Beyond population regulation and limitation

The study of population dynamics addresses three questions that are not always separated in discussions with empirical data. Two questions address population regulation. What stabilises population density is the first question, and, in spite of much theory, little progress has been made in answering this question empirically. The assumption of an equilibrium density is impossible to test and direct experimental tests to answer this question are rare. What prevents population growth is a second question, and is the classic question of population regulation. To answer this question requires an increasing population, and, with adequate experimental manipulations, the densitydependent factors preventing increase can be identified. Surprisingly, answering this question has provided little assistance in solving practical problems in population dynamics, possibly because most populations are rarely in the state of growth and show a limited range of densities. What limits population density in good and poor habitats is a third question, which addresses population limitation rather than regulation, and has been the most useful question for empirical ecologists to ask. Population limitation admits of little theory and no elegant models, and highlights the gap between theory and practice in much of ecology. Defining the question clearly and adopting an experimental approach with clear alternative hypotheses will be essential to avoiding the controversies of the past while building useful generalisations for the practical problems of population management. Introduction The problems of population dynamics contain, without question, the most controversial and long-standing issues in population ecology. The battles between the densitydependent school and the density-independent school began in the 1920s, peaked in the 1950s and early 1960s, and arose again in the 1990s. Long-standing controversies in science are typically caused by scientists who operate under different paradigms (Kuhn 1970), and thus normal types of empirical tests cannot resolve the controversies. Since many of these controversies have been over words that have been used in different ways, I begin by defining terms following Sinclair (1989). Population regulation: the process by which a population returns to its equilibrium density. Population limitation: the processes that set the equilibrium density. Population dynamics: the analysis of the causes for change in population density, including both regulation and limitation. Given that we already have extensive reviews of population regulation and limitation (Sinclair 1989; Krebs 1995, Turchin 1999; Murray 1999; White 2001), the first question that I must answer is what new could possibly be said on this issue. Why should anyone keep reading this paper? The answer is that in this review I will adopt an empirical view of the problems of population dynamics and ask three questions: (1) What are the questions we wish to answer about population limitation and regulation? (2) What data are relevant to answering these questions? (3) How successful have ecologists been in achieving these goals of understanding population changes? I adopt this empirical approach because the theory of population regulation and limitation has been elegantly presented in many papers beginning with Nicholson (1933) and formalised more clearly by Enright (1976), May (1981, 1989), Murdoch (1994), Sinclair (1989) and Sinclair and Pech (1996), among others. The problem does not rest in the theory but in the application to real populations, and that will be my point of departure. What is striking about most theoretical discussions of population limitation and regulation is that they present no data, only idealised graphs (e.g. Berryman et al. 1987; Sinclair and Pech 1996). This is the first clue that there is a gaping chasm between the theory and the real world. We clearly need to close this gap by bringing theory and data together, as was achieved by Caughley et al. (1987) in their study of kangaroo populations. I will discuss here the issues of population dynamics within a single-species population. Some of these same issues arise in community ecology in comparisons between species.