Literature review: Impact of Chilean needle grass ... - Weeds Australia
Literature review: Impact of Chilean needle grass ... - Weeds Australia
Literature review: Impact of Chilean needle grass ... - Weeds Australia
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BIODIVERSITY<br />
“Biodiversity ... one <strong>of</strong> the best descriptors <strong>of</strong> ecosystem condition”<br />
(Aguiar 2005 p. 262)<br />
This section explores the concept <strong>of</strong> biodiversity and aspects <strong>of</strong> its assessment, discusses the range <strong>of</strong> impacts that invasive<br />
plants, and <strong>grass</strong>es in particular can have on biodiversity, and evaluates existing knowledge <strong>of</strong> the impact <strong>of</strong> N. neesiana on<br />
biodiversity in temperate <strong>Australia</strong>n <strong>grass</strong>lands.<br />
Definitions<br />
The United Nations Convention on Biodiversity concluded at Rio de Janeiro on 5 June 1992 defines biological diversity as “the<br />
variability among living organisms from all sources including, inter alia, ... ecosystems and the ecological complexes <strong>of</strong> which<br />
they are a part: this includes diversity within species, between species and <strong>of</strong> ecosystems.” Biodiversity has elsewhere been<br />
defined as “the number, variety and variability <strong>of</strong> living organisms at genetic, population, species, community and ecosystem<br />
levels” (Giles 1994). It is present at every heirarchical level within the purview <strong>of</strong> biology (Mayr 1982): molecular, genetic,<br />
chromosome, organelle, cellular, tissue, organ, organism, taxon, association, etc. At each level, diversity varies spatially and<br />
temporally, in historical origin, functional role and evolutionary significance (Mayr 1982). In broader terms, biodiversity<br />
encompasses not just the biological taxa, but the processes and functions in which the organisms participate (Saunders 2000). It<br />
therefore includes the range <strong>of</strong> interactions organisms have with one another and the physical environment, and the associations<br />
they form, including mutualisms, competitive relationships, guilds, functional groups, successional dynamics and patterns,<br />
trophic relationships and foodwebs (Woods 1997, Saunders 2000). Indeed, “there is hardly any biological process or<br />
phenomenon where diversity is not involved” (Mayr 1982 p. 133), so understanding the impact <strong>of</strong> an invasive species on<br />
biodiversity requires understanding this broader context. Since all individuals differ in their history and precise chemical<br />
makeup, and in sexually reproducing populations in many other ways, every individual <strong>of</strong> every population is a unique part <strong>of</strong><br />
biodiversity (Mayr 1982).<br />
Biodiversity enables ecosystem services, provides direct economic benefits and creates the distinctive milieu in which human<br />
cultures flourish (Saunders 2000, Mansergh et al. 2006b). The concept <strong>of</strong> ecosystems services provides a framework for the<br />
economic quantification <strong>of</strong> chemical and biological reserves and cycles in areas including soil stabilisation and fertility, water<br />
quality and quantity, biological production, etc. (Mansergh et al. 2006b). Biodiversity can also create ecosystem “dis-services”,<br />
including, in general, exotic invasive species (Mansergh et al. 2006b p. 300). However many processes alter ecosystem<br />
functioning, not just alterations to biodiversity, and its contribution to ecosystem services has not been adequately resolved<br />
(Aguiar 2005). Less diverse anthropogenic systems may in some circumstances provide similar levels <strong>of</strong> service to those<br />
provided by diverse natural ecosystems.<br />
Contracting parties to the Convention on Biodiversity, which include <strong>Australia</strong>, are required to identify components <strong>of</strong><br />
biodiversity important for conservation and sustainable use, monitor them through sampling and other techniques, identify<br />
processes and categories <strong>of</strong> activities that have or are likely to have significant adverse impacts on the conservation and<br />
sustainable use <strong>of</strong> biological diversity and monitor their effects. Parties are also required to prevent the introduction <strong>of</strong>, control or<br />
eradicate those alien species which threaten ecosystems, habitats or species (United Nations 1992).<br />
Quantification and indices<br />
Full quantification <strong>of</strong> biodiversity requires enumeration <strong>of</strong> diversity at each heirarchical level, both in terms <strong>of</strong> taxonomy (classes<br />
to subspecies) and levels <strong>of</strong> biological organisation (molecular to ecosystem). Understanding <strong>of</strong> the processes and mechanisms<br />
that alter or maintain biodiversity requires study and measurement <strong>of</strong> the interactions on and between each level (Giles 1994).<br />
Numerous authors caution against the over-reliance on species richness as an index <strong>of</strong> biodiversity (e.g. Aguiar 2005), but<br />
biodiversity assessment must start somewhere, so there has been long historical emphasis on species, and their intrinsic worth,<br />
novelty, or ‘uniqueness’. The dependence <strong>of</strong> eccological processes on biodiversity is a more recent concern, that has come to be<br />
included under the moniker <strong>of</strong> “sustainability”. Fuller capturing <strong>of</strong> these multidimensional attributes <strong>of</strong> biodiversity requires a<br />
series <strong>of</strong> indicators (Aguiar 2005). Biodiversity attributes for which such indicators exist or can be measured include composition<br />
(the identity and variety <strong>of</strong> the component elements at each heirarchial level from gene to landscape), structure (physical,<br />
chemical, biological and geographical organisation <strong>of</strong> these elements) and function (ecological and evolutionary processes that<br />
organise the systems) (Aguiar 2005). Consideration <strong>of</strong> the biodiversity attributes within such a framework enables a much fuller<br />
appreciation and understanding <strong>of</strong> system functioning.<br />
As a general rule, the number <strong>of</strong> species present increases as the total area under consideration is increased (Londsdale 1999).<br />
Assessment <strong>of</strong> biodiversity must therefore standardise for spatial scale. Exotic fraction, the ratio <strong>of</strong> exotic species to native<br />
species, has been widely used as an indicator <strong>of</strong> invasibility, but does not control for scale (Lonsdale 1999).<br />
The most basic measure <strong>of</strong> diversity is species richness, a simple count <strong>of</strong> the number <strong>of</strong> species present. Species richness indices<br />
can be compiled from species richness data. All more complex diversity measures rely on determining the number <strong>of</strong> individuals<br />
<strong>of</strong> each species. When population numbers are known the heterogeneity <strong>of</strong> the community can be determined, and a community<br />
with only two species that are equally abundant is supposedly more heterogeneous than when one species is more abundant than<br />
the other (Krebs 1985).<br />
Species abundance models <strong>of</strong>ten indicate that there are few common species and many rare ones, i.e. the relationship between<br />
the abundance <strong>of</strong> individuals <strong>of</strong> the species in a sample and the number <strong>of</strong> species in a sample is logarithmically related. This<br />
relationship results in the alpha diversity index, which gives an indication <strong>of</strong> diversity that is independent <strong>of</strong> sample size:<br />
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