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An area of speargrass near QDPI's Walkamin
research station outside Atherton, north Queensland
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My thesis 'The impact of pastoral practice on
the genetic diversity of the grass Heteropogon contortus in
a semi-arid Australian rangeland' was submitted in February
2002.
I escaped to New Zealand for a brief holiday
in a part of the world with a conspicuous lack of red dirt,
speargrass and hayfever.
Back in Townsville after the break I did some
part-time tutoring and lecturing at James Cook University and made
final thesis corrections in response to examiners’
reports.
In October 2002 I started work with CSIRO
Sustainable Ecosystems at the Centre for Arid Zone Research in
Alice Springs—more red dirt and hayfever, but not so much
speargrass.
My current work is quite varied, with an
emphasis on biodiversity issues (and proposal writing). Particular
areas of interest include the dispersal and impacts of buffel grass
in central Australia, the sustainability of wild harvesting for
domestic and commercial use and techniques for biodiversity
monitoring in rangelands.
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James Cook University, Townsville: Completed.
Grant Whiteman
Summary | Findings | Effect of human activity | After the PhD | More information
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A study of the ecological genetics of Heteropogon
contortus —completed in 2001—was conducted in the
semi-arid rangelands of north-eastern Australia. H.
contortus , speargrass, is a perennial, tussock grass of the
tribe Andropogoneae with a global tropical and sub-tropical
distribution.
It reproduces through aposporous apomixis—producing seed
which are genetically identical with the maternal plant. Previous
embryological studies have indicated that this mode of reproduction
is obligate, and that consequently the species never reproduces
sexually.
The major land use in the study area is extensive cattle grazing
and spear grass is the dominant forage species. Climate is strongly
seasonal, with a dry, warm winter and a hot summer with variable
but potentially high rainfall. The local prominence of speargrass
is closely tied to land use, especially the level of disturbance
caused by domestic livestock.
It exists as a minor vegetation component in undisturbed areas,
dominates at moderate disturbance levels and is displaced under
high disturbance. It was considered that the combination of clonal
reproduction and large fluctuations in population size under
livestock grazing could lead to a loss of genetic diversity,
leaving the species and the pastoral industry which it supports
vulnerable to future environmental change.
Comparisons of the AFLP phenotypes of sibling seedlings grown
from seeds which were collected throughout the study area revealed
that approximately 15 per cent of the seedlings examined were of
sexual rather than apomictic origin.
Grant hypothesized that differing lengths of days affected the
floral initiation and breeding system and that, under typical
climatic conditions, interactions between the two effects suppress
sexual reproduction in H. contortus . He also proposed that
this interaction between daylength effects may be a common feature
within the tribe, and has ecological and evolutionary
implications.
Genetic studies using DNA molecular markers (RAPDs and AFLPs)
were conducted in parallel with field and glasshouse studies to
assess genetic structure and to relate that structure to factors
generally considered to be important in rangeland vegetation
dynamics. Genetic analyses were conducted across nested spatial
scales ranging from 0.2m 2 arrays of seedlings to
populations separated by hundreds of kilometers. Substantial
genotypic diversity was found at all spatial scales and under
different usage histories. Evidence of short distance dispersal,
between grass patches within paddocks and between paddocks on
individual properties, and long-distance dispersal over distances
in excess of 200km was found.
This study indicates that the genetic structure of H.
contortus in north Queensland is strongly influenced by seed
dispersal, by the occurrence of suitable climatic conditions for
sexual reproduction and seedling establishment and by the
availability of ‘regeneration niches’ generated through
disturbance, rather than by selection for genotypes to suit local
conditions or particular disturbance regimes per se.
It is argued that propagule dispersal is promoted by human
activity, and that local genetic diversity at most spatial scales
has probably increased since European settlement of the rangelands.
It is concluded that pastoralism in Queensland’s rangelands
has not reduced the genetic diversity or long term viability of
H. contortus populations .
