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

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