Effects of livestock grazing on saltmarshes at Ossabaw Island and Sapelo Island, Georgia, USA


Spartina dominated salt marsh may provide grazing for livestock. Grazing or grazing abandonment, have implications for salt marsh plant communities and associated organisms. Productivity of three main constituent plant species in three salt marsh systems (ungrazed, grazed and formerly grazed) in coastal Georgia, southeast USA , were measured over two years; mud fiddler crab Uca pugnax densities were compared; and the effects of simulated grazing were evaluated in a stand of smooth cordgrass S.alterniflora.

Two localities were selected: Ossabaw Island marshes (livestock-grazed for over 30 years; with some areas where grazing ceased on 4 May 1972 - ‘formerly grazed marsh'); and Sapelo Island (ungrazed marsh). The grazed and formerly grazed areas comprised a mixed S.alterniflora, glasswort Salicornia virginica and saltgrass Distichlis spicata community; the ungrazed marsh lacked glasswort.

Plant material was sampled monthly (within 10, 1 x 1 m quadrats). Above-ground plant production, potential detritus (i.e. dead plant material potentially contributing to the esturine food web) and plant mineral concentrations were assessed. During July 1973, mud fiddler crab densities were estimated.

On Sapelo, a pure stand of S.alterniflora was also selected to determine the effects on plant productivity of simulated grazing. Three plots (20 x 150 m) were cut (using a dual wheeled tractor with a sickle bar mower) in 1972, one each on l May, l July and l September. On 24 May 1973, five 1 m² samples of Spartina were cut within each plot and an adjacent control; stem density and height were recorded.

Living material: Yearly aerial crop was highest in the ungrazed marsh (average dry weight 583 g/m²/yr), then formerly grazed (322 g) and grazed marsh (222 g). S.alterniflora production peaked during September (achieving a maximum of about 320 g/m² in ungrazed marsh). D.spicata was most plentiful in August (approx. 120 g/m² formerly grazed; 50 g/m² grazed). S.virginica displayed little annual variation but had consistently higher production in all months in ungrazed (approx. 190-490 g/m²) compared to formerly grazed (40-100 g) and grazed (5-35 g) marsh.
Potential detritus: Annual detritus production was highest in the ungrazed marsh, followed by formerly grazed then grazed. For S.alterniflora average dead material dry weight was highest in late winter (January-April) peaking in January in ungrazed marsh (312 g/m²) compared to the formerly grazed (170 g/m²) and grazed (160 g/m²). For D.spicata it was highest in summer (August) in the formerly grazed marsh (65 g/m²).
Mineral composition: The presence of livestock did not affect plant mineral composition.
Fiddler crabs: Highest crab densities were found in the ungrazed marsh. Average number per m² was: grazed -116 (±16); formerly grazed - 149 (±16); ungrazed - 176 (±17). Presumably livestock trampling degraded crab habitat in the grazed area.
Simulated grazing: Highest Spartina stem densities occurred in the area cut most recently (1 Sept), the lowest in the control area; there were significantly fewer stems more than 0.5 m in height (7 stems/m²) than the control (187 stems).
In summary, grazing of salt marshes by ungulate livestock reduced primary production, detritus production and crab densities.

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