Interannual variation in seasonal predation: Its origin and ecological importance1

Abstract

Between‐year variation in the springtime mortality of the copepod Diaptomus sanguineus is quantified using 8 yr of data from a single population in Bullhead Pond, Rhode Island. The mortality is ascribed to increased fish feeding activity when the water warms. The timing of the onset of predation and its intensity are quantified using the seasonal dynamics of adult copepod sex ratio. Females are more vulnerable to predation than males, and the slope of a plot of sex ratio vs. time is >0 when the fish begin to influence the copepod population. The magnitude of the slope is an index of predation intensity.The annual onset of predation and its intensity are each significantly correlated with fish density, and fish density is a function of pond volume. Pond volume fluctuates between years, driving the dynamics of this predator‐prey interaction. Annual variation in pond volume is related to groundwater level, which in turn is related to annual rainfall. Groundwater and rainfall records from the USGS and NOAA permit estimates of the extent of variation in springtime copepod mortality over a 38–45‐yr period.