We have structured this entire unit using the natural levels of organization seen in ecological interactions. We started by discussing how individual organisms of the same species interact with one another in populations. These interactions led to the distinctive characteristics of populations that ecologists measure and observe, including growth rates, sizes, distributions, and so on. We then moved to a discussion of how populations of different species interact with one another in communities. These interspecific interactions, as well as the intraspecific interactions seen inside populations themselves, are the basis for much of the evolutionary process in the world. Last, we discussed the interactions between communities and their habitats in ecosystems. These biotic-abiotic interactions provide the rest of the substance upon which evolutionary forces generally act.
Each level of organization in ecology has its unique attributes and characteristics, most of which cannot be measured, or even understood, at the other levels. For example, an individual cannot have a growth rate, as this term is understood for a population. A growth rate is calculated by subtracting the death rate from the birth rate. Since an individual only has one birth and one death, the idea of an individual growth rate in this sense is absurd. Absurd, we tell you!
At the same time, it is impossible to measure the life history of a population or community. These are characteristics of individuals. Therefore, recognizing and understanding the different levels of organization in ecology can help the researcher or student focus on the most important aspects of that level. It is impossible to study and comprehend everything at once. You know it, and we know it.
For this reason, too, it is important to identify which level of organization is under consideration. In addition, having a good concept of one level can help you better understand the others. We saw this when we transitioned from populations to communities. Without a good understanding of what a population is and how it functions, your understanding of a community would be greatly reduced. The same is true for understanding an ecosystem. Recognizing the natural levels of organization in the other areas of biology can be equally useful.