Geoscience Reference
In-Depth Information
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 den-
sity 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 com-
mon, 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 ecol-
ogy. It is obvious that unlimited capacity of all animals to increase in popula-
tion 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 pop-
ulation 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 popu-
lation growth that can maintain densities at or near an equilibrium value
indefinitely.
For more than 50 years ecologists have debated whether population densi-
ties 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-
