Swan grazing on seagrass: abundance but not grazing pressure varies over an annual cycle in a temperate estuary

Herbivorous waterfowl such as black swans are significant grazers in temperate waterbodies; their abundance, distribution and grazing rates vary over seasonal cycles. The present study examined spatial and temporal variation in the abundance and grazing rates of black swans in the Lower Swan River estuary, Western Australia, using visual surveys over 1 year (2009), and potential drivers of this variation, food sources and disturbance factors were assessed. We predicted that swan abundance and grazing pressure would be greater in summer and autumn when seasonal wetlands dry and the abundance of food sources would positively influence their distribution, whereas the level of disturbance would have a negative effect. Plant-grazer interactions are dynamic and complex; the present study revealed new findings on the seasonality of this relationship, where swan abundance but not grazing pressure varied over an annual cycle. Maximum swan abundance occurred in autumn (185) with minima in spring (53) but the swan grazing pressure did not vary between seasons, ranging from 6% to 25% of seagrass production consumed. Swan abundance was a function of season and the cover of seagrass. Key hot spots for swan abundance were identified where management efforts could be targeted by minimising human disturbances and protecting seagrass.