Contribution of green turtles Chelonia mydas to total herbivore biomass in shallow tropical reefs of oceanic islands

Abstract

Green turtles are megaherbivores with a key role in the dynamics of tropical seagrass meadows, but little is known about their relevance as herbivores in tropical reef habitats. We conducted underwater censuses of green turtles, herbivorous fishes and sea urchins in two distinct tropical regions: Fernando de Noronha (Western Atlantic Ocean) and the Hawaiian Archipelago (Central Pacific Ocean), to assess the contribution of green turtles to the total herbivore biomass in shallow reef habitats of tropical oceanic islands. Juvenile green turtles ranging 40-60 cm were observed at most of the surveyed sites, and hence, could be considered typical components of the shallow reef fauna of tropical oceanic islands. Furthermore, they were usually one of the most abundant species of roving herbivores in many of the sites surveyed. However, the biomass of green turtles was usually much lower than the aggregated biomass of fishes or sea urchins, which usually constituted most of the total herbivore biomass. Green turtles made a major contribution to the total herbivore biomass only in sheltered sites with low rugosity, low coral cover and high algal cover. Further investigation on the trophic redundancy between herbivores is required to assess the actual relevance of green turtles in reef ecosystems of oceanic islands, compared to herbivorous fishes and sea urchins, because different herbivores may target different algal resources and complementarity may be needed to maintain ecosystem functioning across large, naturally varied reefscapes.

Functional biogeography of marine vertebrates in Atlantic Ocean reefs

“…Yet, different herbivorous species, as bony fishes, marine turtles and mammals may target distinct algal resources (Cardona et al., 2020; Castelblanco‐Martínez et al., 2009; Tebbett et al., 2020). For example, the herbivory pressure exerted by a single Chelonia mydas equals many fish individuals (Goatley et al., 2012), and green turtles have a major contribution in sheltered reefs with low rugosity, low coral cover and high algal cover (Cardona et al., 2020). Unfortunately, this essential reef function delivered by threatened herbivorous reptiles and mammals, as Chelonia mydas and Trichechus spp., is at global risk (Atwood et al., 2020).…”

“…In these reefs, herbivory is mainly performed by bony fishes through diverse feeding modes as browsing, excavating or scraping on the reef substrate (Ferreira & Gonçalves, 2006; Francini‐Filho et al., 2010; Mantyka & Bellwood, 2007). Yet, different herbivorous species, as bony fishes, marine turtles and mammals may target distinct algal resources (Cardona et al., 2020; Castelblanco‐Martínez et al., 2009; Tebbett et al., 2020). For example, the herbivory pressure exerted by a single Chelonia mydas equals many fish individuals (Goatley et al., 2012), and green turtles have a major contribution in sheltered reefs with low rugosity, low coral cover and high algal cover (Cardona et al., 2020).…”

“…Unfortunately, this essential reef function delivered by threatened herbivorous reptiles and mammals, as Chelonia mydas and Trichechus spp., is at global risk (Atwood et al., 2020). Thus, functional complementarity is needed to maintain ecosystem functioning (Cardona et al., 2020).…”

Trophic history of Hawaiian green turtles as revealed by stable isotope ratios (δ13C, δ15N and δ34S) in the bones of museum specimens

“…Human activity dramatically reduced the abundance of many marine species worldwide during the 20th century (McCauley et al, 2015). Changes in population size were often associated with diet shifts (Cardona et al, 2020; Hanson et al, 2009; Zenteno et al, 2015) and hence altered the structure and dynamics of food webs, even when no species went extinct (Bas et al, 2019; Ólafsdóttir et al, 2021). Only recently have some populations started to increase as a result of protection (Duarte et al, 2020).…”

“…Changes in population size were often associated with diet shifts (Cardona et al, 2020;Hanson et al, 2009;Zenteno et al, 2015) and hence altered the structure and dynamics of food webs, even when no species went extinct (Bas et al, 2019;Ólafsd ottir et al, 2021).…”