A trophic cascade regulates salt marsh primary production

Abstract

Nutrient supply is widely thought to regulate primary production of many ecosystems including salt marshes. However, experimental manipulation of the dominant marsh grazer (the periwinkle, Littoraria irrorata) and its consumers (e.g., blue crabs, Callinectes sapidus, terrapins, Malaclemys terrapin) demonstrates plant biomass and production are largely controlled by grazers and their predators. Periwinkle grazing can convert one of the most productive grasslands in the world into a barren mudflat within 8 months. Marine predators regulate the abundance of this plant-grazing snail. Thus, top-down control of grazer density is a key regulatory determinant of marsh grass growth. The discovery of this simple trophic cascade implies that over-harvesting of snail predators (e.g., blue crabs) may be an important factor contributing to the massive die-off (tens of km(2)) of salt marshes across the southeastern United States. In addition, our results contribute to a growing body of evidence indicating widespread, predator regulation of marine macrophyte production via trophic cascades (kelps, seagrasses, intertidal algae).

Fig 1.

Effects of grazer density (G) and marsh zone (Z) on (A) grazing intensity on live cordgrass and (B) aboveground Spartina biomass after 8 months. n = 8 per treatment, and probability values given for two-way ANOVAs testing for main and interactive effects. (Bars = ±1 SE.) All pair-wise comparisons are significant (P < 0.02, Tukey test, all cases).

Fig 2.

A comparison between short-form and tall-form Spartina zones. (A) Predation rates on tethered snails (>10 mm). (B) Natural density of adult Littoraria. (C) Spartina biomass in low density treatments after 22 months. (D) Spartina biomass in medium density treatments (≈ 600 snails per m²) after 22 months. All pair-wise comparisons in B, C, and D were significant at P < 0.01 for the effect of zone in the two-way ANOVA. (Bars = ±1 SE.) For the effect of zone on tethering, see A: x = 121.34, P < 0.01. The proposed mechanism of the marsh trophic cascade is portrayed in the marginal cartoon. Snail predators pictured include (from left to right) a blue crab, terrapin, and mud crab. (Illustrations by Jane K. Neron.)

Fig 3.

Effects of periwinkle grazing on Spartina standing crop and canopy structure in the tall zone after 8 months. (A) Low-density plot. (B) High-density plot. After 20 months, cordgrass in all medium-density plots was reduced wholesale, and the marsh substrate was completely denuded (B).