The organisms of the world comprise and inhabit the biosphere, a realm with diverse, complex, tightly interrelated properties and processes. The biosphere is important because (among other reasons) it feeds life and parts of it release life giving oxygen into the atmosphere. There are many different ways to categorize and subdivide the biosphere into smaller domains, but a simple and commonly used approach partitions the biosphere into a number of smaller systems called biomes based on location, climate, structure and composition (e.g., tropical forests, savannas, agriculture). Within each biome are many smaller ecosystems. An ecosystem is formed by the complex interaction of organisms with each other and their environment. The biosphere can be thought of as the global ecosystem. Organisms in any given ecosystem occupy various functional roles referred to as "niches."

Conservation of energy and the fundamental processes of photosynthesis and primary production are common features among ecosystems. While the following sections focus primarily on terrestrial ecosystems, note that aquatic ecosystems are equally important and complex.

1.1 Photosynthesis

Plants use solar energy in a chemical reaction which converts carbon dioxide and water into carbohydrates. This crucial process is known as photosynthesis. The first step in photosynthesis is the absorption of light, accomplished by chlorophyll pigments found in virtually all plants. Chlorophyll absorbs light in specific wavelength bands. Leaves appear green to the human eye because preferentially more green light is reflected (and hence less absorbed) from the leaf's surface and internal structure than the rest of the visible portion of the spectrum.

Leaves reflect infrared light even more so than green, so if human eyes were sensitive to infrared light, the leaves would appear very bright and reflective to us. Most of the sunlight reflected from leaves is in the near-infrared spectral region. Leaves primarily absorb light in the violet, blue, orange and red wavelengths, and to a lesser extent green wavelengths. This range of wavelengths of light absorbed by green leaves is called photosynthetically active radiation (referred to as PAR or PAR band, from 0.4-0.7 microns or micrometers). Whereas human eyes are limited to observing light in the visible or "optical" part of the spectrum (which coincides with the range of PAR), remote sensing instruments can be designed with detectors sensitive to many different and wide-ranging spectral wavelengths.

PAR energy absorbed by plant leaves is used in photosynthesis to convert water and carbon dioxide into carbohydrates and oxygen. Generalizing this production into one simplified equation you have:

H2O + CO2 + light energy ---> carbohydrate + O2

Even though this formula for photosynthesis looks simple, the process actually consists of a series of complex reactions. The significant point is that the byproducts of photosynthesis, oxygen and carbohydrate molecules, form necessary resources for the continuation of life on Earth. Oxygen released by plants is the primary source of atmospheric oxygen, and plant carbohydrates supply food to many animals.

The reverse of photosynthesis is respiration, the conversion of stored energy (in the form of carbohydrates) into energy needed for maintenance and formation of cell material. The formula for respiration is

carbohydrate + O2 ---> CO2 + H2O + energy for respiration

This released energy is necessary for all metabolic processes. While photosynthesis requires light and therefore only occurs during the day, respiration can occur both day and night.

In the presence of sufficient light, plants assimilate more CO2 than they release. As light levels decrease, the level of CO2 assimilation decreases and the rate of CO2 production (respiration) increases, so in the absence of light there is a net input (to the atmosphere) of CO2.

Respiration is a function present in all living things, including plants, that is largely responsible for the loss of energy in moving up the food chain from primary producers to increasingly higher levels of consumers. As a rule of thumb, approximately 90% of the energy consumed at a level in the chain is respired, leaving only 10% for consumption by all the remaining levels above (see Figure 3.01, discussed in Section 2).

1.2 Primary Production

Plants are the crucial link whereby energy from the Sun's rays gets transformed into essential sustenance for nearly all of the world's consumers, and consequently they are referred to as primary producers. Plants and plant products consist of more than just carbohydrates created by photosynthesis. A number of biochemical processes in plants combine carbohydrates for energy and as a building material with nitrogen, phosphorus, sulfur and magnesium to form other compounds such as protein and fat.

Primary production is quantitatively defined as the rate at which radiant energy is made into and stored as organic substances. It is typically expressed as a mass (or weight) per unit area over a specified time period, usually a year (i.e., tons/acre/yr, kg/m2 /yr). A distinction is made between gross and net primary production. Gross primary production (GPP) is the total rate at which material is produced (energy used for plant respiration is not included in the value). The rate of net primary production or productivity (NPP) accounts for respiration, so the amount of biomass stated is the organic matter produced in excess of what is used by plants (energy used for plant respiration is subtracted from the GPP). Net primary production fuels other forms of life through food web chains.

Net primary productivity (NPP) is directly related to ecosystem conditions. Ecosystems where the environment most favors plant growth will have the highest NPP. Favorable in this context refers to the presence of appropriate and sufficient conditions of sunlight, temperature, water and nutrients. Because conditions around the globe vary, NPP differs between major ecosystems of the world. For instance, in moist tropical areas, NPP is at its largest. Regions at the other extreme of NPP include deserts and arctic tundras, where climate is less conducive to plant growth. On average, NPP tends to decrease from the equator to the poles, but plants have adapted to grow in a wide variety of environments.