Cyclic and Stable Populations: Plankton as Paradigm

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Abstract
We analyzed over 20 study-years of data from populations of Daphnia and algae in a wide variety of field situations. These systems display three types of dynamic behavior: both populations stable; both populations cyclic; and Daphnia cyclic but algae stable. The last pattern occurs whether we analyze the total amount of algae or only edible algae. There is evidence that this range of dynamics arises from the interaction between Daphnia and its food supply, occurring in systems that are structurally the same; that is, differences in biological rates or time delays, alone, can explain the existence of different dynamic classes. This is particularly the case when different classes occur in the same species in the same environment in different years, or in similar and adjacent habitats at the same time. The cycles thus appear to be internally driven, rather than resulting from external, cyclic forcing factors. These findings support a basic premise of most mathematical models in ecology. The broad dynamic patterns in cyclic field populations of Daphnia are similar to those found in laboratory populations. The two sets of cycles have very similar periods and (small) amplitudes; both are single-generation cycles (i.e., the period of a cycle is one generation); and both are caused by dominance and suppression, whereby each cohort suppresses reproduction until its density declines sufficiently to allow production of another cohort. The demographic features of laboratory and field cycles are also similar in detail. Since algae are dynamic in the field but not in the laboratory, we cannot conclude that the mechanisms driving laboratory and field cycles are identical. Our hypothesis is that the presence or absence of cycles is determined by the relationships between time delays in Daphnia and other rates in the interacting populations. There is, however, no obvious environmental factor affecting these rates and delays, thereby determining which dynamic class a particular system fits at a particular time. It does not appear that change in the average temperature is the critical factor. Similar single-generation cycles appear to occur in other systems and may be driven by similar dominance-and-suppression mechanisms.