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The Rangeland Journal, 2006, 28, 27–35
The impacts of invasive plant species on the biodiversity
of Australian rangelands
A. C. Grice
CSIRO Sustainable Ecosystems and Co-operative Research Centre for Australian Weed Management,
Private Bag PO Aitkenvale, Qld 4814, Australia. Email: [email protected]
Abstract. Most parts of the Australian rangelands are at risk of invasion by one or more species of non-native
plants. The severity of current problems varies greatly across the rangelands with more non-native plant species
in more intensively settled regions, in climatic zones that have higher and more reliable rainfall, and in wetter and
more fertile parts of rangeland landscapes. Although there is quantitative evidence of impacts on either particular
taxonomic groups or specific ecological processes in Australian rangelands, a comprehensive picture of responses
of rangeland ecosystems to plant invasions is not available. Research has been focused on invasive species that are
perceived to have important effects. This is likely to down play the significance of species that have visually less
dramatic influences and ignore the possibility that some species could invade and yet have negligible consequences.
It is conceivable that most of the overall impact will come from a relatively small proportion of invasive species.
Impacts have most commonly been assessed in terms of plant species richness or the abundance of certain groups
of vertebrates to the almost complete exclusion of other faunal groups. All scientific studies of the impacts of
invasive species in Australian rangelands have focused on the effects of individual invasive species although in
many situations native communities are under threat from a complex of interacting weed species. Invasion by nonnative species is generally associated with declines in native plant species richness, but faunal responses are more
complex and individual invasions may be associated with increase, decrease and no-change scenarios for different
faunal groups. Some invasive species may remain minor components of the vegetation that they invade while others
completely dominate one stratum or the vegetation overall.
Additional keywords: plant invasion, plant species richness, weeds.
Introduction
Plant invasion presents a serious threat to biodiversity
management and conservation in many parts of the world.
This is certainly the case in Australia where plant invasion
is ranked with habitat clearance, livestock grazing, forestry
and soil degradation as a major source of pressure on native
species and communities (SoEAC 1996). Around 2700 nonnative plant species have become naturalised in Australia
(Groves et al. 2003) out of the 27 000 non-native plant species
that have been introduced (Virtue et al. 2004). There are
frequent statements to the effect that these naturalized species
are a threat to biodiversity, often in reference to particular rare
and threatened species (Vidler 2004), but also in relation to
plant and animal communities or ecosystem processes.
Australian rangelands occupy around 6 million km2 which
equates to about 70% of the continent (Foran et al. 1990).
This means that their management is very important for the
overall conservation of Australia’s biodiversity. For example,
50% of Australia’s bioregions and 75% of its plant ecosystems
© The Rangeland Society
are represented in the rangelands (Smyth and James 2004).
For many faunal groups, including amphibians, birds and
mammals, species richness in the rangelands is generally
lower than in some more coastal parts of the continent
though there are some notable exceptions (SoEAC 1996).
In contrast, species richness of reptiles in the rangelands is
relatively high (SoEAC 1996). The Australian rangelands
support significant numbers of endemic species. Of the
15 biodiversity ‘hotspots’ announced by the Australian
Government in 2003, five are located in the rangelands,
though all are relatively close to the coast. These five
‘hotspots’ are the Einasleigh and Desert Uplands region in
north-east Queensland, the Brigalow region in south-east
Queensland, and the Carnarvon Basin, Hamersley/Pilbara
and North Kimberley regions in the northern half of Western
Australia (Department of Environment and Heritage 2003;
Martin et al. 2006).
Many non-native plant species occur in the Australian
rangelands with over 620 non-native plant species reported
10.1071/RJ06014
1036-9872/06/010027
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The Rangeland Journal
as naturalised (Martin et al. 2006). However, the number of
non-native plant species per one-degree grid cell is generally
lower in the rangelands than in non-rangeland areas in
the south-east and south-west of the continent (SoEAC
1996). Different regions of the rangelands have different
proportions of their modern floras made up of non-native
species as illustrated by a comparison of the ‘Flora of
Central Australia’ (Jessop 1981) and ‘Plants of Western New
South Wales’ (Cunningham et al. 1981). Whereas Central
Australia, covering about 2.3 million ha has 1872 flowering
plant species, of which 6% are non-native, Western NSW,
which covers 383 000 ha, has 1917 flowering plant species,
of which almost 21% are non-native (Grice 2000).
This paper focuses on the impacts of invasive, non-native
plant species on the biodiversity of Australian rangelands.
Clearly, those impacts are not a simple function of the number
of non-native species present. The relative abundance of
non-native species is extremely variable from place to place.
Some invasive species dominate the vegetation that they
invade while others contribute relatively little to total plant
biomass even though they are abundant. It is possible for
a species to be relatively uncommon, or contribute only a
small amount to total biomass and yet have a disproportional
influence on biodiversity or ecosystem function relative to
its abundance. The non-native species that are of greatest
concern are those that contribute a large amount to total plant
biomass or disproportionately influence ecosystem function.
Here I review the impact of invasive species on Australian
rangeland biodiversity by addressing the following questions.
(i) How important is the threat by invasive plant species to
the biodiversity of Australian rangelands?
(ii) What components of biodiversity are under threat?
(iii) What are the impacts of different kinds of invasive plant
species?
(iv) What are the mechanisms whereby invasive plant
species affect biodiversity?
(v) What are the main gaps in knowledge of the relationship
between invasive plant species and biodiversity?
How important is the threat from invasive
plant species?
There are many published statements about the effects
of invasive plant species on the structure and function
of Australian ecosystems, on the integrity of their native
plant and animal communities and on the distribution and
abundance of species. For example, Vidler (2004) listed
41 cases in which a native Australian species is reported to
be threatened by either a single or multiple weed species.
The list was derived by searching species recovery plans
and by consulting ecologists and other people familiar with
particular species. In most cases, there were few if any
quantitative data presented in evidence of the claims that
the native species was detrimentally affected by the weed
A. C. Grice
in question. However, Vidler’s synopsis indicates that expert
opinion recognises invasive plant species are a threat to
individual threatened species in Australia. At least nine
of the 41 cases listed involve species from rangelands.
In six cases, these relate to invasions by specific, nonnative species and in three cases, to annual, non-native
grasses collectively. The individual weeds mentioned in
the nine cases are Mimosa pigra L., Opuntia spp. Mill.,
Acacia nilotica (L.) Delile, Brachiaria mutica (Forssk.) Stapf
(= Urochola mutica (Forssk.) T.Q Nguyen) and Cenchrus
ciliaris L. (Vidler 2004).
The perceived impacts of weeds at an ecosystem level
are illustrated by the analysis of the conservation status of
Queensland’s 13 bioregions (Sattler and Williams 1999).
Ten of these bioregions are entirely or partially covered
by rangelands and non-native plants are listed among the
threats to the biodiversity of nine of them. Fifteen species
are individually identified as threats (Table 1).
The current and potential distributions of 33 of the 71 nonnative plant species that were investigated as potential weeds
of national significance extend into Australian rangelands
to some degree (Thorp and Lynch 2000). Fewer species
currently extend into the more arid rangelands compared with
the semi-arid or more northerly tropical rangelands.
Although expert opinion and general perception is that
non-native plant species present a major threat to Australian
biodiversity at species, community and ecosystem levels,
there are relatively few quantitative data to back these
views. This is illustrated by the nature of references to
‘environmental’ and ‘biodiversity’ impacts of individual
weeds discussed in ‘The Biology of Australian Weeds’
(Groves et al. 1995; Panetta et al. 1998). For 18 species
whose current Australian ranges extend into the rangelands,
there is a reference to detrimental environmental impacts,
though there are quantitative data for only a few of them
(Table 2). In most cases, statements about the environmental
or biodiversity impacts of particular weeds are based on more
or less casual observations of the weed’s abundance.
Grice et al. (2004) reviewed publications providing
quantitative data relating to the effects of weeds on the
biodiversity of Australia. They identified 24 journal papers
that provide quantitative data on the impacts of invasive plant
species in Australian ecosystems. Nine of these described
work undertaken in rangelands. Although the sample is
small, they generally demonstrate that at least some invasive
plant species do have significant impacts on biodiversity in
the rangelands.
What is threatened by invasive species?
From the few quantitative studies of the effects of invasive
plant species on Australian rangeland biodiversity it is
apparent that the impacts are currently or potentially
great. The extensive current or potential ranges of many
rangeland weeds means that most bioregions are prone to
Invasive plant species and biodiversity
The Rangeland Journal
29
Table 1. The principle non-native species that are listed among the threats to the biodiversity of the ten rangeland bioregions
of Queensland
North-west highlands (Mount Isa Inlier) (MII), Gulf Plains (GUP), Cape York Peninsula (CYP), Mitchell Grass Downs (MGD),
Channel Country (CHC), Mulga Lands (ML), Einasleigh Uplands (EU), Desert Uplands (DU), Brigalow Belt north and
south (BBN, BBS), New England Tableland (NET) (Sattler and Williams 1999)
Species
Acacia nilotica
Azadirachta indica
Bryophyllum tubiflorum Harv.
Calotropis procera (Aiton) W.T.Aiton
Cenchrus ciliaris
Cryptostegia grandiflora
Eichhornia crassipes (Mart.) Solms
Lantana camara
Non-native pasture spp.
Parkinsonia aculeata L.
Parthenium hysterophorus L.
Rubus fructicosus L.
Salvinia molesta D.S. Mitch.
Senna obtusifolia (L.) H.S.Irwin & Barneby
Themeda quadrivalvis (L.) Kuntze
Xanthium pungens Wallr.
MII
GUP
CYP
MGD
—
—
—
—
a
—
—
—
—
—
—
—
—
—
—
—
a
a
—
a
—
a
a
—
—
a
—
—
a
—
—
a
—
—
—
—
—
a
—
—
—
—
—
—
—
a
—
—
a
—
—
—
a
—
—
—
—
a
—
—
—
—
—
—
Bioregion
CHC ML
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
a
—
—
a
—
—
—
a
—
—
—
—
—
—
EU
DU
BSN, BSS
NET
—
—
—
—
—
a
—
a
—
—
a
—
—
—
a
—
—
—
—
—
a
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
a
—
—
—
a
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
a
—
—
—
—
Table 2. Non-native plant species found in Australian rangelands and for which environmental/biodiversity impacts are reported (Groves
et al. 1995; Adair and Groves 1998; Panetta et al. 1998)
The nature of the evidence (perception, observation, and quantitative data) of environmental impacts is described
Species
Evidence of environmental impacts
References
Acacia nilotica
Alternanthera phileroxoides
Perceived as significant
Quantified range expansion and increasing abundance;
reported to crowd out other plant species and disrupt
ecological processes
Reduces plant species richness
Increases fire intensity and frequency
Reduces plant species richness
Perceived to seriously damage native communities
Quantified effects on native vegetation
No reference to environmental impacts
Measured effects on aquatic microenvironment, plants
and animals (not Australian)
Noted to be present and abundant in areas of
conservation significance
Reported effects on water flow
Noted as spreading into native forests; affects rare
native species
Reported to reduce native plant densities
Quantified effects on biodiversity; converts aquatic
grassland/sedgelands to shrublands
Reported to dominate pastures
Reported to suppress growth of other plants
Perceived to cause water quality problems and so
affect other plants and animals
Altered trophic structure of avifauna and reduced
habitat availability for reptiles
Reported to invade pastures, creek banks, etc.
No reference to environmental impacts
Mackey (1998)
Julien and Bourne (1988), Julien (1995)
Cabomba caroliniana
Cenchrus ciliaris
Chrysanthemoides monilifera
Cryptostegia grandiflora
Cytisus scoparius
Echium plantagineum
Eichhornia crassipes
Emex australis
Hydrilla verticillata
Hypochaeris radicata
Lantana camara
Mimosa pigra
Parthenium hysterophorus
Rubus fruticosus
Salvinia molesta
Tamarix aphylla
Ulex europaeus
Xanthium spp.
Mackey and Swarbrick (1998)
Latz (1991)
Weiss (1984), Weiss and Noble (1984a, 1984b)
Humphries et al. (1991)
Smith (1994), Waterhouse (1988)
Piggin and Sheppard (1995)
References cited in Wright and Purcell (1995)
Keighery (1996)
Swarbrick et al. (1995)
Amor and Stevens (1976),
Gilfedder and Kirkpatrick (1993)
Swarbrick et al. (1998)
Braithwaite and Lonsdale (1987),
Braithwaite et al. (1989)
Chippendale and Panetta (1994)
Amor et al. (1998)
Room and Julien (1995)
Griffin et al. (1989)
Richardson and Hill (1998)
Hocking and Liddle (1995)
30
The Rangeland Journal
some degree of invasion by one or more plant species
(Table 1). Whether determined by casual observation or
quantitative study, the impacts most frequently identified are
on macrophytes and or vertebrates (Table 2). There are, for
instance, few studies of the effects of invasive plants on
soil flora and fauna or invertebrates in general. Few studies
have quantified how an invasive species influences specific
ecological processes.
Australia wide, and in the rangelands in particular,
some invasive species have a demonstrated capacity to
drastically alter plant species composition and the structure
of native vegetation. Many of the more prominent rangeland
weeds reach very high densities and biomass. For example,
Tomley (1998) refers to densities of the shrubby vine
Cryptostegia grandiflora R.Br. of 5000 plants/ha, the
tree Acacia nilotica reaches densities of >150 plants/ha
(Mackey 1998), and ‘typical’ densities of Mimosa pigra
are around 10 000 plants/ha (Lonsdale 1992). In these
situations other plant species are often largely excluded
(Grice and Campbell 2000). Changes to the composition and
structure of native vegetation doubtlessly has ramifications
for the animal communities that depend on them though
the available evidence suggests that faunal responses to
invasive plant species are more complex than the floristic
responses. This is illustrated by the information available
for responses by northern coastal wetlands to invasion
by Mimosa pigra (Braithwaite et al. 1989). This species
is associated with reduced biomass and species richness
of herbaceous vegetation and reduced densities of woody
species. However, amphibians showed little response, lizards
and many bird species declined and at least some small
mammals increased.
It is not clear how susceptibility to invasion by nonnative plant species varies across Australian rangelands.
Water and nutrient availabilities, which vary widely at
geographical and landscape scales, will influence the species
that can establish. Environments that are in some way more
extreme may be susceptible to fewer weed species. The lower
proportion of non-native species in arid central Australia
(Jessop 1981) compared with semi-arid western New South
Wales (Cunningham et al. 1981) may reflect this. However,
communities can be drastically altered by a single non-native
species so that the biodiversity of the communities found in
more extreme environments may be threatened if it is invaded
by a species well-adapted to that environment. The invasion
of the riparian zones of some central Australian river systems
by Tamarix aphylla (L.) H.Karst. is an example of this (Griffin
et al. 1989). Disturbance is also often seen as a prerequisite
to plant invasions (Fox 1991) but species will differ in how
they are affected by particular disturbance regimes and some
may inhibit invasions by certain species. Sociological and
historical factors can also be important determinants of the
likelihood of plant invasion as they influence which species
are introduced and where (Mack 1999). Current information
A. C. Grice
is insufficient to determine the relative importance of all these
factors in determining susceptibility of different rangeland
types to plant invasions.
A large proportion of Australia’s rangelands are arid or
semi-arid but they incorporate more mesic land units in the
form of riparian zones and various kinds of wetlands. These
parts of rangeland landscapes appear especially prone to
plant invasions. Prominent riparian invaders include Acacia
nilotica, Cryptostegia grandiflora, Jatropha gossypifolia L.,
Tamarix aphylla and Azadirachta indica A.Juss. Even species
that are not riparian or wetland specialists can be more
abundant in riparian or wetland habitats. For example,
in north-east Queensland, Cenchrus ciliaris occurs broadly
across savannas and woodland landscapes but is especially
prolific in the riparian zone. More mesic and more fertile
parts of rangelands are also likely to support distinctive
species and communities and to provide important refuges
in times of drought. This coincidence of important habitat
and susceptibility to weed invasion points to a need for
weed management resources to be directed at more mesic
and/or more fertile environments and landscape positions
(see Martin and van Klinken 2006).
What are the impacts of invasive species?
Grice et al. (2004) proposed that four generalisations emerge
from studies of the effects of invasive species on biodiversity
Australia wide. Firstly, the abundances of individual native,
threatened plant species are negatively correlated with the
abundance of the weed species that has invaded their habitat
(Gilfedder and Kirkpatrick 1993; Sorensen and Jusaitis
1995; Gentle and Duggin 1998). The second generalisation,
based on a very small number of studies, is that the
abundance of common components of invaded communities
is reduced (Weiss and Noble 1984a, 1984b; Waterhouse
1988), though Smith (1994) reported that shade tolerant
native species were more abundant at sites invaded by Cytisus
scoparius (L.) Link.
Thirdly, the abundance/presence of invasive exotic species
is consistently negatively correlated with that of native plant
species (Waterhouse 1988; Braithwaite et al. 1989; Griffin
et al. 1989; Smith 1994; Fensham et al. 1994; Franks 2002;
Houston and Duivenvoorden 2002; McArdle et al. 2004).
Two native species [Pittosporum undulatum Vent. and Acacia
sophorae (Labill.) R.Br.] that have been studied where
they are invading Australian habitats outside their native
ranges yielded similar results (Gleadow and Ashton 1981;
Mullett and Simmons 1995; Rose and Fairweather 1997;
Costello et al. 2000).
Finally, there is great variation among faunal groups
in their responses to invasive plants. For example,
bird communities are affected by invasive plant species
(Braithwaite et al. 1989; Griffin et al. 1989; French and
Zubovic 1997) but different guilds seem to respond in
different ways, total abundance may not change and
Invasive plant species and biodiversity
there is little evidence of consistent changes in numbers
of species present.
These generalisations are based on relatively few studies
and are not rangeland specific though a good proportion of the
studies deal with various types of rangelands. The biology of
the invasive species and the characteristics of the community
being invaded is no doubt, important in governing the strength
and direction of effects on the various faunal groups.
The many non-native plants found in Australia’s
rangelands include species of a wide variety of growth forms
(Grice 2000). Around 50% of species are annual forbs, 15%
are perennial forbs, 17% are annual grasses, and the rest are
perennial grasses, shrubs or trees (Grice 2000). Of the species
identified as having the greatest impacts on biodiversity,
however, 27% are shrubs and 26% are perennial grasses
(Martin et al. 2006). Whether native or alien, different growth
forms play different structural and functional roles in the
ecosystems that they occupy. The impact of a particular weed
species will be influenced by its growth form. The effects
of low-growing species, grasses, forbs and low shrubs, will
be most apparent in the understorey. However, they could
also influence the overstorey of woodland communities, for
example via recruitment processes.
It is perhaps easier for non-native trees and shrubs
to give an impression of having major impacts on the
communities they are invading by dominating the overstorey.
The perception of impact is especially strong when a woody
species such as Acacia nilotica invades naturally treeless
communities, in this case Mitchell grasslands (Mackey
1998). The situation is similar where Mimosa pigra invades
floodplain communities (Lonsdale et al. 1995). Although
trees and shrubs constitute only around 7% of the non-native
species of Australian rangelands, they represent almost 40%
of the rangeland species nominated as Weeds of National
Significance (Thorp and Lynch 2000).
Another prominent growth form group among the weeds
of Australian rangelands is the perennial grasses. Two
wetland species, Hymenachne amplexicaulis Nees and
Urochloa mutica, are serious threats to the biodiversity of
floodplain and riparian communities in the higher rainfall
parts of the northern rangelands. However, quite a number
of species are capable of occupying the drier parts of
various rangeland landscapes. These species include Nassella
trichotoma (Nees) Hack. ex Arechav. (Campbell and Vere
1998) and N. neesiana (Trinius & Ruprecht) Barkworth
(Iaconis 2004), various exotic Sporobolus spp. R.Br. (Bray
et al.1999), Andropogon gayanus Kunth (Rossiter et al. 2003,
2004) and Pennisetum polystachion (L.) Schult. (Setterfield
et al. 2004). Perhaps the most important invasive grass of
Australian rangelands is Cenchrus ciliaris (Franks 2002;
Butler and Fairfax 2003; Jackson 2005), which has a
capacity to thrive in arid and semi-arid areas. It should
be acknowledged that some introduced grass species may
play a role in maintaining or restoring some aspects of
The Rangeland Journal
31
landscape functionality in areas subject to the effects of
livestock grazing, and that landscape functionality is an
important correlate of biodiversity (Noble et al. 1997;
Ludwig et al. 2004). Currently, there are few options for
controlling invasive perennial grasses of rangelands and
development and implementation of such options should be
given high priority.
Vines also are important invaders of rangelands though
their impact is likely to be greater in more mesic environments
and landscape positions. The most notable example is
Cryptostegia grandiflora which is highly invasive in northern
Australian rangeland riparian zones (Tomley 1998).
In rangelands, some growth forms, for example perennial
grasses, are more likely than others to dominate the
vegetation, or at least the stratum that they invade, and so
have serious consequences for the flora and fauna of those
communities. These can be regarded as ‘transformer species’.
Rangeland transformer species include the perennial grasses
Cenchrus ciliaris and Andropogon gayanus, the riparian
vine Cryptostegia grandiflora, and the trees Acacia nilotica,
Tamarix aphylla and Prosopis spp. In contrast, the 70–80% of
non-native rangeland plants that are annuals probably have
more subtle impacts that are nevertheless important. Little
or no effort has been directed at documenting the effects of
invasive annuals on Australian rangeland ecosystems.
The mechanisms of impact
The abundance of a weed, as with any plant, is dependent in
the first instance on its capacity to sequester basic resources:
light, water, carbon dioxide, oxygen and nutrients. The more
resources obtained by members of one plant species, the
fewer are available for competing species. Weed species that
sequester a large proportion of the available resources will
dominate the vegetation that they invade and the severity of
a weed infestation is often subjectively judged in terms of
the degree to which the species dominates. This is probably
a reasonable measure of the impact that a species is likely
to have. However, most attempts to document the effects of
weeds on natural ecosystems depend upon presence-absence
comparisons (Grice 2004).
There are three main ways in which sequestration of
resources by weeds influences the ecosystems that they
invade. These are through (i) direct competition; (ii) effects
on availability of resources for consumers; (iii) modification
of ecological processes.
Inter-species competition in plants is potentially
significant in relation to invasive species from two
perspectives. One of these revolves around the possibility
that intact, native communities are more resistant to plant
invasion. This is consistent with the idea that disturbance
is a prerequisite to invasion because it reduces native
competitors (Fox 1991; Noble and Brown 1997). The other
is that plants are invasive because they have a competitive
advantage (Sheppard 2000). The general conceptual debate
32
The Rangeland Journal
about the relative importance of competition in unproductive
environments (Goldberg and Novoplansky 1997) remains
unresolved in relation to the vulnerability of different
Australian rangeland types to plant invasions.
There are many examples from the Australian rangelands
where a non-native species dominates the vegetation over
extensive areas. These include Mimosa pigra on floodplains
in the Top End of the Northern Territory, Acacia nilotica in
Mitchell grasslands of western Queensland, Cenchrus ciliaris
in central Australian woodlands and Cryptostegia grandiflora
in riparian vegetation of north Queensland. In these cases,
competition is most likely a key factor in the interaction
between the invading species and the native plant species
of the invaded community, and probably accounts for the
general decline of native plant species in the face of invasion.
The exact nature of any competition that occurs, whether
for light, water or nutrients, has not been researched for
Australian rangeland species, though Rossiter et al. (2004)
showed that Andropogon gayanus does reduce nutrient and
water resources available in the soil. The speculation is
that these reductions lead to resource shortages for native
species. Competition for water and nutrients, rather than light,
for example, is likely to be most important in Australian
rangelands where water is often limited and the soils are
often poor in nutrients such as nitrogen and phosphorus
(Johns et al. 1984).
Changes to the structure and species composition of
vegetation affect the availability of resources. The nature
of the resources that are available and the spatial and
temporal patterning of resource availability can all be altered
when an alien plant species invades. The changes in faunal
communities that have been correlated with invasions of
central Australian riparian zones by Tamarix aphylla (Griffin
et al. 1989), of savanna woodlands by Cenchrus ciliaris
(Franks 2002; Butler and Fairfax 2003), and of northern
floodplains by Mimosa pigra (Braithwaite and Lonsdale
1987; Braithwaite et al. 1989), provide what are probably the
most comprehensively studied examples of these resource
availability effects in Australia. The great variation in the
responses by different faunal taxa to invasion by alien plant
species is no doubt due in part to the fact that a particular
alien species may enhance habitat suitability for some
species or higher taxa but degrade it for others because of
their different resource requirements. For example, relatively
high densities of the rare marsupial Sminthopsis virginiae
were recorded on floodplain areas infested with M. pigra
although wetland birds such as magpie geese (Anseranas
semipalmata), brolga (Grus rubicundus) and green pygmy
geese (Nettapus pulchellus) are likely to decline as a result
of this invasion (Braithwaite et al. 1989). The resources in
question may be food, shelter or nest sites, though again there
has been little research quantifying the details of changes to
availability of resources in Australian rangelands as a result
of plant invasions.
A. C. Grice
Changes in resource availability for one species or higher
taxon will have repercussions through the food web though
there are no Australian examples where these flow-on effects
have been examined in detail. Some non-native plants
probably provide food or other resources for particular animal
species. Obvious examples include fleshy-fruited plants such
as Lantana camara L. (Stansbury and Vivian-Smith 2003)
and Ziziphus mauritiana Lam. (Grice 2002).
There has been some research on how certain ecological
processes are influenced by plant invasions. Fire is an
important ecological phenomenon in most Australian
ecosystems; management of fire regimes is important
for the conservation of biodiversity (Keith et al. 2002).
In Australian rangelands, fire has received attention in
relation to invasion by alien perennial grasses. It has been
postulated that one of the ways in which Cenchrus ciliaris
affects the biotic communities that it invades in central
Australia is by increasing fire frequency and intensity
(Griffin 1993). Quantitative data on these affects are
available for Andropogon gayanus in northern Australia.
It has been demonstrated that the presence of this species
increases intensity of fires relative to those based on
native perennial grasses because the alien species produces
higher fuel loads that cure later in the season (Rossiter
et al. 2003, 2004). Although it is accepted that there is a
general link between fire regimes and biodiversity (Keith
et al. 2002), the specific biodiversity consequences of the
demonstrated shifts in fire regime due to invasion have not
been elucidated.
There has been speculation about other impacts of invasive
plants on ecological processes including hydrological effects
as a result of infestations of aquatic weeds in waterways
(e.g. Bunn et al. 1998) and links between invasive plants and
soil erosion.
Conclusions
It is widely accepted that invasive plant species have
generally detrimental effects on the biodiversity of Australian
ecosystems including those of the rangelands. Most parts of
the Australian rangelands are at risk of invasion by one or
more species though the severity of current problems varies
greatly across the rangelands. In general, there are more
non-native plant species in more intensively settled regions,
in climatic zones that have higher and more reliable rainfall,
and in those parts of rangeland landscapes that receive run-on
or are relatively fertile.
For a number of invasive plant species of Australian
rangelands there is quantitative evidence of impacts on
either particular taxonomic groups or specific ecological
processes. However, the paucity of research means that a
comprehensive picture of responses of rangeland ecosystems
to plant invasions is not available. Although invasion by
non-native species is generally associated with declines in
native plant species richness, faunal responses are more
Invasive plant species and biodiversity
complex and individual invasions may be associated with
increase, decrease and no-change scenarios for different
faunal groups.
Species vary enormously in the impacts they have on
the systems that they invade. Some may remain minor
components of the vegetation while others can completely
dominate one stratum or the vegetation overall. In addition,
species can have effects out of proportion to their contribution
to overall biomass. For example, understorey species may
interfere with recruitment of community dominants.
Two areas of emphasis in research are apparent from the
literature. One is the tendency to focus research on species
that are perceived to have important effects. This emphasis
will yield information about some of the effects of weeds
perceived to be important, down play the significance of
species that have visually less dramatic influences and ignore
the possibility that some species could invade and yet have
negligible consequences. It is conceivable that most of the
overall impact will come from a relatively small proportion
of invasive species.
The other area of emphasis concerns the taxa and
ecosystem processes that are measured as indicators. Plant
species richness has been a popular measure and vertebrate
groups have received attention to the almost complete
exclusion of other faunal groups.
All scientific studies of the impacts of invasive species
in Australian rangelands have focused on the effects
of individual invasive species. In many situations native
communities are under threat from a complex of interacting
weed species (Grice et al. 2004). The importance for
ecosystems of these inter-weed interactions can only
be guessed.
Acknowledgments
I acknowledge useful discussion on the impacts of weeds with
participants in the workshop that provided the impetus for
this paper: Nora Brandli, Yvonne Buckley, Shane Campbell,
Steve Csurhes, Dane Panetta, Joe Vitell, Craig Walton,
Richard Carter, Mike Cole, Mic Julien, Rieks van Klinken,
Sandy Lloyd, Rachel McFadyen, John Morley, John Pitt,
Anita Smyth and Helen Spafford-Jacob. Two anonymous
referees also provided helpful comments.
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