Which characteristic is not associated with populations?

Population growth rate is indeed a fundamental characteristic of populations, but it is typically seen as a dynamic measure rather than a static characteristic. The other options, such as population size, population density, and population structure are all specific, measurable characteristics that describe a population at a given time.

Population size refers to the total number of individuals in a given population, providing a clear, quantifiable aspect of that population. Population density indicates how many individuals live in a specified area, giving insight into how crowded a habitat is. Population structure describes the composition of a population in terms of age, sex, and other demographic features, which can provide crucial information about reproductive rates and future growth potential.

While the growth rate is important, it's a measure of how population size changes over time (often related to birth and death rates), rather than a direct description of the population’s current state. Thus, growth rate is not typically categorized with the core static characteristics that define a population at a specific moment.