Published online by Cambridge University Press: 20 May 2010
A long-term objective of population biology is to explain the spatiotemporal variations in abundance of organisms by understanding the factors that limit both distribution and changes in abundance. In general, theory predicts that a major determinant of distribution and dynamics is the instantaneous population growth rate, presented as r (where r = ln(λ) = ln(Nt+1/Nt). Some models reveal the obvious, such that species will tend not to exist where their population growth is consistently negative and there is no immigration (see Chapter 2). But the models also expose intriguing dynamics; for example, simple single-species nonlinear models reveal that dynamics can vary from stability through oscillatory to chaotic behaviour simply by subtle changes in the population growth rate (May 1976). As such, it is not surprising that when we incorporate interspecific interactions, the stochastic vagaries of environmental conditions and dispersal, we reveal a Pandora's box of dynamical behaviours.
In the empirical literature, the population growth rate parameter does not enjoy the same importance as it does in the theoretical literature. Rarely do workers make an estimate of the intrinsic growth rate parameter (r) or its empirical equivalent, the maximum growth rate (rmax) which is simply the maximum rate of growth observed within a time-series. Changes in the observed growth rate at a specific time (rt) may be recorded along with the factors associated with the reproductive output of individuals, but studies tend not to estimate the extent to which the growth rate is reduced by density dependent regulatory factors.