Recovery of a large herbivore changes regulation of seagrass productivity in a naturally grazed Caribbean ecosystem

Gulick, Alexandra G.; Johnson, R. A.; Pollock, Clayton G.; Hillis‐Starr, Zandy; Bolten, Alan B.; Bjorndal, Karen A.

doi:10.1002/ecy.3180

Cited by 14 publications

(44 citation statements)

References 89 publications

(180 reference statements)

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“…However, some of these studies were conducted at subtropical/temperate latitudes, near the northern limit of the T. testudinum range (Fourqurean et al., 2010; Holzer & McGlathery, 2016). Other studies from tropical Caribbean locations found no difference in below‐ground biomass (Gulick et al., 2020; Johnson et al., 2017) or storage of rhizome soluble carbohydrates (Gulick et al., 2020) between naturally grazed and ungrazed areas. Since light availability and depth are critical to supporting below‐ground biomass (Enríquez et al., 2019) and the storage of carbohydrates in seagrasses (Alcoverro et al., 2001; Lee & Dunton, 1997), we propose that the increase in light availability in grazed areas and the tropical latitude could collectively play a role in the sustained below‐ground biomass in grazed areas at our study site.…”

Section: Discussionmentioning

confidence: 92%

“…Light attenuation coefficients within the seagrass canopy (Canopy‐K d , m −1 ), a metric for the degree of leaf self‐shading, were calculated for each exclosure (see Enríquez et al., 2019; Enríquez & Pantoja‐Reyes, 2005). In situ grazing intensity was calculated at the time of exclosure placement, using the following index (ranges from 0 to 1, or 0%–100% removal of blade biomass; Gulick et al., 2020):

Grazing intensity = (\frac{# Grazed shoots false/ m^{2}}{# Total shoots false/ m^{2}}) \times (\frac{Ungrazed blade length ‐ Grazed blade length}{Ungrazed blade length}) .

Grazing intensity ranged from 0.1 to 0.7 at our study site (Gulick et al., 2020); allowing for a thorough assessment of relationships between T. testudinum growth and morphology and the canopy light environment over a gradient of in situ grazing pressure in this study.…”

Section: Methodsmentioning

confidence: 99%

“…Raw data values for growth and morphology were used in each model. Given that water depth (shallow and deep; treated as a two‐level factor) was a significant factor in regulating T. testudinum productivity at this study site (Gulick et al., 2020), we also included depth as an ordered smooth‐factor interaction in addition to a separate component smooth, for each covariate. This allowed us to determine how much variation in T. testudinum growth was explained by morphology alone while still accounting for the effects of depth on each covariate.…”

Section: Methodsmentioning

confidence: 99%

“…Through a previous experiment, we documented a significant compensatory growth response to green turtle grazing in a tropical Caribbean seagrass ecosystem where productivity of the dominant seagrass, Thalassia testudinum, was regulated by both grazing intensity and abiotic factors (Gulick et al, 2020). In this manuscript, we identify a mechanism for compensatory growth responses of T. cultivation grazing and for identifying potential thresholds that may be used to evaluate the sustainability of increased grazing pressure by recovering mega-herbivore populations.…”

Section: Introductionmentioning

confidence: 95%

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Recovery of a cultivation grazer: A mechanism for compensatory growth of Thalassia testudinum in a Caribbean seagrass meadow grazed by green turtles

et al. 2021

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Recovery of green turtles (Chelonia mydas), mega‐herbivores that consume seagrasses, is resulting in dramatic ecosystem‐wide changes as meadows are returned to a natural grazed state. The green turtle grazing strategy, with long‐term cultivation of meadows and high foraging site fidelity, is distinct from other terrestrial and aquatic mega‐herbivores and may affect seagrass compensatory growth responses. Identifying the mechanisms of compensatory growth responses to grazing is essential to understand the functioning of plant systems under natural grazing regimes. In a naturally grazed Caribbean seagrass ecosystem, we identify a mechanism for compensatory growth responses to grazing by evaluating relationships between Thalassia testudinum morphology and growth, grazing intensity, and canopy light dynamics in grazed and ungrazed areas. The morphological characteristics that explain variability in T. testudinum growth differed between grazed and ungrazed areas. In grazed areas, T. testudinum leaf linear growth, leaf area growth, and productivity:biomass (P:B) significantly increased as above‐ground biomass decreased; P:B also increased with shoot density. Mass growth in grazed areas exhibited an increasing trend with shoot density and was maintained above a threshold of 2.5 g dry mass m−2 above‐ground biomass. In ungrazed areas, trends for mass growth and P:B with above‐ground biomass and shoot density were opposite to those in grazed areas. In grazed areas, shoot density significantly increased with grazing intensity while above‐ground biomass decreased and leaf area index (LAI) was not affected. Light availability at canopy height was greater in grazed areas than in ungrazed areas, and canopy light attenuation increased with shoot density in grazed areas. Synthesis. Grazing removes above‐ground biomass, which increases light availability and stimulates leaf growth and turnover (i.e. compensatory growth). Shoot density increases with grazing intensity, maintaining LAI and canopy light harvesting potential. This maximizes the potential for leaf photosynthetic activity and provides the plant with the capacity to sustain mass growth and support a compensatory growth response to grazing. This study presents novel insight for assessing the underlying mechanisms of plant compensatory growth responses to cultivation grazing and proposes potential thresholds that may be used to evaluate the sustainability of in situ grazing pressure by a recovering mega‐herbivore.

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Section: Discussionmentioning

confidence: 92%

Grazing intensity = (\frac{# Grazed shoots false/ m^{2}}{# Total shoots false/ m^{2}}) \times (\frac{Ungrazed blade length ‐ Grazed blade length}{Ungrazed blade length}) .

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 95%

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Recovery of a cultivation grazer: A mechanism for compensatory growth of Thalassia testudinum in a Caribbean seagrass meadow grazed by green turtles

et al. 2021

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“…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.…”

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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Seagrass Tolerance to Simulated Herbivory Along a Latitudinal Gradient: Predicting the Potential Effects of Tropicalisation

Garthwin,

Poore,

Ferretto

et al. 2024

Ecology and Evolution

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The polewards range expansion of tropical herbivorous fish into temperate latitudes is leading to overgrazing of marine habitats and community phase shifts in some regions. Here, we test the potential effects of increased herbivory on the temperate habitat‐forming seagrass Posidonia australis. We used a series of simulated herbivory experiments to predict the potential impacts of climate‐mediated increases in seagrass consumption along P. australis entire latitudinal range (~9° latitude) in eastern Australia (1700 km of coastline). We subjected treatment plots to two levels of simulated herbivory (10% or 80% of leaves clipped) and compared them to unclipped controls. We measured seagrass leaf growth rates and tissue chemical traits: carbohydrates in rhizomes, leaf phenolics, and nutrients (carbon, nitrogen, and C:N ratio) in leaves and rhizomes. At the warmest range‐edge population, we also tested how responses to increased herbivory may vary between summer and winter, or with repeated clipping events. Clipped shoots maintained growth rates similar to unclipped controls despite losing up to 80% of leaf biomass. This was consistent along the full latitudinal range and after repeated simulated herbivory at the northernmost location. One‐off clipping events impacted plant architecture, increasing the number of subdividing shoots. At the species range edge, leaves grew more in winter than in summer, and clipping tended to lower seagrass growth only in winter; however, higher levels of shoot subdivision were produced over summer than in winter. Plant chemical traits could not explain consistently the growth patterns observed despite some traits varying with latitude (e.g., leaf nitrogen content decreased with latitude and C:N ratio increased) and/or simulated herbivory. Synthesis: P. australis growth is not affected by increases in simulated herbivory and may be relatively resilient to future increases in seagrass consumption, suggesting that this species could be a relative ‘winner’ under future climate change conditions that lead to enhanced herbivory.

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Recovery of a large herbivore changes regulation of seagrass productivity in a naturally grazed Caribbean ecosystem

Abstract: Recovery of a large herbivore changes regulation of seagrass productivity in a naturally grazed Caribbean ecosystem.

Cited by 14 publications

References 89 publications

Recovery of a cultivation grazer: A mechanism for compensatory growth of Thalassia testudinum in a Caribbean seagrass meadow grazed by green turtles

Recovery of a cultivation grazer: A mechanism for compensatory growth of Thalassia testudinum in a Caribbean seagrass meadow grazed by green turtles

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

Seagrass Tolerance to Simulated Herbivory Along a Latitudinal Gradient: Predicting the Potential Effects of Tropicalisation

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