Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)


Natural Resources

Department name when degree awarded

Range Science

Committee Chair(s)

Brien E. Norton


Brien E. Norton


D. R. Anderson


M. D. Brennan


Rigorous statistical examination of the population dynamics of a number of long-lived perennial plant species permitted an interpretation of changes in those populations and the establishment of potentially important causal agents. Survival and recruitment of six species from the mulga-zone rangelands of Western Australia varied across three climatically different periods following the removal of domestic livestock.

Two species' populations (Eragrostis xerophila and Maireana glomerifolia) had their rates of increase reduced by kangaroo and euro grazing. Both survival and recruitment of Eragrostis xerophila were reduced by this grazing while Maireana glomerifolia suffered reduced recruitment during all periods. These negative effects were most pronounced during the post-drought period of more "normal" rainfall. Three other species populations responded positively to grazing. Recruitment and survival of Eremophila spectabilis, were affected by grazing, particularly on a poor condition site, while differences in recruitment were more important for Eremophila leucophylla. Frankenia pauciflora had increased recruitment and survival during the postdrought period. The sixth species (Ptilotus obovatus) suffered increased mortality and reduced recruitment during the post-drought period.

Relative rates of change in management related groups of 18 species, including the six examined in more detail, were presented as a viable addition to the analysis of trend in rangeland vegetation. Grazing by kangaroos significantly reduced the rate of recovery of destocked rangeland pastures.

Site and seasonal factors affected all species populations examined. Site induced differences affected survival of all six species examined, highlighting the difficulty of replicating large scale experiments in rangeland vegetation. Different seasonal sequences (very wet, very dry, and more "normal") produced different species population responses. Populations of most species increased during the very wet, and more "normal", post-drought period, but declined, or remained static, during the drought. Potentially competitive relationships were examined at one site where the complexity of juvenile plant survival responses to seasonal conditions, treatment and to neighbor-free-space suggest potential competition, but more importantly reflect a need for more detailed study.

A re-examination of Danthonia caespitosa population data collected by O.B. Williams in rangeland areas of New South Wales indicates the importance of grazing by merino sheep, seasonal conditions, and the cohort within which a plant is recruited. The extreme variability of the data highlights the need for experimental designs that take account of the frequency and importance of causal agent impact, and are related to the dynamism of the population.