Habitat collapse due to overgrazing threatens turtle conservation in marine protected areas

Abstract

Marine protected areas (MPAs) are key tools for combatting the global overexploitation of endangered species. The prevailing paradigm is that MPAs are beneficial in helping to restore ecosystems to more 'natural' conditions. However, MPAs may have unintended negative effects when increasing densities of protected species exert destructive effects on their habitat. Here, we report on severe seagrass degradation in a decade-old MPA where hyper-abundant green turtles adopted a previously undescribed below-ground foraging strategy. By digging for and consuming rhizomes and roots, turtles create abundant bare gaps, thereby enhancing erosion and reducing seagrass regrowth. A fully parametrized model reveals that the ecosystem is approaching a tipping point, where consumption overwhelms regrowth, which could potentially lead to complete collapse of the seagrass habitat. Seagrass recovery will not ensue unless turtle density is reduced to nearly zero, eliminating the MPA's value as a turtle reserve. Our results reveal an unrecognized, yet imminent threat to MPAs, as sea turtle densities are increasing at major nesting sites and the decline of seagrass habitat forces turtles to concentrate on the remaining meadows inside reserves. This emphasizes the need for policy and management approaches that consider the interactions of protected species with their habitat.

Habitat selection by green turtles in a spatially heterogeneous benthic landscape in Dry Tortugas National Park, Florida

“…1). This small area coupled with the high concentration of core areas there underscores the potential issue of habitat degradation due to overgrazing; this has been a problem at other locations such as Indonesian marine protected areas in the Indian Ocean (Agatti Lagoon and Lakshadweep Islands; Lal et al 2010, Kelkar et al 2013, Christianen et al 2014). An aerial survey reported high abundance of green turtles (ca.…”

“…However, currently there are no abundance estimates for green turtles in DRTO and therefore it is uncertain whether there are sufficient resources to sustain the population. Given the reported increases of green turtles due to conservation efforts elsewhere (Christianen et al 2014), a future increase in green turtles at DRTO as a result of current conservation efforts is possible. Our results highlight the importance of maintaining the seagrass Core areas from multiple turtles overlapped each year, which indicates some level of habitat sharing, but overall UDOI was small (< 0.01; Table 3), suggesting a restricted degree of habitat sharing.…”

Physiological Response of Thallasia hemprichii on Antrophogenic Pressure In Pari Island, Seribu Islands, DKI Jakarta

“…Seagrass community is one of coastal ecosystem which has important ecological functionsuch as food source for coastal community and marine sediment stabilization (Koch et al, 2012;Christianen et al, 2013) and as feeding area for several marine biota (Heck et al,2003;Van Tussenbroek et al,2006;Christianen et al, 2014;Kawaroe et al,2016a). Seagrass also plays important role in carbon cycle (Mcleod et al, 2011).…”

New Tools to Identify the Location of Seagrass Meadows: Marine Grazers as Habitat Indicators

“…Similarly in Mayotte in the western Indian Ocean, several seagrass species are thought to form the mainstay of the diet of green turtles (Ballorain et al, 2010). Where the density of green turtles is very high, green turtles may even overgraze seagrass meadows causing habitat degradation (Christianen et al, 2014). However, at some coastal foraging sites green turtles may also consume macroalgae such as in parts of the Torres Strait, Australia (André et al, 2005).…”