A Probabilistic Approach to the Dynamics of Natural Populations of the Chydoridae (Cladocera, Crustacea)

Populations of four chydorids in the littoral of Lawrence Lake, Michigan, were studied through an entire year. Three species appeared in spring after a winter absence. Their population sizes were smallest in summer and greatest in late summer or fall. Numbers of the fourth species, Chydorus sphaericus, peaked in spring and dropped to a low level in the summer, then rose through fall to a winter plateau. Rates of population processes were estimated with the egg—ratio technique. This required monitoring littoral temperature and determining developmental times of chydorid eggs over a range of temperatures. Predicted rates of birth were higher during summer when rates of observed change fluctuated considerably. A probabilistic method, based on reproductive potential, showed relatively large probabilities for changes observed from late fall to early spring in populations of Chydorus. Most population changes in the other three species, and in Chydorus during the summer, had zero probability. Birth alone could explain few population changes. Most population loss appears to be predatory; emigration and “natural” mortality are not important.