Energy flow is the capture of solar energy through photosynthesis, which is the process used by green plants to convert radiant energy from the sun into organic compounds such as glucose. With the known exception of organisms that live along thermal vents in the deep ocean floor, all organisms--including humankind--nurture themselves, directly or indirectly, on the products of photosynthesis.
At each stage or loop in the food chain or energy pyramid, organisms turn the energy into growth, heat, and activity. For example, human thought, feelings, growth, reproduction, and activity are all fueled by radiant solar energy captured initially by green plants.
Often we think of forms of energy as interchangeable or replaceable. This is true of energy in thermal, mechanical, or electrical forms. But there is no replacement for photosynthetic or biological energy.
A small fraction of the solar energy striking the earth is captured by photosynthesis and stored as chemical energy in organic compounds. A fraction of this energy is in turn used by humankind to nurture itself and to provide biomass fuels and shelter.
We can increase energy flow in a given area by lengthening one or more legs of the triangle that forms the base of solar conversion of the energy tetrahedron: length of the growing season, volume of plants, or leaf area. All of these factors are related to other ecosystem processes such as the water cycle, mineral cycles, and community dynamics.
We can increase energy flow into food crops, fiber, and forage by lengthening the growing season (with irrigation, for example). We can increase the volume and leaf area of plants (by adding nitrogen fertilizer, for example). However, long-term solutions to low energy flow must take into account all of the ecosystem processes, and must enhance biodiversity as well.
The shape of the human economy in the United States is like the energy pyramid turned upside down. Less than 2 percent of our Gross Domestic Product is directly based on photosynthesis or biological energy (the solar dollar, or net profit to producers from sales of food and fiber). Yet this biological energy, in the form of human creativity and activity, drives the entire economy.
For many economists, photosynthesis is a minor sector. When the ecosystem is considered as a whole, it is the primary sector.