Turtle boom may cause habitat crash
Green sea turtles (Chelonia mydas) are thriving in a 1.2-million-ha marine protected area (MPA) surrounding the Derawan Archipelago near Indonesia. The population has grown to 20 individuals per hectare – the highest density ever reported globally – over the past decade of protection.
But according to new research, the turtles' success has resulted in over-grazing of the local seagrass habitat they need to survive. The study (P Roy Soc B 2014; doi:10.1098/rspb.2013.2890) even documents a previously undescribed feeding strategy: turtles digging up seagrass rhizomes and roots with their flippers after they have removed the above-ground biomass. “The seagrass meadow is now close to collapse”, says study author Marjolijn Christianen, an ecologist at Groningen University (Groningen, the Netherlands).
By modeling turtle population dynamics and grazing on ecosystem recovery, Christianen found that turtle numbers would have to be reduced to almost zero for the seagrass to recover. The findings call into question whether MPAs are being managed optimally for turtle conservation. “Our research provides a clear example that the use of MPAs for conservation can't focus only on turtles. It has to incorporate habitat protection as well”, insists Christianen. For instance, she continues, MPAs will function most effectively when they are established as networks that are able to ensure sufficient seagrass habitat in addition to protection of large predators.
Seagrass overgrazing in MPAs has been noted elsewhere. For example, fishers in India's Lakshadweep Islands have linked lower fish abundance to turtles overgrazing seagrass (Biol Conserv 2013; doi:10.1016/j.bio con.2013.07.014).
“In some ways, it's a nice problem to have – protecting turtle nesting beaches and stopping international trade has been remarkably successful”, says Brendan Godley, a conservation ecologist at the University of Exeter (Cornwall, UK). “Many sea turtle populations are increasing, so it shouldn't be a surprise that there will be knock-on ecological impacts.” But, he admits, it does call into question efforts to conserve single species rather than whole ecosystems. Christianen says the answer is simple: “To protect a charismatic species, protect its habitat, both inside and outside the reserve”.