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

elated plants have similar defences against herbivores (see references in Ricciardi and Ward 2006) and are likely to share traits
that confer resistance to attack, or have similar physiological adaptations that lessen stress and better enable the operation of their
defences (Parker et al. 2006b). Exotic predators which share little evolutionary history with either the native or exotic plants
show no feeding preference for native species (Parker and Hay 2005).
Most studies framed within biotic resistance and/or enemy release hypotheses have largely ignored the role of plant interactions
with soil microbes and the acceleration of knowledge of soil microecology. There is good evidence that invasive plants in North
America have escaped from the host-specific soil microbes of their homelands and also formed new relationships with nonspecific
microbial mutualists in the invaded territory (Callaway and Maron 2006).
Mack’s (1989) generalisations about the role of exotic livestock in destruction of native caespitose grasslands in western USA,
the South American pampas and Australia, their facilitation of invasion by exotic grasses with which they coevolved and the
history of native grassland degradation in Australia (Moore 1973 1993, Groves and Whalley 2002) mesh neatly with the biotic
resistance hypothesis. In a sense “exotic plants may thrive not by escaping their native enemies, but by following them” (Parker
et al. 1996a p. 1460).
N. neesiana may possess pre-adaptations that minimise predation by Australian native herbivores (suggested perhaps by the
general presence of native Stipeae in invaded areas in Australia), or the native ecosystems invaded now lack their natural
herbivore assemblage (e.g. kangaroos generally absent, extinct macropods and marsupial megafauna). The exotic herbivore
assemblage that has invaded native grassland (including an array of invertebrates as well as mammals) may differentially attack
the native plants (Parker et al. 2006a), or the ecosystem has been otherwise anthropogenically disturbed, or N. neesiana has other
attributes (fecundity, high growth rate etc.) that enable it to overcome the effects of native generalist herbivores (Parker and Hay
2005).
The biotic resistance hypothesis,as it relates to plants, is not challenged by evidence that landscape scale areas with high plant
species diversity also tend to have higher numbers of exotic plant species. Lonsdale (1999) compared exotic and native plant
species richness at 184 landscape-scale sites of wide variation in size and found that the number of exotic plant species, but not
their proportion of the flora, increased with native plant species richness. Similar analyses at large landscape scales show similar
trends, however the areas invaded are not plant communities - the ecological units that are expected to show biotic resistance -
and such trends are at best weak in small areas (Cox 2004).
The biotic resistance hypothesis has been further elaborated under the moniker of functional group theory: the idea that the
diversity of functional groups rather than species enables resistance (Prieur-Richard and Lavorel 2000). Absence of a particular
functional group (e.g. grass seed predators) eases constraints on an invasive plant. The presence of a larger number of functional
groups of plants suggests that a larger proportion of available resources are already efficiently captured, so there should be
greater resistance to invasion.
If there is reduced predator pressure in the new environment, a plant has a lesser requirement for defence and an evolutionary
trade-off may occur in which the ‘freed up’ resources are allocated to other purposes, inclduing reproduction. Trade-offs
resulting in growth increases and higher investment in reproduction are believed to be common in invasive plants (Cox 2004).
The rate at which invasive plants acquire new natural enemies is highly variable, but is generally rapid at first, the diversity of
generalist herbivores reaching a maximum in as little as 100 years, then slows, with host shifting by and evolution of specialist
herbivores gradually occurring over longer periods, up to 10,000 years (Cox 2004). Major factors influencing these rates are the
diversity of the native herbivore and pathogen pools and the extent of their adaptation to phylogenetically related native plants,
the phenological availabilityof the invasive plant to the potential native utilisers, and the innate defences of the plant (Cox 2004).
The abundance or total area occupied by the invasive plant also appears to be important, e.g. as found in a global study of the
arthropod pests of Saccharum officinale, which found that the number of pest species had a linear relationship to the area of
sugar cane under cultivation (Cox 2004 citing Strong et al. 1977).
It can therefore be hypothesised that N. neesiana is preferentially consumed by native generalist species compared with
dominant native grasses, and that exotic generalist plant predators will preferentially consume the dominant native grasses. If this
is the case, grasslands with diverse and abundant populations of exotic plant predators should be more susceptible to invasion
and more highly invaded by N. neesiana. Grasslands that have inherent biotic resistance should be occupied by native generalist
mammalian herbivores including kangaroos, and have a greater diversity of native generalist phytophagous invertebrates.
Grasslands without biotic resistance should lack large native mammal herbivores, have a depauperate guild of native generalist
grass-feeding invertebrates, and an increased complement of exotic grass-feeding mammals Manipulative field experiments
involving predator removal or paired feeding assays (e.g. Parker and Hay 2005) are needed to demonstrate such effects.
Resource-enrichment and fluctuating resources
A successful invasive plant may simply be a superior competitor for basic resources such as light, water and nutrients. Many
biological attributes that engender such superior ability have been identified, for example plants with C 3 and C 4 photosynthetic
pathways are superior convertors of sunlight to sugars in different environments, and any one plant may have higher fecundity or
a faster growth rate than another. But because resources are ‘locked up’ variably in space and time by the plants in the preexisting
community, disturbance that kills or inhibits them and frees up resources is generally required for a successful invasion,
or their must be extrinsic addition of resources at a rate faster than the native plants can use (Herbold and Moyle 1986, Hobbs
1991, Burke and Grime 1996, Cox 2004).
The tenet that disturbance is a prerequisite for invasion is implicity based on the notion that in undisturbed, successionally
mature vegetation, surplus resources are absent (Carr 1993) or minimised or unobtainable by the existing flora. The fluctuating
resources theory posits that a “plant community becomes more susceptible to invasion whenever there is an increase in the
amount of unused resources” (Davis et al. 2000). The community becomes more susceptible to invasion by a particular exotic
plant if the particular resource was previously limiting the growth or survival of that plant (Hobbs 1991). Continuity of the
invasion requires that gains the invader makes are not lost when resource supply contracts (Melbourne et al. 2007).
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