ecologists usually study a specfic populaions of organisms rather than studying the entire species why?

Answer 1

One reason is practicality: unless the species has a very limited distribution, (i.e. consisting of one or a few small populations), it's just not feasible to observe and study in its entirety. Think about a bird like a turkey vulture: it ranges across North America and into Mexico and further south--it would take lifetimes to study all of the populations. Another reason is that ecologists study organisms in their relation to their environment--both living and non-living. The conditions for a given population usually differ, if only slightly, from that of another population, and so the ecologist is always looking at a specific set of conditions in order to understand what is going on. From there he/she may generalize beyond the population, but understanding always starts with local observation and analysis.