Chapter 6 Detecting Stability and Causes of Change in Population Density Other chapters in this volume focus on various methods for quantifying density or other population qualities. Here I focus on the techniques ecologists use to extract the dynamics | Chapter 6 Detecting Stability and Causes of Change in Population Density Joseph S. Elkinton Other chapters in this volume focus on various methods for quantifying density or other population qualities. Here I focus on the techniques ecologists use to extract the dynamics of population systems from such data. Population ecologists seek to explain why some animals are rare whereas others are common as well as what accounts for observed changes in density. They have focused on two analytical questions Are populations stabilized by negative feedback mechanisms and what are the causes of density change Here I examine some of the techniques that have been developed to answer these questions. The concept of a balance of nature goes back to the very early days of ecology. It is obvious that unlimited capacity of all animals to increase in population size or density is inevitably checked by competition for resources or the action of natural enemies. If any of these factors cause systematic changes in survival or fecundity of a population as the density increases they are said to be density dependent. If fecundity or survival decreases sufficiently as the population increases then the per capita birth rate will decline to a value equal to or less than the per capita death rate and population growth will stop. In this manner density-dependent processes constitute negative feedbacks on population growth that can maintain densities at or near an equilibrium value indefinitely. For more than 50 years ecologists have debated whether population densities of most species are stabilized by such density-dependent factors. Howard and Fiske 1911 were the first to articulate the idea that populations cannot long persist unless they contain at least one density-dependent factor that causes the average fecundity to balance the average mortality. Other early pro 192 JOSEPH S. ELKINTON ponents of this idea were Nicholson 1933 1957 and Lack 1954 . In contrast Andrewartha and Birch 1954 .
