Dugongs: Seagrass community specialists

Abstract

Dugongs exploit a relatively wide diet including seagrasses, macroinvertebrates and algae within intertidal and subtidal tropical and subtropical seagrass communities. The importance of seagrass genera to dugongs differs among locations and may change at the same location during times of seagrass loss. Dugongs feed by excavating or cropping, depending on seagrass morphology and the nature of the sediment. An individual dugong can disturb a considerable area of seagrass in a single day, especially in areas with low biomass. The local impact on seagrass biomass can be very significant and cause a loss of over 50% of production. The food quality of the seagrass forage eaten by dugongs is similar to the forage eaten by many wild, large, terrestrial, herbivorous mammals. Dugongs are less effective at masticating fibrous seagrasses than low-fibre seagrasses. This limitation may be most important under lengthy periods of food scarcity, such as at times of seagrass loss. The effect of dugongs feeding on seagrasses is complex and can be measured at several spatial scales including: (1) the individual feeding scar, (2) the area disturbed per day by an individual animal, and (3) the effect of a large group of animals on an individual plant community and using several responses variables: (1) microbial processes, (2) above- and below-ground plant biomass, (3) plant species composition, (4) plant nutrients, (5) invertebrate community composition and detritus, plus (6) the time taken by each of these variables to return to the pre-disturbed condition. Marked temporal fluctuations in dugong mortality and fecundity track major changes in the seagrass communities on which dugongs depend for food.