Population Regulation in Theory and Practice

Population regulation is a fundamental process related to most phenomena in ecology, including evolutionary ecology. I review the conceptual basis for defining regulation as bounded fluctuations in abundance, in contrast to the unbounded fluctuations of random—walk populations. Regulation arises as a result of potentially stabilizing density—dependent processes, even when brought about by “non—equilibrium” mechanisms. Although the phenomena is unambiguous in theory, detecting regulation by finding evidence for density dependence in a series of population 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 regulation is typically achieved by local stabilizing mechanisms or via metapopulation dynamics remains to be determined. I summarize recent studies on a particularly well—regulated system–red scale (Aonidiella aurantii) and its controlling parasitoid, Aphytis melinus. We tested and failed to find evidence for eight hypotheses that might account for the system's stability, including spatial heterogeneity in attack rates, a refuge, and metapopulation dynamics. We also failed to find evidence for density—dependent parasitism, but such density dependence might be still be present. Recent laboratory and modeling studies have uncovered a number of other potentially stabilizing mechanisms centering on the response of individual Aphytis to their size—structured host. This 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—structured models. The latter provide a vehicle for bringing together investigations of selection of life histories, and population dynamics. This is illustrated by a model of Aphytis and red scale dynamics that can explain a dramatic case of competitive displacement. The red scale/Aphytis system exemplifies a particularly challenging problem in population 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 increasing the host or prey population equilibrium. My colleagues and I have shown 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.