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Abundance and habitat use of the Black Swan in the Lower Swan River Estuary

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 grazing interactions between the black swan (Cygnus atratus) and the seagrass Halophila ovalis in, the Lower Swan River estuary, Western Australia.

The study has:

Characterised the spatial and temporal variation in black swan abundance at 45 sites across four seasons (spring, summer, autumn and winter) and at different times of day investigated the changes in grazing pressure exerted by swans over a year; and experimentally examined the strategies seagrasses use to cope with grazing.

Some key findings:

  • There was variation in black swan density among seasons, highest in autumn (185 swans) and lowest in spring (53 swans);
  • The key characteristics of sites with high swan abundance included: vegetation on the river bank; seagrass cover; and a shallow sloping seabed;
  • Swan grazing removed most seagrass biomass in summer and least in winter. However, as a percentage of seagrass production there was no difference across the year;
  • Long-term grazing reduced productivity in winter but had no, or a slightly positive effect in summer productivity;
  • Flowering and seed production were positively associated with grazing.

Plant-grazer interactions are dynamic and complex. This study is revealing that the expression of traits that afford tolerance to grazing varies across a year. It is also showing that changes in the plants’ sexual reproduction may be strategy to cope with grazing.


Gary Choney
Dr Kathryn McMahon
Professor Paul Lavery

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