Ecology studies how organisms interact with one another and with their physical environment. Those interactions can be organized into nested levels.

An organism is one individual living system. A population is a set of individuals of the same species living in the same area and capable of contributing to a shared pool of descendants. A community is the set of populations of different species living and interacting in an area. An ecosystem includes the biological community together with the nonliving physical and chemical environment through which energy and matter move.

These levels answer different questions.

• Organismal ecology asks how an individual tolerates temperature, obtains resources or responds to its surroundings.
• Population ecology asks why the abundance of one species rises or falls.
• Community ecology asks how interactions among species shape diversity and composition.
• Ecosystem ecology asks how energy and chemical elements move through living and nonliving components.

The levels are connected but not interchangeable. A drought can reduce the growth of individual plants, lower plant population size, alter competition among species and change ecosystem productivity. The same event therefore has consequences at several scales.

A useful hierarchy is

$$\boxed{\text{organism} \rightarrow \text{population} \rightarrow \text{community} \rightarrow \text{ecosystem}.}$$

The purpose of these categories is not merely naming levels. They separate different kinds of mechanisms so that ecological explanations can be built without confusing individual physiology, population dynamics and ecosystem flows.