broad categories

The subject matter of ecology is normally divided onto four broad categories:

 physiological ecology, having to do with the response of single species to environmental conditions such as temperature or light.

                                                             
population ecology, usually focusing on the abundance and distribution of individual species and the factors that cause such distribution.
                                                

community ecology, having to do with the number of species found at given location and their interactionsa. 

ecosystems ecology, having to do with the structure and function of the entire suite of microbes, plants, and animals, and their abiotic environment, and how the parts interact to generate the whole.