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

Ma Vanderklift,R Garthwin,L De Wever,Ga Kendrick,K George,A Zavala-Perez,C Sampi,K Dougal,Z Edgar,T Sampi,P Mccarthy,D George,A Vergés,M Hutton,Rd Pillans,A Howard,D Oades

doi:10.3354/meps13657

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.

Highlights

Herbivory is a key ecological process that often determines the composition and abundance of plants on land and in the sea
We found some of the highest rates of herbivory recorded for seagrasses globally, but they were temporally and spatially very variable
Video showed that herbivorous fish, especially the goldenlined rabbitfish Siganus lineatus, were abundant at high tide

Summary

Introduction

Herbivory is a key ecological process that often determines the composition and abundance of plants on land and in the sea. Mar Ecol Prog Ser 665: 63–73, 2021 and rabbitfish) are not usually abundant, sea urchins and waterfowl consume large amounts of seagrass in some places (Valentine & Heck 1999, Heck & Valentine 2006). Especially near the tropics, fish are the main consumers of seagrass (Vergés et al 2018). Kirsch et al 2002, Unsworth et al 2007), while in temperate meadows, other taxa tend to be more important (e.g. the sparid Sarpa salpa in the Mediterranean Sea: Prado et al 2007). Rabbitfish (Siganus spp.) can suppress macroalgae in tropical and temperate ecosystems (e.g. Vergés et al 2014, ZarcoPerello et al 2017), but many species frequently consume seagrass Rabbitfish (Siganus spp.) can suppress macroalgae in tropical and temperate ecosystems (e.g. Vergés et al 2014, ZarcoPerello et al 2017), but many species frequently consume seagrass (e.g. El-Sayed 1994, Pitt 1997), they have not been recorded to suppress seagrass in the same way

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Conclusion