Large herbivores that graze on seagrass, like green sea turtles and dugongs are characterized by being highly elusive, widely dispersed, and globally vulnerable. This raises several challenges for their conservation, as well as the seagrass meadows on which they depend in the marine ecosystems. Given their elusive nature, it is often difficult to determine their principal feeding grounds, and how their foraging is distributed at these locations. This information is a critical conservation need, if we are to identify key locations for structural and spatial management of seagrass meadows important for dugong and turtle conservation. But also, because herbivores in seagrass meadows contribute significantly to their structure by influencing biomass, productivity, leaf nutritional quality, seed dispersal, diseases spread, species assemblage structure and the extent of the meadow.

Approaches that depend on direct sightings or remote animal tracking of these species are time-consuming, logistically and technically complex, expensive, and not entirely reliable. The use of secondary signs can be a powerful alternative, given that many large marine herbivores leave behind unmistakable foraging signs that can be used as evidence of their habitat use.
I explored spatial patterns of foraging by green sea turtles, dugongs, and fish in intertidal seagrass meadows of the Andaman Islands, India. These extensive meadows are typically dynamic and multispecific and are also differentially subjected to a host of anthropogenic pressures, from small-scale fishing and boat anchoring to urban pollution. I studied four seagrass meadows at different locations with contrasting anthropogenic pressures – the relatively isolated Pokkadera (near Mayabunder), the moderately impacted meadows of Dolphin and Kalapathar (both in Swaraj Dweep), and the highly polluted urban meadow of Haddo (in Port Blair). The objective of my study was to evaluate if patterns of use by fish, turtles, and dugongs differ between these meadows, and to determine the distribution of herbivory within each meadow. All three groups (fish herbivores, green sea turtles and dugongs) leave distinct signs of recent foraging on seagrasses. While fish herbivory is characterized by crescent-shaped bites on individual seagrass blades, turtles chew entire shoots down to the base. Dugongs in contrast uproot the seagrass from the roots, leaving behind characteristic feeding trails in meadows in which they have fed. I used these secondary signs to map foraging intensity by all three groups of herbivores in all four seagrass meadows. I first gridded the extent of each intertidal meadow using a grid with each point placed 50m apart, and visually sampled every grid point for signs of active herbivory. At each point, I also recorded the seagrass cover percentage, seagrass species present, seagrass species with foraging signs, substrate type, type of microhabitat and the distance from the shore. My results indicate that herbivory patterns by fish, turtles and dugongs vary considerably between meadows. Overall, the occurrence of herbivory was lowest in the seagrass meadow at Haddo which is located in a highly urbanised environment but was relatively abundant in all other meadows. In general, fish herbivory was evenly distributed across the four meadows but the amount of herbivory differed in each meadow. In contrast, both turtle and dugong herbivory were spatially explicit and strongly associated with the distribution of preferred seagrass species. Dugong herbivory was limited to two meadows and was primarily found in subtidal parts of these meadows, in areas dominated by Halodule sp. or Halophila sp. In contrast, turtle herbivory was more widespread across the meadows and associated with Thalassia sp. and Cymodocea sp. presence. The shallow intertidal meadows of the Andamans are often ignored as an ecosystem but they have shown to be highly diverse ecosystems which are key areas for the conservation of vulnerable marine species of large herbivores. They are also typically multi-use environments that contribute a range of services to coastal communities. The results indicate that, except at the most urbanised locations, large marine herbivores may be able to coexist with moderate levels of human use in seagrass meadows. Using secondary signs can be a valuable proxy of more direct observational studies in these ecosystems, and can help identify hotspots for dugong and turtle conservation.