Conservation planning in a changing world
237
Distributional patterns of native and non - native
species may vary in such a way that they will have
opposing effects on patterns of homogenization. For
example, Qian and Ricklefs (2006) evaluated changes
in differentiation diversity of vascular plants across
North America (excluding Mexico) and found that non -
native species tended to homogenize fl oras in distant
areas whose native plant species differ greatly, but
differentiate neighbouring areas that exhibited more
closely related native fl oras (Figure 9.5 ). Because few
native species have yet been extirpated from state and
provincial fl oras, these authors reason that the pattern
of homogenization and differentiation probably refl ects
the haphazard introduction and establishment of non -
native species with respect to suitable habitats. At play
is also the natural and human - assisted spread of non -
native species with no regard to the ecological constraints acting on native species.
Smart et al . (2006) used botanical data for fl owering
plants in Great Britain to test the hypothesis that plant
communities have become taxonomically and functionally more similar over the past 20 years in human -
dominated landscapes. Although little evidence was
found for the taxonomic homogenization of plant communities, this study revealed that plant traits related to
dispersal ability and canopy height increased in their
occurrence across the communities over time. The
authors suggest that environmental change has caused
different plant communities to converge on a narrower
range of winning trait syndromes (i.e. functional
homogenization), while species ’ identities remained
relatively constant.
Similarly, Castro and Jaksic (2008) reported that
the compositional similarity of the continental fl ora
of Chile has not shown signifi cant modifi cations
over time. Interestingly, this result is not shared for
oceanic island fl oral assemblages off the coast of Chile,
in which present - day islands share a greater number
of species compared to the pre - European condition
(Castro et al. , 2007 ).
9.3.4 Mammals
Interest in the process of biotic homogenization has
thankfully expanded beyond fi shes, plants and birds in
recent years. In a compelling study, Spear and Chown
(2008) examined the effects of ungulate introductions
on biotic similarity across four spatial scales, at three
spatial resolutions within South Africa and among
scale - dependent. At smaller spatial scales (islands
within archipelagos), the expected pattern of low
initial similarity leading to greater homogenization
was observed, whereas this relationship reversed at the
larger spatial scale of islands between archipelagos.
This study illustrates that the spatial extent of investigation and the evolutionary history of the region
under consideration can infl uence patterns of taxonomic homogenization and differentiation within and
across what appear to be equivalent spatial units (i.e.
ocean basins).
9.3.3 Plants
Evidence for fl oral homogenization comes from studies
conducted in many countries at a variety of spatial
scales. However, evidence to date suggests that levels
of fl oral homogenization are considerably smaller than
those observed for freshwater fi shes (Table 9.2 ).
Within the United States, McKinney (2004) found
that non - native plant species contributed signifi cantly
to fl oral homogenization of 20 parks and local conservation areas, although the magnitude was relatively
low and sometimes negative (indicating differentiation). Cosmopolitan plant species most responsible for
the observed homogenization included curly dock
( Rumex crispus ), dandelion ( Taraxacum offi cinale ) and
bluegrass ( Poa annua ). Similarly, Schwartz et al . (2006)
found that the county fl oras of California, USA, have
shown slight homogenization. The establishment of
noxious weeds played a central role in shaping patterns
of homogenization, but the authors suggest that the
greatest potential for future homogenization may come
from extirpations of extant native populations within
counties.
At a fi ner spatial scale, Rooney et al . (2004)
re - surveyed 62 upland forest stands in northern
Wisconsin, USA, to assess the degree of fl oral homogenization of under - storey communities between 1950
and 2000. By incorporating changes in both species
occurrence and abundance, the authors found that
two - thirds of the sites had become more similar in their
composition as a result of declines in rare species and
increases in already regionally abundant native and
non - native species. Interestingly, levels of homogenization were greatest in areas without deer hunting, suggesting that selective grazing by overabundant deer
populations was acting as a key driver of fl oral
homogenization.
237
Distributional patterns of native and non - native
species may vary in such a way that they will have
opposing effects on patterns of homogenization. For
example, Qian and Ricklefs (2006) evaluated changes
in differentiation diversity of vascular plants across
North America (excluding Mexico) and found that non -
native species tended to homogenize fl oras in distant
areas whose native plant species differ greatly, but
differentiate neighbouring areas that exhibited more
closely related native fl oras (Figure 9.5 ). Because few
native species have yet been extirpated from state and
provincial fl oras, these authors reason that the pattern
of homogenization and differentiation probably refl ects
the haphazard introduction and establishment of non -
native species with respect to suitable habitats. At play
is also the natural and human - assisted spread of non -
native species with no regard to the ecological constraints acting on native species.
Smart et al . (2006) used botanical data for fl owering
plants in Great Britain to test the hypothesis that plant
communities have become taxonomically and functionally more similar over the past 20 years in human -
dominated landscapes. Although little evidence was
found for the taxonomic homogenization of plant communities, this study revealed that plant traits related to
dispersal ability and canopy height increased in their
occurrence across the communities over time. The
authors suggest that environmental change has caused
different plant communities to converge on a narrower
range of winning trait syndromes (i.e. functional
homogenization), while species ’ identities remained
relatively constant.
Similarly, Castro and Jaksic (2008) reported that
the compositional similarity of the continental fl ora
of Chile has not shown signifi cant modifi cations
over time. Interestingly, this result is not shared for
oceanic island fl oral assemblages off the coast of Chile,
in which present - day islands share a greater number
of species compared to the pre - European condition
(Castro et al. , 2007 ).
9.3.4 Mammals
Interest in the process of biotic homogenization has
thankfully expanded beyond fi shes, plants and birds in
recent years. In a compelling study, Spear and Chown
(2008) examined the effects of ungulate introductions
on biotic similarity across four spatial scales, at three
spatial resolutions within South Africa and among
scale - dependent. At smaller spatial scales (islands
within archipelagos), the expected pattern of low
initial similarity leading to greater homogenization
was observed, whereas this relationship reversed at the
larger spatial scale of islands between archipelagos.
This study illustrates that the spatial extent of investigation and the evolutionary history of the region
under consideration can infl uence patterns of taxonomic homogenization and differentiation within and
across what appear to be equivalent spatial units (i.e.
ocean basins).
9.3.3 Plants
Evidence for fl oral homogenization comes from studies
conducted in many countries at a variety of spatial
scales. However, evidence to date suggests that levels
of fl oral homogenization are considerably smaller than
those observed for freshwater fi shes (Table 9.2 ).
Within the United States, McKinney (2004) found
that non - native plant species contributed signifi cantly
to fl oral homogenization of 20 parks and local conservation areas, although the magnitude was relatively
low and sometimes negative (indicating differentiation). Cosmopolitan plant species most responsible for
the observed homogenization included curly dock
( Rumex crispus ), dandelion ( Taraxacum offi cinale ) and
bluegrass ( Poa annua ). Similarly, Schwartz et al . (2006)
found that the county fl oras of California, USA, have
shown slight homogenization. The establishment of
noxious weeds played a central role in shaping patterns
of homogenization, but the authors suggest that the
greatest potential for future homogenization may come
from extirpations of extant native populations within
counties.
At a fi ner spatial scale, Rooney et al . (2004)
re - surveyed 62 upland forest stands in northern
Wisconsin, USA, to assess the degree of fl oral homogenization of under - storey communities between 1950
and 2000. By incorporating changes in both species
occurrence and abundance, the authors found that
two - thirds of the sites had become more similar in their
composition as a result of declines in rare species and
increases in already regionally abundant native and
non - native species. Interestingly, levels of homogenization were greatest in areas without deer hunting, suggesting that selective grazing by overabundant deer
populations was acting as a key driver of fl oral
homogenization.