Ecology that focuses on dynamic changes occurring in one species is ecology. 1) population 2) applied 3)systems

1) population

Explanation:

Ecology of population is a sub-discipline of ecology that concerns with the dynamics of population of the species, interaction of the population with the environment, with other species and among members of the same species. It studies how size of the population change over area and time.  

Therefore, on the basis of the above description, ecology that focuses on dynamic changes occurring in one species is called as population ecology.