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

Glyceria maxima (Hartman) Holmb and Brachiaria mutica (Forrskal) Stapf convert shallow water aquatic systems to wet
grassland (Panetta and Lane 1996). Weeds that convert one ecosystem to another, changing its major functional characteristics
have been termed transformers (see above) and are usually ‘canopy dominants’ (Panetta and Lane 1996).
Most invasive plants are not transformer species but are are functional analogues of native species in the invaded systems. Few
invasive grasses have the potential to change a grassland into any other ecosystem, but they frequently appear to simplify the
system by reducing species richness and becoming dominant. For instance Hyparrhenia hirta was found to dominate areas it
occupied, which also had reduced native plant species richness (McArdle et al. 2004).
Weed invasion “can affect invertebrates adversely by elimination of native plant species, habitat alteration and ... the spread of
exotic invertebrates” (Yen 1999 p. 63).
Physical structure and chemistry
Alterations to the biotic structure and composition of a community by an invasive plant affects the spatial and temporal patterns
of resource flow within that community (Grice 2006). Altered soil chemistry can result from allelopathy, pH changes or changes
in the availability of minerals, particularly major nutrients and salts (Woods 1997, Levine et al. 2003).
Legumes (Fabaceae, Caesalpineaceae, Mimosaceae) such as Lupinus arboreus Sims, may increase soil N levels (Adair and
Groves 1998) and have a direct effect on soil fertility. On young volcanic soils in Hawaii, the invasive tree Morella faya (Aiton)
Wilbur (Myricaceae) and its microbial symbionts increased soil N fixation to levels 90 times that of all native plants combined
and increased the rates of N mineralisation, which created a cascade effect through the system and altered its subsequent
development (Vitousek et al. 1987, Vitousek and Walker 1989). Bacterial symbionts in Sorghum halepense enable it to invade
N-poor soils, partly explain its ability to form dense near-monocultures that exclude other plants, and are largely responsible for
its ability to significantly alter soil biogeochemistry (Rout and Chrzanowski 2009).
Other plants can deposit salt on the surface, e.g. Mesembryanthemum crystallinum L. (Aizoaceae) (Woods 1997). In Argentine
grasslands Pinus spp. alter the pH and other properties of soils to the detriment of some native grasses (Amiotti et al. 2007).
Alterations to the soil chemistry in turn commonly result in alteration to nutrient cycles.
Many weeds were introduced to combat soil erosion. Chrysanthemoides monilifera was widely planted and became a major
weed because it is an efficient sand binder (Groves 2004). McIlroy et al. (1938) advocated the use of three now severely invasive
grasses Paspalum dilatatum, Pennisetum clandestinum Hochst. ex Chiov. and Cynodon dactylon L. for erosion control in
Victoria.
The many physical, chemical and biotic effects of the production and deposition of litter by invasive plants can impact markedly
on biodiversity. Presence of litter can reduce seedling establishment (Lenz et al. 2003). Litter alters the microclimate, surface
conditions and soil properties including temperature, water infiltration, retention and evaporation, and may produce chemical
leachates. These changes in turn can modify competitive interactions between organisms, alter rates of seed and seedling
predation, favour proliferation of fungal pathogens, etc. (Lenz et al. 2003). Avena litter at 400 g m -2 was found to significantly
reduce maximum soil temperature (by c. 3ºC in early November) but not minimum temperature or soil moisture (Lenz et al.
2003). Ens (2002a) found that dense litter mats produced by N.neesiana altered the species composition and activity levels of
invertebrate communities with flow-on effects through the system. Dense grass litter in grasslands generally results in reduced
plant biodiversity (Lenz et al. 2003). Buildup of perennial native grass litter in the higher productivity temperate south-eastern
Australian grasslands, paticularly T. triandra grasslands, has the same effect, so regular biomass reduction through fire, grazing
or other management is necessary to maintain vascular plant diversity (Stuwe and Parsons 1977, McIntyre 1993, Morgan 1997,
Henderson 1999, Lunt and Morgan 2002). For example Morgan (1995a 1995b) found that seedling establishment of Rutidosis
leptorrhynchoides in dense T. triandra grassland required large canopy gaps, mortality of young seedlings in smaller gaps being
due to shading and increased herbivory. In contrast, litter experiments in Dry Themeda grasslands in the ACT by Sharp (1997)
found that retention of the T. triandra litter resulted in higher native forb richness and cover than when litter was removed, with
the opposite effect for exotic forbs. The effects of grass accumulation on plant productivity differs seasonally and from site to
site and species to species (Lenz et al. 2003). Generally plants with smaller seeds are inhibited more by litter because the
germinants have inadequate energy reserves (Lenz et al. 2003). Annuals are thus more likely to be negatively impacted.
In pot experiments using soils from native grassland in South Australia Lenz et al. (2003) found that Avena arbata litter at
different densities had complex, time-dependent effects on the emergence and biomass of seedlings, that varied between taxa.
The establishment or growth of some exotic dicots was reduced by dense litter, but after 4 months the biomass of annual grasses
was positively affected , and total biomass fell markedly in the dense litter treatment, mainly due to suppression of Trifolium
spp.. Medium and high litter levels had little effect on the only native plants that emerged in sufficient numbers to assess,
Austrodanthonia spp., but after 4 months all had died due to fungal disease. Field experiments, in which all standing biomass
was removed and dry Avena litter added, demonstrated similar complex effects. In winter, c. 4 months after treatment, growth of
annual grasses was increased by dense litter and there were significant suppressive effects on Austrodanthonia spp., and some
exotic herbs, but no effect on Austrostipa eremophila (Reader) S.W.L. Jacobs and J. Everett. After c. 9 months, the biomass of
exotic annual grasses was slightly increased by high litter levels, there was no siginficant effect on native perennial grasses, but
the biomass of all other species combined (mostly exotic forbs) was significantly decreased.
Invasive plants can severely modify hydrological cycles (Vidler 2004). Tamarix in inland Australia lowers the water table, alters
stream flow and flooding regimes and ultimately salinity levels (Griffen et al. 1989). Andropogon virginicus L., develops a high
biomass of dead shoots that reduce evaporation rates from the soil and also passes through an inactive senescent phase during
which transpiration is reduced (Mueller Dombois 1973). In rainforest communities this results in excess water in the soil,
increased runoff and accelerated erosion.
Altered microclimates (Vidler 2004) are probably commonplace, particularly through increased shading.
Alterations to animal health, habitat, food chains and community trophic structure
Invasive plants frequently have impacts on consumers and decomposers, including their community composition, diversity and
behaviour (Levine et al. 2003). The impacts of invasive plants on fauna are more complex than on plants, and can be positive or
86