Abstract The resilience, or sustainability, of an environmental system depends on key factors remaining within critical thresholds. Current approaches to assessing the condition and trend of environmental systems rely on expert knowledge of system performance and subjective interpretation. Computer simulation models of natural resource systems offer a way to integrate system properties, and ecological theory and relationships, with long-term climate and rainfall information to simulate system performance within a consistent framework. Financial accounting methods, such as balance sheets and ratio analysis, have been developed to assess overall businesses viability and offer a potential tool for assessing the sustainability of natural systems, providing key accounting principles and assumptions can be reasonably met. This thesis explores the integration of accounting and ecological theory in a balance sheet framework for sustainability accounting using non-financial terms with a view to contributing to the sustainable management of natural resources within dynamic systems. A generic approach to constructing environmental balance sheets was developed and tested at a range of scales (field to catchment). Sensitivity analysis of the models was used to determine key factors and critical thresholds relating to system resilience. These values were then used to construct the balance sheets. The current ratio was then used to identify if the system was being managed sustainably. A current ratio (assets/liabilities) greater than 1.0, derived from the balance sheet, was shown to denote more resilient, and hence sustainable, systems. Case studies used were wheat cropping in the Maranoa area of Queensland, Australia, and the Bonogin Valley in the Gold Coast, Queensland Australia The same approach to constructing balance sheets worked across all scales from farm to catchment. . The approach was then used to develop a sustainability assessment of the Coomera catchment of the Gold Coast to consider how natural resource management and urbanisation is affecting catchment resilience. A series of models was used to develop the accounts: a grazing systems model – SGS; a cropping systems model - APSIM-sugar; and an existing catchment hydrology and water quality model - EMSS. The approach demonstrated that sugarcane cropping systems within the catchment were not likely to be sustainable without significant input of nitrogen, but that the grazing systems were. Furthermore, the overall catchment was likely to be sustainable (2002). This finding is consistent with an independent field-based assessment of the catchment conducted by the Healthy Waterways Partnership of South East Queensland. The urban development anticipated in the catchment by 2020, did not appear to have a significant affect as measured by long-term trends in flow frequency and water quality. The use of ratio analysis provided a dimensionless variable that related to the resilience of a parcel of land or catchment. These values were able to be spatially integrated, using an area weighted median, to provide an overall estimate of resilience of land use for a farm or a catchment. However, it was considered simpler to model the catchment of interest as a whole rather than to combine ratios from a series of catchment sub-models. The availability of appropriate comprehensive systems models may prove a limitation for application to all land uses especially native bushland systems. However, the approach developed in this thesis provides a robust and consistent framework for exploring system resilience and sustainability in a way that can augment existing approaches to natural resource assessments of condition and trend.

The Silver Gull is a small gull (265 - 450g), which exhibits sexual dimorphism, with males larger than females. It has a protracted laying period of about 8 months over the winter on Penguin Island in Western Australia. The Silver Gull was studied on Penguin Island from 2000 to 2002. Completed clutches were removed from breeding pairs to induce repeat laying in order to determine the effect of increased reproductive effort on maternal body condition, egg production ability, offspring sex ratio and chick rearing capacity. Increased egg production had no significant effect on maternal body condition as measured by condition index, derived from mass divided by a measure of skeletal size. The seasonal period, divided into thirty-day intervals, had a significant impact on female condition index, with a decline in condition toward the end of the breeding season. While male condition also appeared to decline at the end of the season, this pattern was not significant. The initiation of laying varied between the three years of the study. The earliest occurred in 2000, which also experienced earlier rainfall than the later two years. Egg size and mass decreased throughout the breeding season although the number of eggs in a clutch did not decline. The size and mass of the eggs was significantly affected by the laying history of the parents, although this effect was dependent on the year in which the eggs were produced. The minimum interval required by Silver Gulls to replace a lost clutch is about 14 days. This interval increased from the start of the breeding season, but then declined toward the end, as summer was approaching. Laying interval increased significantly as the number of clutches produced by the parents also increased, up to 4 clutches in total. As more clutches were produced past this point, the laying interval became shorter. The probability of a replacement clutch being produced after clutch removal, declined as the clutch number increased and as the season progressed. Individuals that laid clutches with a larger mean mass were more likely to lay a replacement clutch. Increasing reproductive output caused a decline in the proportion of clutches that were replaced after clutch removal. The proportion of clutches that were replaced also varied between the years with the highest rates of replacement seen in 2000 which was also the year that experienced the earliest start in laying. The size of the original clutch in terms of its mean mass and volume was related to the size of the replacement clutch, but this relationship varied according to the timing of laying. During 2000 and 2001 male offspring predominated in the first two clutches produced by Silver Gulls. Further clutches that were produced demonstrated a sex ratio skewed toward females, the smaller sex in this species. Offspring sex ratio was close to equality in 2002 with very little effect caused by increased egg production. There was no effect of year, season or the laying history of the parents on hatching success. Growth rate in chicks was influenced by the year in which the chick hatched, the period during the season in which the chicks hatched, its sex and the laying history of the parents. The relationship between chick growth and the laying history, however, was complex with no consistent pattern emerging in terms of the performance of chicks from each treatment group. While the chicks from control groups generally grew faster than the chicks from manipulated parents, those individuals that were laid or raised by manipulated parents that had laid at least three clutches in total also performed well. Using the two main measures of reproductive success in the current study, egg production and chick rearing, those birds that were induced to lay multiple replacement clutches, were able to maintain a high level of condition and reproductive success. It is proposed that in the Silver Gull, only those individuals with a high level of condition continue to lay replacement clutches. If the female is unable to produce well provisioned eggs with a high chance of success, the breeding attempt is abandoned. Despite no loss of condition detected in female Silver Gulls with increasing clutch number, there was a significant shift in the offspring sex ratio toward females, indicating that strategies were in place to cope with the increased reproductive effort incurred as a consequence of repeat laying. Protracted laying in this species allows replacement of lost clutches only after maternal condition has been regained after laying.

Thesis(M.S.)--Auburn University, 2006. / Abstract. Vita. Includes bibliographic references.

Thesis (M.S.)--University of Nevada, Reno, 2007. / "December, 2007." Includes bibliographical references. Online version available on the World Wide Web.

Thesis--Auburn University, 2008. / Abstract. Vita. Includes bibliographical references (p. 98-100).

Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references (leaves 147-152). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references (leaves 147-152). Also available on the Internet.

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 99-104).

Bachelor-Arbeit Univ. St. Gallen, 2007.

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Includes bibliographical references (p. 126-136).