POPULATION REGULATION IN THEORY AND PRACTICE - THE ROBERT-H-MACARTHUR-AWARD-LECTURE PRESENTED AUGUST 1991 IN SAN-ANTONIO, TEXAS, USA

Population regulation is a fundamental process related to most phenome na in ecology, including evolutionary ecology. I review the conceptual basis for defining regulation as bounded fluctuations in abundance, i n contrast to the unbounded fluctuations of random-walk populations. R egulation arises as a result of potentially stabilizing density-depend ent processes, even when brought about by ''non-equilibrium'' mechanis ms. Although the phenomenon is unambiguous in theory, detecting regula tion by finding evidence for density dependence in a series of populat ion estimates faces unsolved statistical problems. So, while there is growing evidence for widespread regulation, severe detection problems remain. I illustrate these with data from bird populations. Whether re gulation is typically achieved by local stabilizing mechanisms or via metapopulation dynamics remains to be determined. I summarize recent s tudies on a particularly well-regulated system-red scale (Aonidiella a urantii) and its controlling parasitoid, Aphytis melinus. We tested an d failed to find evidence for eight hypotheses that might account for the system's stability, including spatial heterogeneity in attack rate s, a refuge, and metapopulation dynamics. We also failed to find evide nce for density-dependent parasitism. but such density dependence migh t be still be present. Recent laboratory and modeling studies have unc overed a number of other potentially stabilizing mechanisms centering on the response of individual Aphytis to their size-structured host. T his plethora of size- and stage-dependent interactions leads naturally to a consideration of the factors controlling Aphytis' size-dependent behavioral decisions, and consequently to the elaboration of size-str uctured models. The latter provide a vehicle for bringing together inv estigations of selection of life histories, and population dynamics. T his is illustrated by a model of Aphytis and red scale dynamics that c an explain a dramatic case of competitive displacement. The red scale/ Aphytis system exemplifies a particularly challenging problem in popul ation regulation, namely to account for the co-occurrence of stability and severe suppression of the prey population. A potentially generic solution is to assume stabilizing density dependence in the parasitoid or predator population; however, this has the consequence of increasi ng the host or prey population equilibrium. My colleagues and I have s hown that observed prey densities in a plankton system are too low for such a mechanism to be operating. Further work is needed to test this and other hypotheses.