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Electron micrograph of one of the thousands of species of mites.
Mites may eventually help land managers keep track of their soil
condition
Photo: David Scharf Photography
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Link between mites
and healthy soil
Mites and ticks belong to the class Arachnida
and the order Acarina or Acari. They are one of the most diverse
groups of living organisms comprising around 48,200 species. They
have exploited an incredible array of habitats and because of their
small size (some are truly microscopic) most go totally unnoticed.
Many live freely in the soil but there is also a vast array of
species that live as parasites on plants or animals.
The soil-dwelling mites contribute to plant
litter decomposition, soil formation and nutrient cycling. They
feed on fungi and nematodes and are extremely important organisms
in maintaining soil health and fertility.
A number of studies on the impact of sheep and
cattle grazing on soil mesofauna have shown that mite species
richness and densities decrease in soil under significant grazing
pressure (Battigelli et al., 2003; Kinnear & Tongway, 2004;
Smith and McSorley, 2002; Clapperton et al., 2002). Kinnear &
Tongways’ work is particularly relevant because they studied
soil mites in sheep grazing lands in Western Australia and the
impact of heavy grazing and poor pasture condition was accompanied
by substantial reductions in the abundance of mites and the
richness of species. — Karen Gibb
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New research may pave the way for cheaper and
quicker tests for soil condition and ecosystem health on savanna
grazing lands—using the tiny soil mite.
Link between mites and healthy
soil | Mites are mighty | Research questions | Counting the
mites vs fluorescent labels | Success with DNA
approach | Future directions | References |
In savanna grazing lands uneven grazing can lead to
deterioration in pasture cover and soil quality. While there are
above-ground monitoring and biodiversity studies under way, these
are indirect measures of soil condition. Because biology of soil is
such a critical component, one research team decided to focus on
trialling methods that would allow direct measurement of the
productive potential of soil.
During summer last year, a research team led by Associate
Professor Karen Gibb from Charles Darwin University, with support
from the Tropical Savannas CRC, conducted a six-month trial of
various tests using soil mites to show condition and health of the
soil. The trial took place at the Wambiana cattle station, west of
Charters Towers in north Queensland.
Mites are one of the most diverse groups of living organisms,
and soil dwelling mites contribute to plant litter decomposition,
soil formation and nutrient cycling. They feed on fungi and
nematodes and are extremely important organisms in maintaining soil
health and fertility.
Past research shows that soil mites can be good indicators of
soil health, as well as indicators of disturbance. They may also
have potential as early-warning indicators of alterations to soil
structure because they live in the macro-pores and root channels of
soil. In hot dry environments like northern Australia they are also
the dominant soil animals comprising 70 to 90% of the mesofauna
(medium-sized soil animals from 0.2 mm to 3 mm long).
Unfortunately, focusing on soil health in this way is often
avoided because it is too labour-intensive and requires high levels
of expertise.
But what if a straightforward and economic method could be found
to measure ecosystem health?
Karen and her research team decided to compare two monitoring
methods: conventional analysis that involves extracting mites from
the soil, identifying and counting them and a molecular approach
which analyses the DNA of mites and promises to be quicker and
easier to use.
The research team—Dr Peter O’Reagain from Queensland
Department of Primary Industries, Dr Jenny Beard from the
University of Queensland, Dr Kathy Ophel-Keller, South Australian
Research and Development Institute, as well as valuable input from
Dr John Ludwig from CSIRO Sustainable Ecosystems—had several
research questions they wanted to examine:
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Is there an association between vegetation cover and soil mite
assemblage?
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Are molecular tools a viable option in soil biology?
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To what extent do soil mites have a place in assessing soil
condition?
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To what extent does molecular analysis of soil communities
have a place in assessing soil condition?
“We have no landscape-scale soil monitoring tools to help
land managers decide whether changes are due to cattle vegetation
interactions or whether there is a more serious rehabilitation
issue,” explained Karen. “Most importantly, soil
monitoring may provide an early warning system.”
The team used 16 soil samples to compare two cover types from
grazed paddocks: perennial grass and litter and bare ground,
annuals and litter. Firstly, they analysed the mites in soil using
conventional methods, finding that there was a relationship between
cover type and the types of mites found in the soil. Next came the
DNA analysis to test if it could also find a relationship between
the mite assemblages and the type of vegetation cover. In this
technique (terminal restriction fragment length polymorphism,
T-RFLP) a gene from a mite is “amplified” using
fluorescent labels to differentiate it from the soil DNA. When the
data was analysed, the research team found that there were distinct
differences in the mite assemblages found in different vegetation
covers, just as in the conventional analysis.
The trial study confirmed that mites would be a useful indicator
of soil health: the team found that mite abundance and diversity
were greater in soils with perennial grass cover than in soils with
bare ground and annual grass cover. Secondly, it found that the
molecular T-RFLP approach was extremely effective in measuring mite
assemblages and eventually could be used instead of conventional
analysis.
“Until now T-RFLP has been used to monitor soil microbes
but we have shown that mesofauna species diversity and abundance
data can be obtained quickly and from as little as 10 grams of soil
per sample,” said Karen. “The implication is that once
validated, T–RFLP analysis will significantly reduce the time
and expertise required to monitor soil mesofauna and will change
the way we view our soil biology resources in soil
monitoring.”
The scene is now set for the possibility of a full-scale project
that would refine the use of the molecular approach to analysis.
Questions a larger project would look at are the use of molecular
approach to examine changes in mite abundance and community
structure under different levels of grazing; the extent to which it
can be used as an early warning indicator of change, as well as
assessing the benefits of soil conditioners.
"However, this approach also lends itself to measuring other
taxa and other disturbance systems," said Karen. “For
example, it could be used to assess the impact of mining and the
successional changes resulting from rehabilitation programs.
—Kate O’Donnell; adapted from an initial report
on the study by Karen Gibb.
Battigelli, J.P., McIntyre, G.S., Broersma, K. & Krzic, M.
2003, 'Impact of cattle grazing on prostigmatid mite densities in
grassland soils of southern interior British Columbia', Canadian
J of Soil Science , 83:533–5.
Clapperton, M.J., Kanashiro, D.A., Behan-Pelletier, M. 2002,
‘Changes in abundance and diversity of microarthropods
associated with Fescue Prairie grazing regimes’ ,
Pedobiologia 46:496–511.
Colloff, J., Halliday, R.B. 1998, Oribatid Mites: A Catalogue
of the Australian Genera and Species , Mono on Inv Tax Vol 6,
CSIRO Publishing, 223 pp.
