Energy Flow Through Ecosystems • The ultimate source of energy for biological processes is the sun. • In other words, the basis of living systems is the solar energy captured through the light-dependent reaction of photosynthesis. • The use of energy to manufacture living material is called productivity.
Productivity Definitions • Primary productivity- the rate at which energy is bound by photosynthesis. It is often measured in kcal/m2/year. • Gross production- the amount of production before metabolic costs (the energy required to sustain life) are subtracted. • Net production- the amount of production after metabolic costs (respiration) are subtracted. • Biomass- the amount of living material present at a given time. It is measured in g/ m2.
Trophic Levels • Energy captured by plants is passed along to other organisms as plants are consumed by them. These organisms are in turn consumed by other organisms. • Food chain- the steps in this transfer of energy. • Trophic level- each step in this transfer. • Primary producers (autotrophs)- the organisms that form the base of the food chain. They include photosynthetic organisms and organisms that derive energy from chemical compounds (chemoautotrophs). • Primary consumers- organisms that eat primary producers. All organisms that eat other organisms are heterotrophs. • Secondary consumers- organisms that eat primary consumers. • Tertiary consumers (top carnivores)- organisms that eat primary and secondary consumers. • Decomposers- organisms that break down the remains of other organisms.
Trophic Efficiency • In nature, food chains interconnect to form food webs. For example, more than one kind of animal may eat the same plant. • Only about 1% of solar energy is captured by living organisms. • From 30-75% of gross primary production is used in metabolic activities. • Due to such metabolic costs, only about 10% of the energy at one trophic level is incorporated into net production at the next level. • When the activities of decomposers (bacteria and fungi) are taken into account, trophic efficiency is much greater than 10%, however.
The Energy Pyramid • If a figure is constructed of the amount of energy at each trophic level, the figure appears as a pyramid. This is due to the metabolic losses in energy that occur at each level. • Primary producers form the wide base of the pyramid, primary consumers form the next layer, and secondary and tertiary consumers form the apex of the pyramid. • Not only energy, but also numbers of individuals in the trophic levels tend to follow a pyramid shape.
Measuring Primary Productivity • Measuring production involves harvesting and weighing biomass produced/ unit time. In the case of plants, underground portions must be considered in measures of productivity. • Factors that influence primary productivity include temperature, intensity of sunlight (which varies seasonally), and the age of the community. • As communities such as forests age, productivity may increase at first, but as they reach old age productivity may decline.
Generalizations about Community Productivity • The tropics have among the most productive systems, in part because incident solar radiation is greater toward the equator. • The most productive temperate systems are marshes. Nutrients are continually brought in and wastes are flushed out as water moves through them. • Deserts and oceans are among the least productive systems. Little water in deserts and few nutrients in open oceans are responsible. Thus, 80% of the Earth’s surface is covered by the least productive systems.