Nature today | The sea turtle is coming back, what does this mean for the functioning of ecosystems?

For example, seagrass – their source of nutrition – is grazed again in a natural way. In some places, sea grass is even overgrown. What does the return of the sea turtle mean for the functioning of coastal ecosystems?

A research team has analyzed the value of seagrass ecosystems in an area with grazing sea turtles, for example looking at coastal protection and carbon storage. Through experiments, they determined the effect of the restored turtle populations on the functioning of the ecosystem. The team, led by Wageningen University with colleagues from the Netherlands and the Caribbean, recently published the findings in Global Change Biology.

Seagrass fields provide us with various services. For example, they provide habitat and food for many fish species that are commercially important. Seagrasses also help protect the coast by stabilizing sediment and reducing wave action. Now that the large grazer – the sea turtle – has returned, it is important to determine what impact it will have on the services a seagrass ecosystem can provide. Often these services are measured separately, but the new study examined key services together to assess ecosystem multifunctionality.

Combining ecosystem services into one index

“Our approach of measuring multiple functions simultaneously and combining them into one ecosystem multifunctionality index is new for marine habitats,” said Marjolijn Christiansen, lead researcher of the study. “It led to striking results. We saw that carbon storage and nutrient cycling were higher in seagrass fields with average grazing pressure. On the other hand, the amount of fish and other benefits turned out to be higher in ungrazed fields. But more importantly, we saw , that all these services succumbed to violent grazing pressure.”

In tropical seagrass ecosystems where green turtles are mega-herbivores, three levels of grazing intensity are observed worldwide (a).  After centuries of decline, the turtle's ecological role is rapidly unfolding in several foraging areas, where populations are recovering through conservation.  An increase in the number of overdrives is recorded here.  Researchers have used field experiments to assess the effects of simulated grazing intensity (at different levels) on ecosystem functions and on multifunctionality (b).  These experiments took place over 18 months in a tropical seagrass ecosystem in the Caribbean.

Integrated approach to a balanced ecosystem

Grazing sea turtleBased on the findings, the authors argue that a successful return of the sea turtle must go hand in hand with the protection of their habitats – seagrass beds – and their natural enemies. These predators can influence the grazing behavior of the turtles through the effect of fear. “Recently, studies have been done in the Bahamas. Based on that insight, we believe that spatial variation in the resulting grazing pressure is likely to prevent habitat collapse,” says Fee Smulders, co-author of the study. Christianen: “With an integrated approach to ecosystem management, we can achieve the high multifunctionality of ecosystems. And this leads to balanced ecosystems that can support natural numbers of the sea turtle.”

Pristine fields of sea grass

Green sea turtles are considered the major herbivores in the ocean. Since their populations have thinned out, scientists have become accustomed to studying non-grazing seagrasses. These lush fields – dominated by hardy tribes – are considered healthy, ‘pristine’ and highly valuable. But thanks to successful conservation efforts, the green sea turtle has made a comeback in many coastal areas in recent decades. As a result, the seagrass fields have once again become grazed areas.

  a) To simulate the 'no grazing' scenario, cages were used to prevent turtles from accessing the seagrass.  b) Innovative underwater flow channels, developed by co-authors from the Netherlands Institute for Marine Research (James et al. 2020), were used to measure the stability of the sediment - one of the ecosystem functions studied

An earlier article, published in Nature, ecology and evolution by Wageningen researchers, described how we should adjust our idea of ​​’pristine’ seagrass. This probably consists of a mosaic of grazed areas and heavily grazed areas, alternating with non-grazed areas with a higher density of plant species and a higher biodiversity in general. This mosaic may be more natural, although the ecosystem value is slightly lower than with ungrazed seagrass. The results of these studies become even more relevant and urgent worldwide as the different levels of grazing – many turtles, few or sometimes extremely high – converge in the three sea basins where turtles live. The results can also be used to predict ecosystem impacts that may drive future shifts.

This research is funded by the Netherlands Organization for Scientific Research.

Text: Wageningen Environmental Research
Photos: P. Lindgren, Wikimedia Commons; Marjolijn Christianen
Figures: Christians et al, 2022

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