Ecological Productivity
One of the most important properties of an ecosystem is its productivity, which is a measure of the rate of incorporation of energy by plants per unit area per unit time. In terrestrial ecosystems, ecologists usually estimate plant production as the total annual growth—the increase in plant biomass over a year. Since productivity reflects plant growth, it is often used loosely as a measure of the organic fertility of a given area.
The flow of energy through an ecosystem starts with the fixation of sunlight by green plants during photosynthesis. Photosynthesis supplies both the energy (in the form of chemical bonds) and the organic molecules (glucose) that plants use to make other products in a process known as biosynthesis. During biosynthesis, glucose molecules are rearranged and joined together to become complex carbohydrates (such as cellulose and starch) and lipids (such as fats and plant oils). These products are also combined with nitrogen, phosphorus, sulfur, and magnesium to produce the proteins, nucleic acids, and pigments required by the plant. The many products of biosynthesis are transported to the leaves, flowers, and roots, where they are stored to be used later.
Ecologists measure the results of photosynthesis as increases in plant biomass over a given time. To do this more accurately, ecologists distinguish two measures of assimilated light energy: gross primary production (GPP), which is the total light energy fixed during photosynthesis, and net primary production (NPP), which is the chemical energy that accumulates in the plant over time.
Some of this chemical energy is lost during plant respiration (R) when it is used for maintenance, reproduction, and biosynthesis. The proportion of GPP that is left after respiration is counted as net production (NPP). In an ecosystem, it is the energy stored in plants from net production that is passed up the food chain/web when the plants are eaten. This energy is available to consumers either directly as plant tissue or indirectly through animal tissue.
One measure of ecological productivity in an ecosystem is the production efficiency. This is the rate of accumulation of biomass by plants, and it is calculated as the ratio of net primary production to gross primary production. Production efficiency varies among plant types and among ecosystems. Grassland ecosystems which are dominated by non-woody plants are the most efficient at 60-85%, since grasses and annuals do not maintain a high supporting biomass. On the other end of the efficiency scale are forest ecosystems; they are dominated by trees, and large old trees spend most of their gross production in maintenance. For example, eastern deciduous forests have a production efficiency of about 42%.
Ecological productivity in terrestrial ecosystems is influenced by physical factors such as temperature and rainfall. Productivity is also affected by air and water currents, nutrient availability, land forms, light intensity, altitude, and depth. The most productive ecosystems are tropical rain forests, coral reefs, salt marshes and estuaries; the least productive are deserts, tundra, and the open sea.
Ecological Consumers; Ecology; Habitat; Restoration Ecology
This is the complete article, containing 494 words
(approx. 2 pages at 300 words per page).
