Approaching a Tipping Point? Herbivore Carrying Capacity Estimates in a Rapidly Changing, Seagrass-Dominated Florida Bay
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SPECIAL ISSUE: SEAGRASSES TRIBUTE TO SUSAN WILLIAMS
Approaching a Tipping Point? Herbivore-Carrying Capacity Estimates in a Rapidly Changing, Seagrass-Dominated Florida Bay Alexandra R. Rodriguez 1
&
Kenneth L. Heck Jr 1,2
Received: 31 March 2020 / Revised: 23 October 2020 / Accepted: 29 October 2020 # Coastal and Estuarine Research Federation 2020
Abstract Herbivory is a key structuring force that can have negative, positive, or mixed impacts on seagrasses. The identity and abundance of herbivores, as well environmental conditions such as solar insulation, can influence the impacts of herbivory. To understand the impact of herbivores on turtlegrass in warm, temperate St. Joseph Bay (SJB) in the northeastern Gulf of Mexico, we estimated densities of the most common herbivores: variegated sea urchins and green turtles. We also estimated SJB’s herbivore-carrying capacities by estimating annual average turtlegrass production and variegated urchin consumption rates and by using previously calculated juvenile green turtle consumption rates and seagrass area. During summer, turtlegrass exhibited compensatory growth due to urchin herbivory, but during winter, turtlegrass growth was negatively affected by urchin grazing. Given our calculated annual turtlegrass production, urchin ingestion rates, and the average urchin density of 5.89 ± 0.09 (SE) urchins ha−1, the turtlegrass production not consumed by urchins can sustain 278.0 juveniles ha−1, or about 556,010 turtles, putting current average green turtle abundance of 26 ± 3.7 individuals ha−1 at 4.0–14.6% of carrying capacity. Previous work in this system has shown that simulated turtle herbivory can reduce turtlegrass productivity—with this potential reduction in production, SJB could not sustain a green turtle population at current urchin densities. Given the rapid global green turtle increase, evaluating densities and future carrying capacities of herbivores in higher latitude seagrass pastures is critical for updating and achieving recovery goals for both green turtles and globally declining seagrass meadows. Keywords Plant-herbivore interactions . Carrying capacity . Chelonia mydas . Thalassia testudinum . Lytechinus variegatus
Introduction Seagrasses, marine angiosperms that underpin highly diverse and productive coastal ecosystems, provide a wide range of ecosystem services. Perhaps the best known is their function as nurseries for finfish and shellfish and for providing links to commercial fisheries (Beck et al. 2001; Unsworth et al. 2019). Other important services include stabilizing sediments and sequestering vast amounts of carbon. With increased awareness of the importance of seagrass comes increasing concern Communicated by Melisa C. Wong * Alexandra R. Rodriguez [email protected] 1
Dauphin Island Sea Lab, 101 Bienville Blvd, Dauphin Island, AL 36528, USA
2
Department of Marine Sciences, University of South Alabama, 5871 USA Drive North, Mobile, AL 36668, USA
about their global decline (Orth et al. 2006; Short and WyllieEcheverria 1996; Waycott et
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