A forest researcher once mentioned the dialogue between trees and the atmosphere. What does this mean? The word “dialogue” refers to interactions between living organisms and nonliving matter, the gases in the atmosphere. In a broad sense, such interactions are a fundamental property of ecosystems, the highest of several structural levels into which life is organized. An ecosystem (for example, a Cascade Mountain rain forest), as well as all the nonliving, physical components of the environment that affect the organisms, such as air, soil, and sunlight. The ecosystem and the structural levels below it form a hierarchy, with each level building on the ones below it. Below the ecosystem level, all the organisms in a rain forest are collectively called a community. Below the community, a group of interbreeding individuals of one species, a group of flying squirrels in our example, is called a population. Below population in the hierarchy is the organism, an individual living thing. (The flying squirrel, our organism, is gliding, its skin pulled flat between its outstretched legs.)

Below the organism level, life’s hierarchy unfolds within the individual organism. The flying squirrel’s body consists of several organ systems, such as a circulatory system, an excretory system, and a nervous system, shown here. Each organ system consists of organs. For instance, the main organs of the nervous system are the brain, the spinal cord, and the nerves, which transmit messages between the spinal cord and other parts of the body.

As we continue downward through the hierarchy, each organ is made up of several different tissues, each of which consists of a group of similar cells. A cell is unit of living matter separated from its environment by a boundary called a membrane. Each tissue has a specific function, which is performed by the cells that compose it. The nervous tissue that makes up most of the brain, for example, consists of nerve cells. The nervous tissue in the squirrel’s brain has millions of microscopic nerve cells organized into a communication network of spectacular complexity. The nerve cells transmit signals that coordinate the squirrel’s body parts, such as the muscles that stretch out its legs during a glide.

Finally, we reach the molecular level in the hierarchy. We show as our example DNA (deoxyribonucleic acid). DNA molecules provide the blueprints for constructing the organism’s other important molecules and transmit this information, as genes, from parents to offspring. A molecule is a cluster of atoms, the smallest particles of ordinary matter. In the computer graphic which illustrates only segment of one DNA molecule, each of the spheres represents an atom. The overall three-dimensional shape of a DNA molecule is a very long double helix, two chains coiled around each other.

Life’s hierarchy builds from molecules to ecosystems. It takes many molecules to make a cell, many cells to make a tissue, several kinds of tissues to make an organ, and so on. Most biologists specialize in the study of life at a particular level. For instance, a researcher analyzing the body postures of a gliding squirrel focuses on the organism level. However, the same researcher often works at a higher or lower level as well. For example, under standing gliding posture may require studying, at the organ-system level, the interaction between muscles and bones. Indeed, the deepest biological insights often come from discovering new connections between the different levels in life’s hierarchy. The full spectrum of the hierarchy, from molecules to ecosystems, encompasses the scope of biology. With this in mind, let’s see how biological scientists go about their work. Although we focus on examples of outdoor research in forest ecosystems, the same underlying scientific approach is used in all types of biological research.