The effect of Mating Behavior and Temperature Variation on the Critical Population Density of a Freshwater Copepod

At low density, population growth rates of dioecious zooplankton depend on the encounter rate of potential mates, resulting in a demographic Allee effect and a critical density for population establishment and persistence. Empirical evidence confirms a critical density for the calanoid copepod Hesperodiatomus shoshone , but existing estimates of the critical density span an order of magnitude. Combining three-dimensional video analysis of mating behavior with life history data from natural populations we estimated H. shoshone critical density to be 0.44–1.44 m−3. The critical density was highly dependent on body size, primarily as a result of the latter's influence on swimming speed. Swimming speed also depended on temperature, increasing > 25% as temperature increased from 5°C to 16°C. Rapid swimming (1.25–2.4 cm s−1) and the ability to follow pheromone trails greatly improved the ability of H. shoshone to find mates. The large effect of temperature on mating behavior means that environmental variation can have a major effect on critical density and indicates that recovery or colonization events may be more likely to succeed in warmer lakes and/or warmer years. Considering the potential for critical densities to vary with environmental conditions is important for understanding how these thresholds determine population establishment and persistence in sexually reproducing aquatic organisms and other populations subject to Allee effects.