An aggregation of turtles in a Florida spring yields insights into effects of grazing on vegetation

AbstractAn aggregation of Suwannee Cooters (Pseudemys concinna suwanniensis) prompted an investigation of their effects on vegetation in Blue Spring, a 2nd-magnitude spring in Gilchrist County, Florida. We counted turtles and documented % cover and wet biomass of vegetation in September and October 2013. The maximum mean density of turtles (1566 ± 319 turtles/ha) was recorded near the spring vent during the 1st survey. Mean density among all reaches of the spring run decreased from 421 ± 133 to 145 ± 58 turtles/ha during the 30-d period between surveys. Percent cover and wet biomass of Hydrilla verticillata, Sagittaria kurziana, and Vallisneria americana decreased significantly between surveys, which indicated that turtles reduced the quantity of vegetation in the spring. Relatively little vegetation was lost downstream as turtles grazed, so removal rates were converted to grazing rates. Mean grazing rates on H. verticillata, S. kurziana, and V. americana were estimated to be 850, 275, and 78 g wet mass (...