The population ecology of Oryza meridionalis Ng on the South Alligator River floodplain, Kakadu National Park, monsoonal Australia

Abstract

The abundant and widespread emergent annual grass, Oryza meridionalis, is crucial to the floodplain food chain on wetlands in monsoonal Australia. The floodplains are a dynamic habitat, driven by the annual cycle of wetting and drying. They are also under increasing pressure from anthropogenic change. Although the annual wet season is a reliable occurrence, variability between years in the timing of onset, total amount and the pattern of rainfall as well as the duration of flooding, potentially present risks for aquatic biota. This thesis investigates the population ecology of this important plant species, in order to understand the strategies by which populations persist, despite the expected risks associated with the annual wetting and drying cycle.

Field monitoring and experiments showed that great plasticity allows O. meridionalis plants to survive and reproduce under a range of inundation regimes. Increased flooding was shown to significantly increase seedling mortality and reduce plant seed production. However, benefits from increased flooding were protection of seed from post-dispersal predators, and potential for prolonged seed production by stems recumbent on lingering floodwaters in years of greater rainfall. Thus, there was a trade-off between advantage and disadvantage for different stages of the life-cycle, in various inundation regimes. A simple linear stochastic population model confirmed that this trade-off could result in population persistence, even when factors such as flooding events, inundation level and post-dispersal seed loss were increased well above those levels recorded in the field. Modelling demonstrated that density-dependent population regulation underpins population stability and persistence for O. meridionalis.

This work focussed on the behaviour of mono-specific populations. Future studies could focus on the behaviour of O. meridionalis populations in multi-species settings.

Date of Award	Dec 1998
Original language	English
Supervisor	Naomi Rea (Supervisor)