  The golden age of theoretical ecology (1920-1940)—to borrow the title of a book edited by Scudo and Ziegler—consisted primarily of population  ecology.[10]  The next generation saw a shift of theoretical interest to community ecology. In recent years, interest has reverted to population ecology, sometimes in the form of metapopulation models (consisting of a set of populations with migration between them).  Models in population ecology are based on representing an ecological system as the set of populations (of the same or different species) it consists of. Each population, in turn, consists of potentially interacting individuals of a species. Populations may be characterized by their state variables (parameters representing properties of the population as a whole, for instance, size, density, growth-rate, etc.) or by individual variables, that is, the properties of the individuals in them (for instance, individual fecundity, interactions,  etc.).[11]  Classical population ecology was restricted to the study of state-based models primarily because of the requirement that models be tractable, so as to permit predictions and  explanations.[12]  State-based models will be the focus of this section; individual-based models will be discussed in  Section 5.1.  Classical population ecology is the part of ecology that is theoretically the most developed. The central issue of interest in population ecology is the change in the size of populations (their “abundances”) over time.