Archie Carr Center for Sea Turtle Research and Department of Biology, University of Florida, Gainesville, Florida, 32611, USA.
Division of Resource Management and Research, Buck Island Reef National Monument, National Park Service, Christiansted, U.S. Virgin Islands, 00820, USA.
Ecology. 2020 Dec;101(12):e03180. doi: 10.1002/ecy.3180. Epub 2020 Oct 6.
What happens in meadows after populations of natural grazers rebound following centuries of low abundance? Many seagrass ecosystems are now experiencing this phenomenon with the recovery of green turtles (Chelonia mydas), large-bodied marine herbivores that feed on seagrasses. These seagrass ecosystems provide a rare opportunity to study ecosystem-wide shifts that result from a recovery of herbivores. We evaluate changes in regulation of seagrass productivity in a naturally grazed tropical ecosystem by (1) comparing Thalassia testudinum productivity in grazed and ungrazed areas and (2) evaluating potential regulating mechanisms of T. testudinum productivity. We established 129 green turtle exclusion cages in grazed and ungrazed areas to quantify T. testudinum growth (linear, area, mass, productivity : biomass [P:B]). In each exclosure, we recorded temperature, irradiance, water depth, nitrogen : phosphorus ratio (N:P) of blade tissue, grazing intensity before cage placement, and T. testudinum structural and nutrient characteristics. Thalassia testudinum exhibited compensatory growth in grazed areas via stimulated blade linear growth, blade area growth, and P:B across seasonal high and low growth periods and in shallow (3-4 m) and deep (9-10 m) seagrass meadows. Irradiance, depth, and N:P ratios had significant roles in regulating mass growth and P:B of T. testudinum in ungrazed areas. Depth was a significant regulating factor of mass growth and P:B in grazed areas; rates were higher and more variable in shallow meadows than in deep meadows. Grazing intensity was also a significant regulating factor for P:B, stimulating tissue turnover with increasing grazing pressure. This study provides important insights into how recovery of a large marine herbivore can result in dramatic, sustainable changes in the regulation of seagrass productivity. We also highlight the need for a historical perspective and use of appropriate indicators, including P:B and grazing intensity, when evaluating seagrass response to green turtle grazing as meadows are returned to a natural grazed state. In an age of green turtle recovery and global seagrass decline due to anthropogenic threats, a thorough understanding of green turtle-seagrass interactions at the ecosystem level is critical to ensure the restoration of seagrass ecosystems and continued recovery of green turtle populations.
在经历了数个世纪的低丰度后，自然食草动物的数量反弹，接下来会在草地上发生什么？随着绿海龟（Chelonia mydas）的恢复，许多海草生态系统正在经历这种现象，绿海龟是一种大型海洋草食动物，以海草为食。这些海草生态系统为研究食草动物恢复所导致的整个生态系统范围的变化提供了一个难得的机会。我们通过（1）比较放牧区和未放牧区的塔希提草（Thalassia testudinum）生产力，以及（2）评估塔希提草生产力的潜在调节机制，来评估热带自然放牧生态系统中海草生产力调节的变化。我们在放牧区和未放牧区建立了 129 个绿海龟围栏，以量化塔希提草的生长情况（线性、面积、质量、生产力：生物量 [P:B]）。在每个围栏中，我们记录了温度、光照强度、水深、叶片组织的氮磷比（N:P）、围栏放置前的放牧强度，以及塔希提草的结构和营养特征。在季节性高生长和低生长时期，在浅水区（3-4 米）和深水区（9-10 米）的海草草地中，放牧区的塔希提草通过刺激叶片线性生长、叶片面积生长和 P:B 表现出补偿性生长。光照强度、水深和 N:P 比在调节未放牧区塔希提草的质量生长和 P:B 方面发挥了重要作用。在放牧区，水深是影响质量生长和 P:B 的重要调节因素；浅水区的生长速度和变异性均高于深水区。放牧强度也是 P:B 的一个重要调节因素，随着放牧压力的增加，刺激组织更新。本研究为大型海洋草食动物的恢复如何导致海草生产力调节的显著、可持续变化提供了重要见解。我们还强调，在评估海草对绿海龟放牧的反应时，需要从历史角度出发，并使用适当的指标，包括 P:B 和放牧强度，因为海草草地正在恢复到自然放牧状态。在海龟恢复和全球海草因人为威胁而减少的时代，深入了解生态系统层面上的绿海龟-海草相互作用对于确保海草生态系统的恢复和绿海龟种群的持续恢复至关重要。
