Population Regulation" Old Arguments and a New Synthesis

Population regulation is one of the central organizing themes in ecology. Ecological populations are dynamical systems, in which state variables change with time, or fluctuate. An unregulated system is not characterized by any particular mean level around which it fluctuates. In 1994, DJIA mostly fluctuated between 3600 and 4000, but in the next year or a year after that, DJIA increased beyond 4000 and then, eventually beyond 5000. As a result of infusion of ideas from nonlinear dynamics, the notion of equilibrium in ecological theory has been replaced with a more general concept of attractor. The idea of stochastic boundedness, however, is more general than stationarity, because it does not imply convergence in distribution. Density dependence is a dependence of per capita population growth rate on present and/or past population densities. In practice, the population regulation debate has focused primarily on whether density dependence in the per capita growth rate can be demonstrated statistically and is of the right sign. The primary engine driving the density-dependence controversy has always been a lack of empirical support for population regulation. Novel approaches, influenced by recent developments in nonlinear dynamics, give quantitative tools to probe the structure of population regulation. Demonstrating density dependence is no longer a key issue in population ecology, but plenty of unanswered questions remain. Most importantly, although it is known that most field populations are characterized by equilibria, little is known about the relative frequency of mechanisms that bring these equilibria about.