Final answer:
A change in the economy does not directly affect a population's size. Biological factors, availability of resources, reproductive success, and environmental conditions like temperature are direct determinants.
Step-by-step explanation:
The factor that does not affect a population's size among the options provided is a change in the economy (option b). While the economy can indirectly influence a population's growth or decline through human activities, it is not a direct biological factor like the others listed. In contrast, the availability of resources (option a) affects a population's size by influencing the carrying capacity and how many individuals an environment can support. Reproductive success (option c) directly affects population growth because a high reproductive rate usually translates to an increase in the population size. Lastly, temperature fluctuations (option d) are an example of an environmental factor that can impact survival and reproduction, thus influencing the population's size.
Overall, when considering the various factors affecting population size, one must include the availability of nutrients, the speed of water flow, the density of the population, the frequency of beneficial alleles, the effect of chance on a population's gene pool, and successful reproduction. Natural events, like earthquakes, competition for resources, and demographic characteristics such as birth and death rates also play significant roles in determining the size of a population.