Chapter 7 – Using Density-Manipulation Experiments to Study Population Regulation

This chapter discusses that the nature of population stability has long been a subject of debate in ecology. Experimental studies of regulation discuss general methodological issues such as the choice of appropriate time scales, spatial scales, and treatment levels. Experiments lasting many generations are ideal, because these can show not only whether density-dependent forces operate, but also whether their action in fact leads to convergence and stability. The spatial scale of density manipulations is a key and difficult issue. Ideally, densities are manipulated within replicated areas that correspond to closed populations of the target species, and of any natural enemies that are also subjects of the experiment. Since a realistic choice of initial densities is crucial to meaningful results, it is essential to present data on the range of densities encountered in the field. Analytical techniques must of course be tailored to each study. Compared with resources, predation may be a more difficult regulating force to detect with experiments. Cannibalism and interference competition can readily be studied with density manipulations, although a problem mentioned earlier may arise: these processes often occur among siblings, during the aggregated immature stages of a species that has mobile adults. Clearly, some experiments will turn up little evidence for density dependence, even if they are well designed and have adequate statistical power. Ideally, the kinds of studies described in the chapter will someday be done on enough species, some eruptive and others more stable, that one can begin to generalize the prevalence of various modes of population regulation in nature.