title: 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.
authors.affiliation: Department of Environmental Science, Institute for Water and Wetland Research, Radboud University Nijmegen, , Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands, Department of Aquatic Ecology and Environmental Biology, Faculty of Science, Institute for Water and Wetland Research, Radboud University Nijmegen, , Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands, Spatial Ecology Department, Royal Netherlands Institute for Sea Research (NIOZ), , PO Box 140, 4400 AC Yerseke, The Netherlands, Community and Conservation Ecology Group, Centre for Ecological and Evolutionary Studies (CEES), University of Groningen, , PO Box 11103, 9700 CC Groningen, The Netherlands, Marine Spatial Ecology Lab, School of Biological Sciences, University of Queensland, , St Lucia Campus, Brisbane, Queensland 4072, Australia, Division of Marine Sciences and Conservation, Nicholas School of the Environment, Duke University, , 135 Duke Marine Lab Road, Beaufort, NC 28516-9721, USA, Research Centre for Oceanography, Indonesian Institute of Sciences, , Jl. Pasir Putih No. 1, Ancol Timur Jakarta Utara, Indonesia.
authors.forename: Marjolijn J A
authors.initials: MJ
authors.lastname: Christianen
authors.forename: Peter M J
authors.initials: PM
authors.lastname: Herman
authors.forename: Tjeerd J
authors.initials: TJ
authors.lastname: Bouma
authors.forename: Leon P M
authors.initials: LP
authors.lastname: Lamers
authors.forename: Marieke M
authors.initials: MM
authors.lastname: van Katwijk
authors.forename: Tjisse
authors.initials: T
authors.lastname: van der Heide
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Mumby
authors.forename: Brian R
authors.initials: BR
authors.lastname: Silliman
authors.forename: Sarah L
authors.initials: SL
authors.lastname: Engelhard
authors.forename: Madelon
authors.initials: M
authors.lastname: van de Kerk
authors.forename: Wawan
authors.initials: W
authors.lastname: Kiswara
authors.forename: Johan
authors.initials: J
authors.lastname: van de Koppel
mesh_terms: D000818:Animals; D003247:Conservation of Natural Resources; D017753:Ecosystem; D005247:Feeding Behavior; D007214:Indonesia; D008954:Models, Biological; D011156:Population Density; D014426:Turtles / Q000502:physiology*
keywords: alternate stable states; marine reserves; plant–herbivore interactions