Energy Flow

The flow of energy in ecosystems is from energy source to autotroph to het-erotroph. For most systems, the energy source is the sun and the autotrophs are green plants. The use of energy sources such as inorganic C, N, and S occurs under limited, but not unrealistic, circumstances and is currently of great interest because of the roles of these processes in the production of greenhouse gases such as nitrous oxides and methane. The use of molecules that do not contain C as a source of energy is largely carried out by bacteria and is of great importance for discussing nutrient cycles, but is of lesser importance when discussing energy flow pathways because the contribution to biomass production is a fraction of that produced from solar energy. The solar energy that is captured within an ecosystem is based on the amount of photosynthesis that occurs there. The most commonly used term to begin describing this flow is net primary productivity (NPP), which is the total energy uptake by plants in an ecosystem that is available for use by other trophic levels. NPP is calculated as

NPP = GPP - Rp, where gross primary productivity (GPP) is all of the energy assimilated through photosynthesis, and respiration (Rp) is the sum of energy loss through oxidation of organic compounds. The amount of NPP in an ecosystem can be predicted most easily at the largest scale by state factors, but even more simply, patterns of NPP can be described by climate or moisture and temperature alone (Fig. 8.3).

Biomes, the major ecosystem types based on dominant vegetation, are easily plotted along moisture and temperature gradients showing corresponding increases in NPP. At shorter time scales, such as across seasons, NPP is controlled by leaf area, N content, season length, temperature, light, and CO2.

Secondary production, the amount of biomass produced by consumers, is dependent on the amount of energy made available by primary producers. This has consequences for the complexity of trophic dynamics of an ecosystem. The complexity of a food web is likely dictated by the amount of energy that is contained in the producers. The most complex food webs are in the tropical systems where production is not limited by moisture or temperature. Biological systems are inefficient in the transfer of energy. Plants transform only 1 to 5% of the sunlight that is available to them for photosynthesis into biomass. The transformation

ShoE hB

ShoE hB

wo A

Temperature grassland

Temperature shrublands m

Aridisol

Tropical grassland i

Tropical shrubland

Aridisol

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

Get My Free Ebook


Post a comment