Life History Patterns in Zooplankton

Abstract

Freshwater zooplankton exhibit a continuum of compromises in their means of meeting the presumably conflicting demands of maximizing the potential for rapid increase, predator avoidance, and competitive ability. Rotifers and cladocerans both alternate parthenogenetic and sexual reproduction and attain high rates of increase relative to copepods. The rapidly developing small-clutch rotifers surpass more slowly developing large-clutch cladocerans in rmax. Copepods appear to consume food in a wider range of particle sizes, exhibit a greater capacity for selective feeding, and possess a greater ability in predator avoidance. Hence, ranking of three major taxa of freshwater zooplankton with respect to opportunism is: rotifers > cladocerans > copepods. These results are utilized to interpret some principal biogeographic trends in plankton composition, including the rarity or nearly complete absence of rotifers and cladocerans from the open oceans and deeper waters of large lakes. Patterns in the timing and sharpness of seasonal abundance peaks are also consistent with this interpretation, although complexities in timing occur owing to intrataxon variation in keying to seasonal fluctuations.