Salt marsh resilience compromised by crabs along tidal creek edges


Date:
August 9, 2021
Source:
University of California - Santa Cruz
Summary:
Coastal marshes are vulnerable to erosion caused by rising seas,
pounding waves, and tidal flows. In California's Elkhorn Slough,
these vulnerabilities are made worse by superabundant crabs found
at their highest densities along the estuary's tidal creeks,
according to a new study.



FULL STORY ========================================================================== Coastal marshes are vulnerable to erosion caused by rising seas,
pounding waves, and tidal flows. In Elkhorn Slough on the Central Coast
of California, these vulnerabilities are made worse by superabundant
crabs found at their highest densities along the estuary's tidal creeks, according to a new study published August 8 in Ecosphere.


==========================================================================
The striped shore crab (Pachygrapsus crassipes) is a small crab found
all along the West Coast of North America, and it is extremely abundant
in Elkhorn Slough. The study demonstrated the dual role of these crabs
as both consumers of salt marsh vegetation and as ecosystem engineers.

"Their burrowing weakens the creekbank edges, so that whole chunks of
marsh will sometimes calve off, and by lowering biomass they are reducing
the ability of marsh plants to prevent erosion," said lead author Kathryn Beheshti, who earned her Ph.D. in ecology and evolutionary biology at UC
Santa Cruz in 2021 and is currently a California Sea Grant State Fellow
at the Ocean Protection Council's Climate Change Program.

Beheshti and her coauthors conducted a five-year field experiment to
assess the effects of crabs on the vegetation and sediments along eroding creekbank edges.

Using fencing and traps made of empty tennis-ball cans to exclude crabs
from experimental enclosures, they found that reducing crab abundance
led to increased growth of salt marsh vegetation and enhanced sediment
density.

The researchers also found that the number of burrows did not change
over the study period, even with researchers experimentally removing
crabs. The unexpected persistence of the burrows highlights the value
of long-term field experiments. The experiment was maintained for five
years thanks in large part to the efforts of a team of over 50 UC Santa
Cruz undergraduate students and high school interns.

"Field experiments that span multiple seasons and years are rare,"
said coauthor Kerstin Wasson, research coordinator of the Elkhorn
Slough National Estuarine Research Reserve and an adjunct professor at
UC Santa Cruz. "This work demonstrates the value of long-term studies
by showing that burrows, which weaken the stability of tidal creek
banks, persist despite the near absence of the crabs that build them."
Coauthor Brent Hughes, assistant professor at Sonoma State University,
noted that the crabs were most abundant in spring and summer, when the pickleweed marshes are at peak production. "This synchrony suggests
that the effect of crabs as consumers is more punctuated than their more chronic effect as engineers," he said.



========================================================================== Elkhorn Slough is one of the largest estuaries in California, with the
largest tract of tidal salt marsh in the state outside of San Francisco
Bay. It has been highly altered by human activities, however, and erosion
along the edges of the tidal creeks and main channel is steadily eating
away at the marsh.

"It's a big issue, because when the marsh erodes away along the tidal
creeks it's a permanent loss," Beheshti said.

The impacts of crabs on marsh biomass and soil structure near tidal
creek banks are likely to make the marsh less resilient to erosion and sea-level rise, presenting a unique challenge to managers. Restoring populations of crab predators, such as herons, racoons, and sea otters,
may be one way to mitigate these negative effects.

"In this system, top-predator recovery is key," said coauthor Brian
Silliman, distinguished professor at Duke University.

This collaborative study brought together marsh ecologists from both
the East and West Coasts who have led the field in exploring how animals
affect the marshes they inhabit. Over the past few decades, the U.S. East
Coast has been the epicenter of studies exploring top-down effects in
salt marshes, and this study is one of the few to explore such effects
in a West Coast salt marsh.

"Southeastern U.S. marshes appear to be a harbinger of what's to come
for marshes along the Pacific coast, with sea-level rise amplifying the
effects of what would otherwise be considered an innocuous crab," said
coauthor Christine Angelini, associate professor at University of Florida.

The authors called for similar long-term studies to be conducted in
other West Coast marsh systems to determine how widespread these crab
effects are. "It'd be great for contextualizing our findings," Beheshti
said. "We'd like to know if Elkhorn Slough is the canary in the coal mine, signaling yet another pathway for accelerated marsh edge loss for one
of California's rarest coastal habitats." This work was supported in
part by grants from the David H. Smith Conservation Research Fellowship,
the Myers Ocean Trust, and Friends of Long Marine Laboratory.

========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Cruz. Original written by Tim
Stephens. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Kathryn M. Beheshti, Kerstin Wasson, Christine Angelini, Brian R.

Silliman, Brent B. Hughes. Long‐term study reveals
top‐down effect of crabs on a California salt
marsh. Ecosphere, 2021; 12 (8) DOI: 10.1002/ecs2.3703 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210809144027.htm

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