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

S = α log e (1+N/α)
where S = the number of species in the sample and N = the number of individuals.
However the logarithmic model is applicable only to a limited set of communities with few species. A larger number of
communities are better represented by a log normal distribution, which is the generally expected statistical distribution (Krebs
1985). Determining the total number of species present in a community takes a large amount of sampling effort. But if such a
distribution is assumed, it is possible to predict the total number of species present without having to sample intensively to detect
the rare species.
The Shannon-Wiener index is based on information theory and incorporates the concept of evenness in the population size of the
species. The more even the numbers of the species the higher the diversity. Other indices include Simpson’s index and
Margalef’s index (Krebs 1985).
The usefulness of various indices is debatable (Adair and Groves 1998). They often rely on assumptions that remain untested and
provide few benefits in terms of enabling ready interpretation of comparitive data. In practice the basic information on the
number of species in a sample and abundance of each summarises most of the diversity information (Krebs 1985). Indices
assume fixed relationships between numbers of individuals and numbers of species, but the number of insect individuals is so
temporally and spatially variable that the indices may only become meaningful after prohibitively extended sampling efforts
(Farrow 1999). The same arguments apply to plant sampling, where the number of individuals may be prohibitively large when
the species is of small size, or the determination of what constitutes a single individual may often be difficult. Farrow (1999)
therefore argued that simply enumerating the species present by extending sampling over a longer period was a superior
approach for grassland invertebrate biodiversity assessment because of the major effort involved in counting what may often be
superabundant organisms, and that simple presence/absence assessments are appropriate in some circumstances.
Diversity indices also tend to perpetuate the emphasis on species richness, which they all require for their computation, and
divert attention from the structural and functional attributes of biodiversity that may be more important and valuable, and from
the compositional attributes at other heirarchical levels – genetic, association, community, etc.
Diversity studies also require that important and unimportant species be identified, that exotic species are distinguished from
natives (Greenslade 1994), pest from beneficials, etc., i.e. the identification and appropriate weighting of desirable and
undesirable elements (Driscoll 1994). Proper contextualisation also requires precise identification of all taxa.
Assessments of overall biodiversity of a diverse ecosystem or community is always difficult, and beset with temporal and spatial
scaling problems. Simple species richness assessments based on very few higher taxa (e.g. mammals or vascular plants) are of
little value because no high level taxon appears to adequately indicate biodiversity in any other high level taxon (Melbourne
1993). Furthermore, the biodiveristy significance of a particular area can only be adequately assessed in the context of other
similar areas and overall regional biodiversity (Melbourne 1993).
Impacts of weeds on biodiversity
General considerations
An impact is “a disruption to a particular set of ecosystem services or functions” (Mathison 2004), or more simply, any change in
the diversity or abundance of one organism that is caused by another. Impacts of invasive species may be predicted by introvert
and extrovert measures (Williamson 2001). Introvert assessments are made from study of the invasive organism, its range,
abundance and ecology, to predict likely effects. Gardener and Sindel (1998) predicted impact on Button Wrinklewort Rutidosis
leptorrynchoides F. Muell. and Kirkpatrick et al. (1995) on Sunshine Diuris Diuris fragrantissima D.L. Jones and M.A.
Clements using this approach (Ens 2002a). Extrovert measures involve direct quantification of impact on affected organisms,
processes or communities.
In the trivial sense, any invasive species initially increases the biodiversity of the area it invades. Many invasive plants “integrate
smoothly” (Woods 1997) into the invaded ecosystem and are recognised as having minimal impact (Kirkpatrick et al. 1995,
Woods 1997, Grice 2006). Most Australian temperate grasslands have large inventories of alien vascular plant species and all
areas have at least some exotics (Kirkpatrick et al. 1995). Invasion by multiple weed species, together or sucessively, is usual,
particularly in southern Australia (Adair 1995). A relatively small number of native plant species have largely disappeared, but
on the broad scale the overall vascular plant species diversity is much higher than before European occupation. ’Xenodiversity’
is the richness of a community in exotic species and of new communities dominated by, or assembled from alien species (Cox
2004). Xenodiversity of plants in general increases total species biodiversity, and on a world basis, the rate of new aliens
entering communities much exceeds the rate of extinction of native species. A central problem is that similar sets of alien species
are entering all the world’s biogeographical regions, so the world flora is being homogenised (Cox 2004). Invasions are “blurring
the regional distinctiveness of Earth’s biota” (Vitousek et al. 1997 p. 6) and grasslands everywhere are being invaded by similar
sets of species.
The minimum impact of an exotic plant that integrates smoothly into a native community might possibly be very small:
conceivably it might use resources that would not otherwise be used by the native plants and space that they would not occupy.
In general however resource must be used and displacement is usual. Larger impacts involve displacement of more species over
wider areas. Major impact may involve preemption of the niche of a community dominant. The highest levels of impact involve
alteration to the community properties – the invader is a so-called ‘transformer species’ (Henderson 2001). Typically these
dominate by forming a high proportion of the biomass in the community or stratum or have disproportionate influences on
ecosystem function (Grice 2006). In order to measure the impact of an invasive species on biodiversity it is necessary to examine
the effects on native biota and ecosystem functioning, determine any threshold below which impact is minimal and determine the
management factors that influence the degree of impact (Adair and Groves 1998). The interactions between the invader and the
invaded system are complex, and of many types, and are often indirect (Groves 2002).
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