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

niche cannot be defined if it is infinitely malleable: if the niche is an autecological attribute of a species, the niche moves with
the species and there are never empty niches, if it is a synecological attribute, the invasive organism must modify the niches of
other organisms to sequester niche hyperspace, so again there is no preformed or definable vacant niche (Herbold and Moyle
1986). An invasive species must rearrange the community and sequester flows or resources previously used by other organisms,
and therefore must effect them negatively. In general all invasive species which have been investigated have some negative
effects (Herbold and Moyle 1986). The mechanisms by which this happens, what resources are diverted, etc., is the real question
of interest (Seabloom et al. 2003), and is dependent on particular circumstances.
Current consensus, at least in some schools, appears to be that niche theory is a sterile framework, lacking explanatory and
predictive power. However this non-mechanistic approach, seemingly based on the tenet that ‘diversity confers stability’ and the
false but “commonly accepted ecological truism” that richer communities are less invasible (Lonsdale 1999 p. 1533) continues
under a new guise, with ‘functional groups’ replacing niches as a focus of investigation. Functional diversity, “the number of
functional groups with different behaviours for a particular process”, rather than species diversity, supposedly determines “major
thresholds in ecosystem processes” and the properties of systems that control their invasibility (Prieur-Richard and Lavorel 2000
p. 5). At least, if the composition of functional groups is based on similarity of resource use, the processes and mechanisms that
could cause displacement or facilitiate invasion are more explicit and accessible in these functional group approches than in
simple diversity-resistance approaches.
Many mechanisms might be involved in competitive superiority and these are mostly addressed in this essay within the
frameworks of the main theories. Competive superiority could result from possession of a particular unique characteristic (“novel
weapons” below), general superior adaptation evolved in the native environment (superior ‘invasive potential’, see above),
release from natural enemies (above) or the ability to better exploit disturbance (‘resource enrichment and fluctuating resources’,
above). Dominance by the invader in some but not other areas, suggestive of competitive superiority, may actually be just a
priority effect (Seabloom et al. 2003): the exotic cannot outcompete an equilibrium population of competitors, but is the first to
invade after strong disturbance, and establishes dominance, leading to “multiple stable equilibria” (Seabloom et al. 2003 p.
13384).
‘Novel weapons’
Successful invaders may possess ‘novel weapons’ that enable them to kill, suppress or outcompete native species. Novel
weapons possessed by invasive plants may include the ability to produce chemicals that are toxic to native herbivores (see for
example McBarron 1976) or to native plants. More broadly, they may consist of “competitively unique traits” (Seabloom et al.
2003) or functional attributes, not possessed by the native species, that enable access to unexploited resources (Callaway and
Maron 2006).
Grasses have many adaptations that deter predation. These include anatomical structures such as narrow leaves, mechanical
structures such as leaf phytoliths, and chemical toxins. In many cases involving grass poisoning it is not the grass but it’s
parasites (usually fungi) that are the source of the toxin. If these adaptations are possessed by an exotic species but not the native
species, and are effective against predators in the invaded system, then the invader has a novel weapon that facilitates its
invasion.
Allelopathy
Chemicals produced by invasive plants while growing or decomposing may also have detrimental effects on other plants – the
plant possesses allelolpathic properties (Gill and Davidson 2000). The study of chemical interactions between organisms is still
in its infancy, as has been amply demonstrated by recent major advances in the study of tri-trophic interactions between
predators, their herbivores and host plants, and chemical communication between individuals within a plant population (Baldwin
et al. 2002, Reddy and Guerrero 2004). Plants release a complex range of complex organic compounds into both the soil and the
air that enable above- and below-ground communication between individual plants in a population and probably between
populations, and that influence a range of other plants and animals in the environment.
Simple allelopathy between plant species is known to be widespread, but in most cases the precise chemicals are unknown and
the effects have been determined using plant extracts or residues (Gill and Davidson 2000). Probably all plants are more or less
allelopathic (Gill and Davidson 2000), and possibly all plants also release chemicals that are beneficial to other plants.
Centaurea maculosa Lam. (Asteraceae) in North America is probably the best studied example of allelopathy (Ridenour and
Callaway 2001). C. maculosa root exudates reduce the plant size and root elongation rates of a native tussock grass of invaded
areas, Festuca idahoensis Elmer by half, and allelopathy accounts for the major proportion of the total interference of C.
maculosa with the grass. The allelopathic activity of the litter of Vulpia spp. (Poaceae) against crop and pasture plants has been
demonstrated in laboratory and glasshouse experiments in Australia (Gill and Davidson 2000). Shoot extracts of different
cultivars of wheat Triticum aestivum L. have been found to have startlingly different impacts on radicle elongation of another
grass Lolium rigidum Gaud., that is a major weed of wheat crops in Australia (Lemerle and Murphy 2000). The rapid spread of
the African grass Eragrostis plana Nees in southern Brazil is due in part to its allelopathic effects (Overbeck et al. 2007).
Similarly, aqueous leachates of seeds, roots and leaves of the African grass Brachiaria decumbens Stapf, invasive in Brazilian
cerrado, have been found to reduce germination of potentially competing plants (Barbosa et al. 2008). Allelopathic activity of
Sorghum halepense (L.) Pers. partly explains its success as an invader in the USA (Rout and Chrzanowski 2009). Invasive
European plants present in North America have greater allelopathic effects on North American native plants than European
natives, and on the continental scale chemical co-adaptation amongst members of native plant communities may possibly be
commonly disrupted by alien invaders (Callaway and Maron 2006).
Allelopathic effects of species in the Stipeae appear to have rarely been investigated. Ruprecht et al. (2008) found that leaf
leachate of Stipa pulcherrima C. Koch, a dominant species in abandoned continental European grasslands, reduced seed
germination, radicle elongation and delayed germination of co-occurring species.
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