Date of Award

2012

Degree Type

Thesis

Degree Name

Master of Science

School

School of Natural Sciences

Faculty

Faculty of Computing, Health and Science

First Advisor

Dr. Kathryn McMahon

Second Advisor

Professor Paul Lavery

Abstract

Grazing is an important ecosystem process, influencing community structure and rates of ecosystem processes. Although grazing on seagrasses is generally considered to be minor in many temperate regions of the world, waterfowl are often considered significant grazers in temperate lagoons and estuaries. This study examined spatial and temporal variation in swan abundance, grazing pressure and the impact grazing has on seagrass. Little is known on how grazing rates vary on larger water bodies in the southern hemisphere at different times of year and whether temporal changes in grazing rates affect the ability of seagrasses to tolerate grazing. The plant response to grazing may not be consistent seasonally, in light of recent terrestrial studies suggesting changes in environmental factors over a year such as light, temperature and nutrient supply can influence the ability of plants to cope with grazing. Furthermore, studies have suggested changes in sexual reproduction can be considered a trait to cope with grazing. However, very few studies have investigated this relationship, particularly in seagrasses. With these three main knowledge gaps in mind, this study examined grazing interactions between the black swan (Cygnus atratus) and the seagrass Halophila ovalis in a temperate, estuarine seagrass ecosystem, the Lower Swan River estuary, Western Australia. Firstly, spatial and temporal variation in black swan abundance was documented across 45 sites in four seasons (spring, summer, autumn and winter) and at two times of day. Further investigations sought to determine whether there were changes in grazing pressure over a year. This was conducted at three high “swan use” sites in each season. Finally, the strategies seagrasses use to cope with grazing, and how these vary temporally were assessed using an observational approach across a natural grazing gradient and experimental manipulations (simulated grazing). There was significant variation in black swan density among seasons, with the highest number of swans present during autumn (185 swans), intermediate numbers in summer (104 swans) and winter (80 swans) and the lowest in spring (53 swans). Swans may move to ephemeral wetlands during times of low swan abundance on the estuary. An analysis of the temporal variation in swan abundance on the surrounding wetlands on the Swan Coastal Plain does not show a consistent pattern of seasonal variation on the wetlands. Movement of black swans to ephemeral wetlands is likely to be due to a variety of factors including water height, food availability and the breeding needs of the black swan.

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