Tropical Savannas CRC > Education & Training > PhD Projects: 1996-2001 > Genetic Diversity of Spear Grass

TS-CRC Student project - The impact of livestock grazing on the genetic diversity of the grass Heteropogon contortus in a Queensland tropical savanna

Spear grass

An area of speargrass near QDPI's Walkamin research station outside Atherton, north Queensland

After the PhD

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.


James Cook University, Townsville: Completed.

Grant Whiteman

Summary | Findings | Effect of human activity | After the PhD | More information |

Summary

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.

Findings

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.

Effect of human activity on seed dispersal

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 .

Contacts

Dr Grant Whiteman
Goldfields-Nullarbor Natural Resource Management Strategist
Centre for the Management of Arid Environments
Tel: 08 9088 6046

Fax: 08 9088 6034

Locked Bag 22
KALGOORLIE, WA 6433