Seagrass Herbivory Levels Sustain Site-Fidelity in a Remnant Dugong Population

Herds of dugong, a largely tropical marine megaherbivore, are known to undertake long-dis-tance movements, sequentially overgrazing seagrass meadows in their path. Given theirdrastic declines in many regions, it is unclear whether at lower densities, their grazing isless intense, reducing their need to travel between meadows. We studied the effect of thefeeding behaviour of a small dugong population in the Andaman and Nicobar archipelago,India to understand how small isolated populations graze seagrasses. In the seven years ofour observation, all recorded dugongs travelled either solitarily or in pairs, and their use ofseagrasses was limited to 8 meadows, some of which were persistently grazed. Thesemeadows were relatively large, contiguous and dominated by short-lived seagrasses spe-cies. Dugongs consumed approximately 15% of meadow primary production, but there wasa large variation (3–40% of total meadow production) in consumption patterns betweenmeadows. The impact of herbivory was relatively high, with shoot densities c. 50% higherinside herbivore exclosures than in areas exposed to repeated grazing. Our results indicatethat dugongs in the study area repeatedly graze the same meadows probably because theproportion of primary production consumed reduces shoot density to levels that are stillabove values that can trigger meadow abandonment. This ability of seagrasses to cope per-haps explains the long-term site fidelity shown by individual dugongs in these meadows.The fact that seagrass meadows in the archipelago are able to support dugong foragingrequirements allows us to clearly identify locations where this remnant population persists,and where urgent management efforts can be directed.