Sequential overgrazing by green turtles causes archipelago-wide functional extinctions of seagrass meadows

Gangal, Mayuresh; Gafoor, Al-Badush; D’Souza, Elrika; Kelkar, Nachiket; Karkarey, Rucha; Marbà, Núria; Alcoverro, Teresa

doi:10.1016/j.biocon.2021.109195

Cited by 25 publications

(44 citation statements)

References 44 publications

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“…For sea turtles, upward trends in abundance have been recorded in a range of species and across the globe. There are likely to be a range of ecosystem and demographic consequences of such population recoveries [ 6 ] with the resulting trophic cascades sometimes reshaping communities [ 9 , 10 ]. For example, increasing numbers of green turtles have been associated with overgrazing of seagrass beds [ 10 ].…”

Section: Introductionmentioning

confidence: 99%

“…There are likely to be a range of ecosystem and demographic consequences of such population recoveries [ 6 ] with the resulting trophic cascades sometimes reshaping communities [ 9 , 10 ]. For example, increasing numbers of green turtles have been associated with overgrazing of seagrass beds [ 10 ]. However, overall the demographic changes occurring as part of species recoveries are poorly understood.…”

Section: Introductionmentioning

confidence: 99%

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Changes in mean body size in an expanding population of a threatened species

Hays

Taxonera²,

Renom³

et al. 2022

Proc. R. Soc. B.

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With some taxa, a reduction in the mean size of individuals may reflect over-harvesting and/or trophy hunting. However, we show that in sea turtles, a reduction in the mean size of breeding individuals may be part of the good news story of an expanding population. We describe a 70-fold increase in annual nest numbers on the island of Sal (Cape Verde, North Atlantic) between 2008 and 2020 (from 506 to 35 507 nests), making this now one of the largest loggerhead ( Caretta caretta ) nesting aggregations in the world. We use 20 128 measurements of the size of nesting turtles to show that their mean annual size has decreased by about 2.4 cm, from 83.2 to 80.8 cm. This decrease in the mean size of nesting turtles was not caused by the removal of larger turtles, for example by selective harvesting. Rather we develop a theoretical model to show than this decrease in mean size can be explained by an influx of first-time nesters, combined with a decrease in the size of those first-time nesters over time. A reduction in mean size of nesting turtles has been reported across the Atlantic, Pacific and Indian Oceans, and may be a common feature of population recoveries in sea turtles.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Changes in mean body size in an expanding population of a threatened species

Hays

Taxonera²,

Renom³

et al. 2022

Proc. R. Soc. B.

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“…Overgrazing is a mismatch between the food requirements and food availability in an ecosystem ( Durant et al, 2005 ), and has been observed in aquatic environments through waterfowl ( Kollars et al, 2017 ), sea urchins ( Eklöf et al, 2008 ), and turtle grazing ( Fourqurean et al, 2019 ). This can be the result of increased herbivore abundance and, in extreme cases, has caused the functional extinction of submerged aquatic vegetation (SAV) within an ecosystem ( Gangal et al, 2021 ). In many regions, predicted changes in hydrology have the potential to lead to increased herbivore densities in some aquatic ecosystems, with implications for the plants they depend on.…”

Section: Introductionmentioning

confidence: 99%

Increased extent of waterfowl grazing lengthens the recovery time of a colonizing seagrass (Halophila ovalis) with implications for seagrass resilience

et al. 2022

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Herbivore distributions and abundance are shifting because of climate change, leading to intensified grazing pressure on foundation species such as seagrasses. This, combined with rapidly increasing magnitudes of change in estuarine ecosystems, may affect seagrass resilience. While the overall resilience of seagrasses is generally well-studied, the timeframes of recovery has received comparatively little attention, particularly in temperate estuaries. We investigated how the recovery time (RT) of seagrass is affected by simulated grazing in a southwestern Australian estuary. Whilst excluding swans, we simulated different grazing intensities (25, 50, 75, and 100% removal from 1 m2 plots) at four locations in the Swan-Canning Estuary, Western Australia during summer and tracked the recovery of seagrass over 3 months, using seagrass cover as the main measure of recovery. We found that seagrass recovered within 4–6 weeks from the lower grazing intensities (25 and 50%) and 7–19 weeks from the higher grazing intensities (75 and 100%) across the estuary. Increased grazing intensity led to not only longer recovery times (RTs), but also greater variability in the RT among experimental locations. The RT from the higher grazing intensities at one location in particular was more than double other locations. Seagrass recovery was through vegetative mechanisms and not through sexual reproduction. There was a significant grazing treatment effect on seagrass meadow characteristics, particularly belowground biomass which had not recovered 3 months following grazing. As the pressure of climate change on estuarine environments increases, these quantified RTs for seagrass provide a baseline for understanding grazing pressure as a singular disturbance. Future work can now examine how grazing and other potentially interacting pressures in our changing climate could impact seagrass recovery even further.

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Section: Discussionmentioning

confidence: 99%

“…Foraging green turtles exert pressure on seagrass densities and health over time-both positively through grazing (Gulick et al 2020(Gulick et al , 2021 and negatively by consuming large patches of seagrass (Heithaus et al 2014;Gangal et al 2021). Since the discovery of the large densities of green turtles in the Lakshadweep archipelago of India (Tripathy et al Gangal et al (2021) found that green turtle densities reached numbers above what seagrass beds on the foraging grounds can support. With recent increases in sea turtle nest numbers in Florida, particularly from eastern Florida rookeries (Chaloupka et al 2008;Seminoff et al 2015), it is currently expected that these increases will soon be realized within foraging aggregations.…”

Section: Discussionmentioning

confidence: 99%

Intraspecific spatial segregation on a green turtle foraging ground in the Florida Keys, USA

Welsh

Mansfield

2022

Mar Biol

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Assessing the distribution, density, and abundance of organisms is essential for conservation and management of imperiled species. Simple counts of sampled individuals are often inadequate to make such estimates. This is especially true for highly mobile marine animals like green sea turtles (Chelonia mydas). We used distance sampling and density surface model techniques to generate estimates of green turtle abundance on foraging grounds at the Eastern Quicksands, located west of Key West, Florida, USA. From 2006 to 2018, we conducted 18 surveys of six standardized transect lines from which we estimated abundances, plotted spatial distributions, and identified spatial segregation of life stages using density surface models and null model analysis. The Eastern Quicksands represent one of the densest foraging aggregations of green turtles worldwide. Spatial segregation of Large Juvenile and Adult turtles was evident, which we hypothesize may be due to benthic habitat preferences and differing predator detection and avoidance strategies among the sea turtle life stages. Given the high green turtle densities in this foraging area, and recent increases in Florida green turtle nesting, combined with anthropogenic stressors to seagrass pastures, we foresee management concerns for long-term sustainability of seagrass at the Eastern Quicksands and recommend that this area be given Critical Habitat designation.

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Sequential overgrazing by green turtles causes archipelago-wide functional extinctions of seagrass meadows

Cited by 25 publications

References 44 publications

Changes in mean body size in an expanding population of a threatened species

Changes in mean body size in an expanding population of a threatened species

Increased extent of waterfowl grazing lengthens the recovery time of a colonizing seagrass (Halophila ovalis) with implications for seagrass resilience

Intraspecific spatial segregation on a green turtle foraging ground in the Florida Keys, USA

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