How does ecosystem functioning change now megaherbivores are returning? In our new paper today in Global Change Biology we show that the return of the green turtle has strong effects on seagrass ecosystem functions and can even reduce multifunctionality with ~25%. Read the short & simple summary below.
Consequences of changing marine megaherbivore densities for ecosystem functioning and services. Three scenarios of megaherbivore grazing intensity, can be observed in tropical seagrass ecosystems with green turtles as megaherbivores across the world (Figure 1b). In a situ experiment, three different levels of sea turtle grazing intensity were simulated as found in the literature. Treatment 1—no turtle grazing (megaherbivores ecologically extinct, yellow squares), Treatment 2—intermediate turtle grazing (return of megaherbivores to intermediate levels, purple circles), Treatment 3—intensive turtle grazing (megaherbivores accumulation, red triangles). Megaherbivore grazing intensity affects the seagrass biomass, shoot density, and canopy structure which has implications for ecosystem functioning. The impact of megaherbivore grazing intensity for single ecosystem functions and their integrated overall effect, ecosystem multifunctionality, was determined over the range of remaining seagrass biomass at the end of the 18-month experimental period and is summarized in gray bars. Image credit (vector graphics): Joanna Woerner, Tracey Saxby, Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/imagelibrary/). Images were customized by the authors.
Green sea turtles are considered the megaherbivores of the coastal seas. Due to successful turtle conservation – many seagrass meadows across the globe have been transformed into their natural grazed state. Some are even becoming overgrazed show recent studies in India and Bermuda.
(a) The number of publications on seagrass and green turtle grazing in peer-reviewed journals is accelerating over time (Web of Science, Scopus, Google Scholar 1960–2022, Supplementary information text S1) mirroring the recovery of green turtle populations. Arrow 1: (McRoy & Helfferich, 1977; Thayer et al., 1977); arrow 2: (Jackson, 1997), arrow 3: (Chaloupka et al., 2008; Mazaris et al., 2017; Weber et al., 2014). (b) A selection of sites illustrates that all three different grazing scenarios for green turtles occur in coastal (sub-)tropical seagrass ecosystems around the world, in all three ocean basins where green turtles are found. Green dots: global seagrass distribution (UNEP-WCMC & Short, 2021), blue: distribution of the green turtle, Chelonia mydas, (Kot et al., 2022). (Scenario 1) (Gaubert-Boussarie et al., 2021; Jackson, 1997; Jones et al., 2018; van der Laan & Wolff, 2006; Vonk et al., 2008); (Scenario 2) (Ballorain et al., 2010; Christianen et al., 2019; Gulick et al., 2020; Molina Hernández & van Tussenbroek, 2014; Rodriguez & Heck, 2020; Scott et al., 2020); (Scenario 3) (Christianen et al., 2014; Fourqurean et al., 2019; Gangal et al., 2021).
In an experiment along a grazing gradient from high to low turtle grazing, seven ecosystem functions where measured (e.g. coastal protection #carbon storage, nutrient cycling #biodiversity) simultaneously after 18mo. These were combined in 1 ecosystem-multifunctionality-index. Which is a new approach for marine habitats. This led to striking results.
Cages kept turtles from grazing the seagrass to mimic the –no-grazing scenario. Underwater field flumes, developed by the Royal Netherlands Institute of Sea Research, were used to measure sediment stability.
We found that medium turtle grazing pressure increased carbon storage and nutrient cycling of seagrass. While, fish biomass and other services were higher in meadows without turtle grazing. More importantly, we found simultaneous collapse of all services under severe grazing.
Results of experimental manipulation simulating differential megaherbivore grazing intensities on seagrass ecosystem functions and ecosystem multifunctionality, following the three megaherbivore grazing scenarios (Figure 3) with aboveground seagrass biomass as a proxy for the outcome of grazing intensity (x-axis). The best fitting models determined by AICc are shown (Dataset S2; Christianen et al., 2022). (a) Net leaf nitrogen uptake rate, (b) Tea bag decomposition rate. (c) Sediment organic carbon storage. (d) Biomass of fish species. (e) The taxonomic richness of macroinvertebrates. (f) Sediment stabilization, measured as threshold shear velocity, the speed at which sediment became mobile in a unidirectional-flow field flume. (g) Resilience against invasive species expansion, measured as change in % cover of the invasive seagrass Halophila stipulacea (not significant). (h) Ecosystem multifunctionality index, the average of the seven standardized functions in percent. (i) Several functions (max seven functions) exceed threshold levels in each plot against aboveground seagrass biomass, for thresholds ranging from 10% to 90% of the maximum indicated on the color scale below. Colors and symbols correspond to the three grazing intensities Treatment 1—no turtle grazing (megaherbivores ecologically extinct, yellow squares), Treatment 2—intermediate turtle grazing (return of megaherbivores to intermediate levels, purple circles), Treatment 3—intensive turtle grazing (megaherbivores accumulation, red triangles). Solid line: significant results. Dotted line: results not significant.
We argue that the return of the sea turtle should be accompanied with the protection of their habitat, seagrass meadows, as well as their predators, sharks, who can influence turtle grazing behaviour through fear effects. Watch this space for an upcoming paper by Fee Smulders et al.
By taking such integrative ecosystem approach to management we can maintain high ecosystem multifunctionality, as well as balanced ecosystems that can sustain natural densities of charismatic sea turtles.
Big thanks to Fee Smulders Arie Vonk Lisa Becking Tjeerd Bouma Rebecca James Jaco de Smit Jurjan van der Zee Per Palsboll Liesbeth Bakker et al. Wageningen Environmental Research NIOZ Royal Netherlands Institute for Sea Research Institute for Biodiversity and Ecosystem Dynamics – University of Amsterdam STINAPA Bonaire and our funders NWO Science
