Submersed macrophytes and grazing crayfish: an experimental study of herbivory in a California freshwater marsh

Grazing by the crayfish Procambarus clarkii (Girard) on the abundance of submersed sago pondweed, Potamogeton pectinatus (L.), was quantified during 1984 through 1986 at Coyote Hills Marsh (Alameda County, California, USA). Annual marsh surveys indicated that P. pectinatus abundance declined (from 70% to 0% of the marsh surface), whereas P. clarkii abundance increased (from 1.6 to almost 3.0 crayfish trap⁻¹ week⁻¹). Crayfish in a 1425 m² trapping grid were non-aggregated when pondweed was abundant but became aggregated in the remaining pondweed beds as pondweed abundance decreased. In situ exclusion and enclosure experiments using crayfish densities of 0 to 3 individuals m⁻² were used to determine if crayfish reduced pondweed. Exclusion of crayfish from areas without pondweed, beginning in midsummer 1984, resulted in plant regrowth; addition of crayfish to these enclosed plots reduced pondweed. Exclusion of crayfish for the entire growing season in 1985 resulted in pondweed persistence in exclusion plots until August: in contrast, pondweed within adjacent, open control plots was eliminated by mid-June. Enclosure experiments showed a strong positive relationship between crayfish density and pondweed clearance. This study demonstrates that grazing crayfish can reduce and, in some cases, eliminate macrophytes from freshwater marshes.