIOTIC HOMOGENIZATION AND
CONSERVATION
Biotic homogenization is an important dimension of
the modern biodiversity crisis, with signifi cant ecological, evolutionary and social implications (McKinney &
Lockwood, 1999 ; Olden et al. , 2004; 2005 ). It extends
beyond the narrow focus on elevated extinction rates
to incorporate the other side of the equation: the establishment of non - native species. Biotic homogenization
conjures the prospect of Kunstler ’ s (1993) The
Geography of Nowhere , in which biotic distinctiveness is
gradually dissolving over time. Consequently, a major
challenge within conservation biogeography is to identify and understand present - day patterns of biotic
homogenization to guide policy aimed at mitigating its
future effects (Rooney et al. , 2007 ).
Clearly, the most effective conservation of biodiversity involves reducing and, where possible, preventing
the two processes generating biotic homogenization –
species invasions and extinctions. The conundrum is
determining the best way to achieve this goal. Because
the key factors facilitating homogenization include
people and habitat transformation (through extinctions or the establishment of non - native species), a fi rst
step towards achieving biodiversity conservation goals
is to focus efforts in areas subject to human activities
and to reduce human - related impacts.
Unfortunately, there is a strong correlation between
human population density and species richness, and
the areas of high biotic diversity that are under the
greatest threat are often in the most populated areas
(Chown et al. , 2003 ; McDonald et al. , 2008 ). Indeed, at
a fi ner scale of analysis, designated conservation areas
may often attract people to them through perceived
benefi ts of employment, market access and foreign aid
(Wittmeyer et al . 2008 ). The increased external threat
from accelerated human population growth does not
bode well for the native biota in these areas, which consequently face the risk of increased homogenization.
In the past, purposeful homogenization was undertaken within countries such as Australia and some
Pacifi c island territories by acclimatization societies
within colonist human societies who, for a variety of
reasons, wanted to surround themselves with familiar,
colourful or (regarding birds) tuneful species. Even
today, some conservation organizations encourage the
intentional movement or translocation of species,
which may also have the unintended consequence of
promoting homogenization.
most urbanized sites – an outcome of the replacement
of local endemic species (often urban - sensitive species)
by ubiquitous non - native species (urban - adapted
species).
By contrast, Clergeau et al . (2006) found that avifaunal similarity of town centres in Europe was actually
lower than in less urbanized habitats – a result that
may have been connected to the larger size of towns
and, thus, greater types of potential habitat in this
study system. The results from this study also suggested that urbanization might cause homogenization
by decreasing the abundance of ground - nesting bird
species and bird species that preferred bush/shrub
habitats. Schwartz et al . (2006) reported fl oristic
homogenization of urbanized counties in southern
California, whereas they found no change in more
rural areas of northern California. The study of K ü hn
and Klotz (2006) , on the other hand, found no overall
relationship between patterns of homogenization and
urbanization across Germany.
In summary, although urbanization undoubtedly
plays a role in shaping patterns of biotic homogenization, the exact nature and generality of this relationship is still unclear (McKinney, 2006 ).
Environmentally mediated interactions between
species may also be an important driver of biotic
homogenization. Holway and Suarez (2006) examined
native ant communities in scrub and riparian habitats
of mediterranean California to test the hypothesis that
the invasion of Argentine ant ( Linepithema humile ) has
caused biotic homogenization. By comparing invaded
and un - invaded sites across similar habitats, the
authors showed that sites invaded by Argentine ants
have lower beta diversity compared to un - invaded sites.
Specifi cally, functional homogenization of ant communities occurred via shifting community dominance to
smaller - bodied workers with lower thermal tolerance
and a reduced diversity of behaviours (i.e. nesting
habits, dispersal strategies and foraging behaviours).
Because Argentine ant abundance in seasonally - dry
mediterranean environments is positively correlated
with soil moisture, the authors hypothesized that the
homogenizing effects of the Argentine ant are facilitated by inputs of urban and agricultural water run - off
that acts to create mesic soil conditions. This observation supports the notion that anthropogenic modifi cations to the environment indirectly cause biotic
homogenization by creating opportunities for the invasion of the Argentine ant, as opposed to threatening
the persistence of native ants directly.
Biological invasions and the homogenization of faunas and fl oras
9.5 BIOTIC HOMOGENIZATION AND
CONSERVATION
Biotic homogenization is an important dimension of
the modern biodiversity crisis, with signifi cant ecological, evolutionary and social implications (McKinney &
Lockwood, 1999 ; Olden et al. , 2004; 2005 ). It extends
beyond the narrow focus on elevated extinction rates
to incorporate the other side of the equation: the establishment of non - native species. Biotic homogenization
conjures the prospect of Kunstler ’ s (1993) The
Geography of Nowhere , in which biotic distinctiveness is
gradually dissolving over time. Consequently, a major
challenge within conservation biogeography is to identify and understand present - day patterns of biotic
homogenization to guide policy aimed at mitigating its
future effects (Rooney et al. , 2007 ).
Clearly, the most effective conservation of biodiversity involves reducing and, where possible, preventing
the two processes generating biotic homogenization –
species invasions and extinctions. The conundrum is
determining the best way to achieve this goal. Because
the key factors facilitating homogenization include
people and habitat transformation (through extinctions or the establishment of non - native species), a fi rst
step towards achieving biodiversity conservation goals
is to focus efforts in areas subject to human activities
and to reduce human - related impacts.
Unfortunately, there is a strong correlation between
human population density and species richness, and
the areas of high biotic diversity that are under the
greatest threat are often in the most populated areas
(Chown et al. , 2003 ; McDonald et al. , 2008 ). Indeed, at
a fi ner scale of analysis, designated conservation areas
may often attract people to them through perceived
benefi ts of employment, market access and foreign aid
(Wittmeyer et al . 2008 ). The increased external threat
from accelerated human population growth does not
bode well for the native biota in these areas, which consequently face the risk of increased homogenization.
In the past, purposeful homogenization was undertaken within countries such as Australia and some
Pacifi c island territories by acclimatization societies
within colonist human societies who, for a variety of
reasons, wanted to surround themselves with familiar,
colourful or (regarding birds) tuneful species. Even
today, some conservation organizations encourage the
intentional movement or translocation of species,
which may also have the unintended consequence of
promoting homogenization.
most urbanized sites – an outcome of the replacement
of local endemic species (often urban - sensitive species)
by ubiquitous non - native species (urban - adapted
species).
By contrast, Clergeau et al . (2006) found that avifaunal similarity of town centres in Europe was actually
lower than in less urbanized habitats – a result that
may have been connected to the larger size of towns
and, thus, greater types of potential habitat in this
study system. The results from this study also suggested that urbanization might cause homogenization
by decreasing the abundance of ground - nesting bird
species and bird species that preferred bush/shrub
habitats. Schwartz et al . (2006) reported fl oristic
homogenization of urbanized counties in southern
California, whereas they found no change in more
rural areas of northern California. The study of K ü hn
and Klotz (2006) , on the other hand, found no overall
relationship between patterns of homogenization and
urbanization across Germany.
In summary, although urbanization undoubtedly
plays a role in shaping patterns of biotic homogenization, the exact nature and generality of this relationship is still unclear (McKinney, 2006 ).
Environmentally mediated interactions between
species may also be an important driver of biotic
homogenization. Holway and Suarez (2006) examined
native ant communities in scrub and riparian habitats
of mediterranean California to test the hypothesis that
the invasion of Argentine ant ( Linepithema humile ) has
caused biotic homogenization. By comparing invaded
and un - invaded sites across similar habitats, the
authors showed that sites invaded by Argentine ants
have lower beta diversity compared to un - invaded sites.
Specifi cally, functional homogenization of ant communities occurred via shifting community dominance to
smaller - bodied workers with lower thermal tolerance
and a reduced diversity of behaviours (i.e. nesting
habits, dispersal strategies and foraging behaviours).
Because Argentine ant abundance in seasonally - dry
mediterranean environments is positively correlated
with soil moisture, the authors hypothesized that the
homogenizing effects of the Argentine ant are facilitated by inputs of urban and agricultural water run - off
that acts to create mesic soil conditions. This observation supports the notion that anthropogenic modifi cations to the environment indirectly cause biotic
homogenization by creating opportunities for the invasion of the Argentine ant, as opposed to threatening
the persistence of native ants directly.