Mesopredator release moderates trophic control of plant biomass in a Georgia salt marsh

Abstract

Predators regulate communities through top‐down control in many ecosystems. Because most studies of top‐down control last less than a year and focus on only a subset of the community, they may miss predator effects that manifest at longer timescales or across whole food webs. In southeastern US salt marshes, short‐term and small‐scale experiments indicate that nektonic predators (e.g., blue crab, fish, terrapins) facilitate the foundational grass, Spartina alterniflora, by consuming herbivorous snails and crabs. To test both how nekton affect marsh processes when the entire animal community is present, and how prior results scale over time, we conducted a 3‐year nekton exclusion experiment in a Georgia salt marsh using replicated 19.6 m² plots. Our nekton exclusions increased densities of plant‐grazing snails and juvenile deposit‐feeding fiddler crab and, in Year 2, reduced predation on tethered juvenile snails, indicating that nektonic predators control these key macroinvertebrates. However, in Year 3, densities of mesopredatory benthic mud crabs increased threefold in nekton exclusions, erasing the tethered snails' predation refuge. Nekton exclusion had no effect on Spartina biomass, likely because the observed mesopredator release suppressed grazing snail densities and elevated densities of fiddler crabs, whose burrowing alleviates soil stresses. Structural equation modeling supported the hypotheses that nektonic predators and mesopredators control invertebrate communities, with nektonic predators having stronger total effects on Spartina than mud crabs by controlling densities of species that both suppress (grazers) and facilitate (fiddler crabs) plant growth. These findings highlight that salt marshes can be resilient to multiyear reductions in nektonic predators if mesopredators are present and that multiple pathways of trophic control manifest in different ways over time to mediate community dynamics. These results highlight that larger scale and longer‐term experiments can illuminate community dynamics not previously understood, even in well‐studied ecosystems such as salt marshes.

Full text

ARTICLE
Mesopredator release moderates trophic control of plant
biomass in a Georgia salt marsh
Joseph P. Morton
1,2
| Marc J. S. Hensel
3
| David S. DeLaMater
1
|
Christine Angelini
2
| Rebecca L. Atkins
4
| Kimberly D. Prince
2
|
Sydney L. Williams
4
| Anjali D. Boyd
1
| Jennifer Parsons
5
|
Emlyn J. Resetarits
6
| Carter S. Smith
1
| Stephanie Valdez
1
|
Evan Monnet
5
| Roxanne Farhan
7
| Courtney Mobilian
5
| Julianna Renzi
8
|
Dontrece Smith
7
| Christopher Craft
5
| James E. Byers
4
| Merryl Alber
7
|
Steven C. Pennings
9
| Brian R. Silliman
1
1
Duke University Marine Lab, Beaufort,
North Carolina, USA
2
Department of Environmental
Engineering Sciences, Center for Coastal
Solutions, University of Florida,
Gainesville, Florida, USA
3
Department of Biological Sciences,
Virginia Institute of Marine Sciences,
College of William and Mary, Gloucester,
Virginia, USA
4
Odum School of Ecology, University of
Georgia, Athens, Georgia, USA
5
O’Neill School of Public and
Environmental Affairs, Indiana
University, Bloomington, Indiana, USA
6
Department of Biological Sciences,
Barnard College, Columbia University,
New York, New York, USA
7
Deptartment of Marine Sciences,
University of Georgia, Athens,
Georgia, USA
8
Department of Ecology, Evolution, and
Marine Biology, University of California
Santa Barbara, Santa Barbara,
California, USA
9
Department of Biology and Biochemistry,
University of Houston, Houston,
Texas, USA
Correspondence
Joseph P. Morton
Email: j.morton@ufl.edu
Abstract
Predators regulate communities through top-down control in many ecosys-
tems. Because most studies of top-down control last less than a year and focus
on only a subset of the community, they may miss predator effects that mani-
fest at longer timescales or across whole food webs. In southeastern US salt
marshes, short-term and small-scale experiments indicate that nektonic preda-
tors (e.g., blue crab, fish, terrapins) facilitate the foundational grass, Spartina
alterniflora, by consuming herbivorous snails and crabs. To test both how
nekton affect marsh processes when the entire animal community is present,
and how prior results scale over time, we conducted a 3-year nekton exclusion
experiment in a Georgia salt marsh using replicated 19.6 m
2
plots. Our nekton
exclusions increased densities of plant-grazing snails and juvenile
deposit-feeding fiddler crab and, in Year 2, reduced predation on tethered juve-
nile snails, indicating that nektonic predators control these key macroinver-
tebrates. However, in Year 3, densities of mesopredatory benthic mud crabs
increased threefold in nekton exclusions, erasing the tethered snails’predation
refuge. Nekton exclusion had no effect on Spartina biomass, likely because the
observed mesopredator release suppressed grazing snail densities and elevated
densities of fiddler crabs, whose burrowing alleviates soil stresses. Structural
equation modeling supported the hypotheses that nektonic predators and
mesopredators control invertebrate communities, with nektonic predators hav-
ing stronger total effects on Spartina than mud crabs by controlling densities
of species that both suppress (grazers) and facilitate (fiddler crabs) plant
growth. These findings highlight that salt marshes can be resilient to multiyear
reductions in nektonic predators if mesopredators are present and that
multiple pathways of trophic control manifest in different ways over time to
Received: 4 September 2023 Revised: 26 June 2024 Accepted: 26 August 2024
DOI: 10.1002/ecy.4452
Ecology. 2024;105:e4452. https://onlinelibrary.wiley.com/r/ecy © 2024 The Ecological Society of America. 1of13
https://doi.org/10.1002/ecy.4452