Literature review: Impact of Chilean needle grass ... - Weeds Australia

negative impacts mentioned include limitation or prevention of recruitment of native taxa, alteration to fire regimes, hydrological
cycles, nutrient cycling and other processes, increased soil erosion, genetic pollution, alterations to structure and floristics of
native vegetation communities, competition, and niche modification.
Approaches to impact assessment
Assessments of the biodiversity impacts of weed are of four main types (Downey and Cherry 2005): 1. scientific studies of
individual weed species and the systems they have invaded; 2. reviews or meta-analyses of such studies; 3. reviews of threatened
species databases, and 4. detailed consultation with biodiversity stakeholders as part of a process of threat assessment and
abatement planning. The former two methods approach the problem primarily from the individual weed perspective, the latter
two more from the perspective of the impacted biodiversity.
In terms of scientific studies, knowledge is particularly poor about how the biodiversity impacts of weeds vary in space and time
(Grice 2004a). Assessment is complicated by communities and systems being in disequilibrium or being dependent in their
evolution or dynamics or historical factors, rather than having a single stable state or successional pathway (Woods 1997). In
general very little is known of the impact of invading species at different stages of community succession, or of the permanence
of the occupation, i.e. the potential for community recovery if the factors initially permitting invasion are mitigated (Woods
1997). Furthermore, the balance of negative and positive impacts can shift dramatically over time and across habitats (Groves
2004). Knowledge about how weeds alter fire regimes and other ecological processes, and scientific understanding of the
responses of a range of different taxa in the same area to a weed invasion is also very limited (Grice 2004a). Impacts on fauna
are more complex than those on vascular plants and are therefore more difficult to determine (Grice 2006). In addition, the
impact of measures to control the weed upon biodiversity is rarely known or investigated, although Coutts-Smith and Downey
(2006) found that 7% of species threatened by alien plants were at risk from inappropriate control measures.
A simple example representing a combination of the metanalysis/review approach to impact assessment is that of Carr et al.
(1992), who identified 166 taxa threatened by environmental weeds in Victoria, including many found in grasslands. A slightly
different approach identified the communities at risk: FFG SAC (1996) listed numerous communities threatened by weed
invasion in Victoria, including Northern Plains Grassland, Plains Grassland (South Gippsland) and Western Basalt Plains
Grassland.
Much information about specific impacts is available but has never been adequately compiled and mobilised. Coutts-Smith and
Downey (2006) demonstrated the great utility of a more comprehensive review of such existing threat information. The authors
found that weeds posed a threat to 45% of threatened species, populations and communities in New South Wales and were the
most important single threat after land clearing. However details of the specific biodiversity threatened by particular weeds,
including Weeds of National Significance, in Australia was found to be almost entirely lacking, and only a very small proportion
of the threat information obtained came from scientific studies.
Downey and Cherry (2005) demonstrated the utility of the consultation approach in assessing weed impact for the coastal dune
weed Chrysanthemoides monilifera subsp. rotundata (erroneously called subsp. monilifera by the authors). They found the
number of species threatened by it to be 25 times higher than previously suggested.
Types of impact
Weed impacts can be harmful or beneficial (Adair and Groves 1998, Williams and West 2000, Low 2003, Richardson and van
Wilgen 2004). Weeds can provide food, fodder, building materials, nectar, shade and numerous other benefits (Richardson and
van Wilgen 2004). Weeds can contribute to conservation of biodiversity, for example by protecting other plants from herbivores
and acting as refuges. Invasive plants may become food for native fauna, which ‘host-shift’ to feed on them, or already have
wide host preferences. The possibility of host range expansion is one of the most important hazards in classical biological control
of weeds (Hopper 2001): the deliberately introduced invader may prefer a non-target plant. Shapiro (2002) documented the case
of the city of Davis, California, where a diverse, highly valued urban butterfly fauna is largely dependent on naturalised and
cultivated alien plants, and where, in consequence, efforts to control the alien species conflict with biodiversity goals. Low
(2002) provided numerous Australian examples of native animals, including endangered species, benefitting from alien plant
invasions.
In evolutionary time, the interactions of invasive species with other species in the invaded community changes selection
pressures and ultimately results in evolutionary change, with new species arising (Cox 2004). Thus invasive species eventually
tend to “become integrated into the new biotic community in such a way that their initial impacts are softened. Integration occurs
through the processes of coevolution and counteradaptation” with the ecological adjustments tending to precede the evolutionary
(Cox 2004 pp. 246-247).
Food webs are one conceptual basis for comprehending the interactions of invasive species on the invaded community (Strong
and Pemberton 2002 p. 59). Those that develop around animals introduced for biological control “are simpler than in natural
communities” (Strong and Pemberton 2002 p. 57) and similar simplified systems may be expected around invasive plants.
Unfortunately there is a general lack of detailed information about the food webs of even the most abundant native plants in
Australia and the complexities of such interactions greatly complicate scientific assessment.
Further complications are usually provided by weed management activities, since the weeds most worthy of study for
biodiversity impact are generally those that are subject to control activities. Weed control activities themselves may impact
negatively on biodiversity. In native grasslands herbicidal control in particular can have detrimental effects on native flora and
lead to proliferation of non-target weeds (Lunt 1991, Slay 2002c, Brereton and Backhouse 2003). Such impacts are, by default,
attributable to the particular weed that is being targetted, but little quantitative information on off-target damage is available. The
effects of weed management activities on native invertebrates is unkown (Yen 1999).
Specific threats posed by weeds to biodiversity
Invasive plants potentially influence the structure, function and composition of ecosystems by impacting on growth, recruitment
and survival (Grice 2004a Vidler 2004). These impacts are “ovewhelmingly negative”, but positive impacts also occur (Groves
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