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-
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-