Organisms use the energy available to them as currency to acquire, concentrate, and organize chemical resources for growth and reproduction (Sterner and Elser 2002; see Chapter 4). Even sedentary organisms living in or on their material resources must expend energy to acquire resources against chemical gradients or to make these resources useable (e.g., through oxidation and reduction reactions necessary for digestion and assimilation). Energy gains must be greater than energy expenditures, or resource acquisition, growth, and reproduction cannot be maintained.
Energy and matter are transferred from one trophic level to the next through consumption; however, whereas energy is dissipated ultimately as heat, matter is conserved and reused. Conservation and reuse of nutrients within the ecosystem buffer organisms against resource limitation and contribute to ecosystem stability. The efficiency with which limiting elements are recycled varies among ecosystems. Biogeochemical cycling results from fluxes among biotic and abiotic storage pools.
Biogeochemical cycling occurs over a range of spatial and temporal scales. Cycling occurs within ecosystems as a result of trophic transfers and recycling of biotic materials made available through decomposition. Rapid cycling by micro-bial components is coupled with slower cycling by larger, longer-lived organisms within ecosystems. Nutrients exported from one ecosystem become inputs for another. Detritus washed into streams during storms is the primary source of nutrients for many stream ecosystems. Nutrients moving downstream are major sources for estuarine and marine ecosystems. Nutrients lost to marine sediments are returned to terrestrial pools through geologic uplifting. Materials stored in these long-term abiotic pools become available for extant ecosystems through weathering and erosion. The pathways and rates of nutrient movement can be described by ecological stoichiometry (Sterner and Elser 2002).
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