Population densities can be estimated from samples: estimating population densities is easiest for sedentary organisms, counting mobile organisms is much more difficult because individuals move into and out of census area. (write equation here). Birth and death rates can be estimated from population density data (write equation here). Individuals born at the same time cohort: the number of a population still alive at a given period of time in the future. 54. 2 how do ecological conditions affect life histories: an organism"s life history describes how it allocates its time and energy among the various activities that occupy its life, ecological interactions influence the evolution of life histories. All populations have the potential for exponential growth: all populations have the potential for explosive growth. As the number of individuals in a population increase, the number of new individuals added per unit of time accelerates (even if the rate of increase is expressed on a per individual basis remains constant).