Introduction

For more than 100 years, ecologists have been estimating populations of animals, beginning with those of economic value, and have tried to make sense of the resulting data. How to make sense of quantitative population data is not immediately clear. Once an ecologist has two successive estimates of population size, he or she follows the first law of quantitative ecology, which is to divide one number by the other, producing the finite population growth rate (λ) that Sibly & Hone (Chapter 2) described. However, what to do next?

This is the critical step. Being good scientists, most ecologists would wish to predict the size of the population growth rate and would proceed in one of two directions to do this. First, they could adopt the density paradigm of Sibly & Hone (Chapter 2) and plot population growth rate against population density. (The concept of a paradigm as promulgated by Thomas Kuhn (1970) has been used in many ways, and one might argue that the paradigms discussed here are better labelled as ‘conceptual approaches’. I have no quarrel with this comment and I use the term ‘paradigm’ as shorthand for what ecologists do (cf. den Boer & Reddingius (1996).) Alternatively, they could adopt the mechanistic paradigm and plot population growth rate against an ecological factor, such as the amount of food available per capita, which may explain the change. What are the problems and what are the advantages of going in one direction rather than another?