High rates of herbivory in remote northwest Australian seagrass meadows by rabbitfish and green turtles

Abstract

Herbivory is a key ecological process that often determines the composition and abundance of plants. Estimates of herbivory in seagrass meadows are typically lower than those in other vegetated coastal ecosystems, but herbivory can be intense when large herbivorous vertebrates are abundant. We surveyed rates of herbivory on 2 species of tropical seagrasses (Thalassia hemprichii and Enhalus acoroides), the abundance of herbivorous vertebrates, and the diet of 2 abundant herbivorous vertebrates (the green turtle Chelonia mydas and the rabbitfish Siganus lineatus) in lagoons adjacent to remote islands off northwestern Australia. Rates of herbivory in some deployments of tethered seagrass were more than 1000 times higher than rates of production and were among the highest recorded. Consumption exceeded production in half the deployments (9 of 18). Remote underwater video revealed that S. lineatus was the most abundant herbivore. Stomachs of S. lineatus contained mostly seagrass, and models based on stable isotopes indicated that seagrass was the primary source of nutrition. Stomach contents of C. mydas were more variable, containing seagrass and macroalgae (although the sample size was low), but models based on stable isotopes indicated that seagrass was likely the primary source of nutrition. Multiple lines of evidence suggest that the high rates of herbivory on the seagrasses T. hemprichii and E. acoroides are mainly due to direct consumption by the abundant S. lineatus, and perhaps also C. mydas. Seagrass is the primary contributor to the nutrition of both species.

Coastal dolphins and marine megafauna in Exmouth Gulf, Western Australia: informing conservation management actions in an area under increasing human pressure

“…Dugongs and turtles were commonly sighted in similar areas over shallow reef, seagrass and algal patches (such as at Neale's Cove and north of Qualing Pool), which is likely to be due to shared seagrass food resources (e.g. for green turtles and dugongs; Preen 1995;Vanderklift et al 2021). Dugongs, listed as vulnerable (Marsh and Sobtzick 2019), were mainly sighted as singles or mothercalf pairs.…”

Ontogenetic changes in green turtle (Chelonia mydas) diet and home range in a tropical lagoon

“…However, there are also multiple examples of populations in which large green turtles consume other diets (e.g., see review by Jones and Seminoff, 2013), so the ecological context appears to play an important role in diet. Patterns in consumption of macroalgae are complex, with reports of greater consumption by small individuals (Howell and Shaver, 2021), by large individuals (Veĺez-Rubio et al, 2016), or even selectively by only some individuals in a population (Burkholder et al, 2011;Vanderklift et al, 2021). Jellyfish appear to be important to the diet of green turtles, and the proportion of jellyfish in the diet appears to increase with size.…”

“…It is perhaps less obvious that the food sources in the lagoon are more nutritious. Seagrass is consumed by green turtles throughout much of their range (Esteban et al, 2020), but macroalgae is also consumed even in places where seagrass is abundant (e.g., Burkholder et al, 2011;Vanderklift et al, 2021). Green turtles appear able to extract nutrition from seagrass and macroalgae through hindgut fermentation in which structural carbohydrates (such as cellulose) are broken down by microbes into compounds that can be digested (Bjorndal, 1979;Bjorndal et al, 1991).…”