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

According to Woods (1997 p. 61) “there have been few cases where competition from invaders has been shown unambiguously
to be responsible for significant alteration of communities. Most of the extensive literature suggesting such effects is based on
correlative studies, historical records, or anecdotes”. Byers et al. (2002) considered that much of the research purportedly
demonstrating detrimental impacts fails to clearly demonstrate that the invasive organism is the cause of supposed effects. The
presence of weeds where native plants once grew may be due to their ability to invade without disturbance, or be a consequence
of damage to the native species by disturbance. Correlations between weed density and reduction in cover and abundance of a
native plant implies a direct negative interaction. However the affected species could be reacting in an opposite way to some
independent environmental factor such as an altered disturbance regime resulting from human activity. It is generally difficult to
determine if the invading species or the altered conditions are the cause of such changes (Weiss and Noble 1984, Huenneke et al.
1990, Woods 1997). If anthopogenic disturbance is the cause, management should address the disturbance, rather than the weed.
Despite such difficulties there is wide consensus that “introduced alien species are the most rapidly growing cause of extinction
and extirpation of endemic, native species” worldwide (Cox 2004 p. 220) and currently “a significant cause of global
biodiversity decline” (Downey and Coutts-Smith 2006 p. 803). Environmental weeds apparently cause fragmentation of habitat,
disintegration of plant communities and extinctions, but the details of how this occurs and what is impacted have been scanty
(Adair 1995) and little quantitative information of effects on native species and ecosystem functioning has been published (Byers
et al. 2002). The severity of impact generally increases with the extent of weed cover (Carter et al. 2003).
In Australia, even the simplest data on the proportion of the landscape or habitat invaded and the relationship of weed density to
impact has been lacking for most weeds (Adair 1995). “Remarkably few” studies have attempted to quantify the impact of weeds
on biodiversity, in part because the effects are viewed as “obvious” (Adair and Groves 1998 p. 3). Groves (2002 p. 18)
considered it “surprising” that the impacts of some of Australia’s worst invasive plants on species richness was “still unknown”
and noted only “few well-documented” studies of biodiversity impact. Most published statements about impact “are based on
more or less casual observations” (Grice 2006 p. 28) and this reliance on anecdotal and subjective information is a worldwide
problem (Byers et al. 2002).
Overall in Australia the mechanisms by which weeds impact on ecosystem structure and function - “how” weeds affect
biodiversity - have not been widely quantified (Grice 2004a, Grice et al. 2004), although a few studies have examined land use
and environmental factors associated with invasion (Adair and Groves 1998). There is little data specifically dealing with the
effects of weeds on the biodiversity of Australian rangelands (Grice 2004a 2006). Most studies have focused on vascular plants
and some on vertebrates (Grice 2006) but there are much fewer studies of the effects on animals than on plants (Grice 2004a
2006) and very few on invertebrates and soil biota (Grice 2006). However most fauna studies indicate marked reductions in
diversity and abundance of vertebrates and invertebrates (Adair and Groves 1998).
In general, information has not been readily available on the species and communities acually impacted in Australia, nor has
there been an adequate compilation of the weeds causing the impacts (Downey and Cherry 2005, Downey and Coutts-Smith
2006, Downey 2008). As with the worldwide information ( Byers et al. 2002), causal relationships between invasion and impact
have “generally [been] implied but not demonstrated” (Grice 2004a p. 54). For example, Chejara et al. (2006) claimed that
Hyparrhenia hirta “greatly reduced the species richness of native flora”, although they only compared sites where the grass was
present or absent, and undertook no manipulative experimentation. McArdle et al. (2004 p.50), studied the same species using
“matched plots” with “similar ... apparent disturbance history”, and reached a similar conclusion: “demonstrated impacts on plant
diversity” (p. 54), despite little attempt to determine the mechanisms that enabled the invasion or its history at invaded study site.
Impact on threatened species and communities
Leigh and Briggs (1992) compiled data on the proportion of threatened plants in Australia that were threatened by “weed
competition” and found that alien plants had been dentified as a threat to 69 species. Adair and Groves (1998) found that weeds
had been cited as a major cause of extinction of four plant species and an endangering process for 57 nationally endangered plant
species. They also found that 23 entities listed under the Victorian Flora and Fauna Guarantee Act 1998 were at risk from 22
exotic weed species. Groves and Willis (1999) found that environmental weeds have been implicated in the extinction of four
native species.
In terms of detailed data on the impact of weeds on threatened species or communities in Australia 19 of the 20 studies of
environmental weed impact on plant communities in Australia examined by Adair and Groves (1998 p.7) “demonstrated a
decline in either species richness, canopy cover or frequency of native species”. A brief review by Vidler (2004 p. 652) found a
general absence of quantitative information, but concluded that weeds are “a major threat to at least 41 threatened plant and
animal species”. A much more thorough review (Coutts-Smith and Downey, 2006, updated by Downey and Coutts-Smith 2006)
found negative impacts on 283 plant species (including algae and fungi), 63 animal species, 15 threatened populations and 71
endangered ecological communities (90% of all officially recognised endangered communities) in NSW alone. Weeds threatened
45% (or 44% according to Downey 2008) of the 970 entities listed under the NSW Threatened Species Conservation Act 1995.
127 weed species were on record as biodiversity threats. Unspecified weed species accounted for 51% of the threats and 43% of
threats comprised multiple weed species. Of the major animal groupings, invertebrates had the highest proportion of listed
‘threatened entitites’ at risk from weed invasions. Competition (as opposed to habitat degradation by weeds and weed control
activity) was determined to be the main threatening factor, and accounted for 81% of the threats. The number of native species at
risk from alien plants in one State alone was found to be an order of magnitude larger than previous estimates for the whole of
Australia (Downey 2008). Furthermore, these assessments only considered threats to species formally listed by the State, so
considerably underestimated the real threat (Downey 2008).
Downey(2008) applied similar methodology as used in NSW by Coutts-Smith and Downey (2006) and by Downey and Coutts-
Smith (2006) to the biodiversity listed under the national Environment Protection and Biodiversity Conservation Act 1999, and
found that alien plant invasions were a threat to 291 theatened species. Specific weed species were identified for only 33% of the
threats, and these totalled 57 species.
Invasion of native vegetation by environmental weeds is recognised as a threatening process under the Victorian Flora and
Fauna Guarantee Act 1988 (Department of Sustainability and Environment 2009b) but no action plan has been developed. The
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