Energy Flow in Ecosystems

The thermodynamic assessment of an ecosystem starts with the recognition that an ecosystem is an open system, in the sense of physics, such that it receives energy and matter input from outside its borders and transfers output back to this environment. Thus, every ecosystem must have a system boundary and must be embedded in an environment that provides low-entropy energy input and can receive high-entropy energy output. In addition to the external resource source-sink, there is another internal, within-system boundary environment with which each organism directly and indirectly interacts. Patten proposed the concept of these two environments, one external and mostly unknowable (other than the input-output interactions), and the second internal and measurable (i.e., external to the specific organismal component but within system boundary) as a systems approach to quantify indirect, yet within-system interactions. This approach - called environ analysis - relying on the methodologies of input-output analysis has developed into a powerful analysis tool for understanding complex interactions and dependencies in ecological networks. For now though, let us concern ourselves more generally with what occurs within the ecosystem boundary.

Photosynthesis

Plant respiration

Ecosystem boundary

Respiration

Immigration/ emigration

Photosynthesis

Plant respiration

Ecosystem boundary

Respiration

Immigration/ emigration

Figure 1 Conceptual diagram of a simplified ecosystem. Clear arrows, energy; dark arrows, biomass; blue arrows, water.

Energy flow in ecosystems begins with the capture of solar radiation by photosynthetic processes in primary producers (eqn [1]). Note, there are also chemoautotrophs that capture energy in the absence of sunlight, but while biologically fascinating, contribute negligible energy flux to the overall global ecological energy balance

The accumulated organic matter, first as simple sugars then combined with other elements to more complex molecules, represents the gross primary production in the system, some of which is released and used for the primary producers' growth and maintenance through respiration:

The remainder, or net primary production, is available for the rest of the ecosystem consumers including decomposers. Secondary production refers to the energetic availability of the heterotrophic organisms, which accounts for the energy uptake by heterotrophs and the energy used for their maintenance. Overall ecosystem production is supported by the primary producers, whereas ecosystem respiration includes the metabolic activity of all the ecosystem biota (Table 3). In this manner, plants provide the essential base for all ecological food webs. Since it is often difficult to make direct measurements of ecological production, the change in biomass measures growth, which can be used as representative of production.

The captured energy moves through a reticulated network of interactions forming the complex dependency patterns known as food webs. In a simplified food chain, and as first described by Lindeman, the trophic concept is used to assess the distance away from the original energy importation, but in reality the multiple feeding pathways found in ecological food webs make discrete trophic levels a convenient yet inaccurate simplification. Elton observed that one typically finds a decreasing number of organisms as one proceeds up the food chain from primary producers to herbivores, carnivores, and top carnivores - leading him to propose a pyramid of numbers. One can control for the individual variation in body size by considering the biomass at each trophic level rather than the number of individuals - resulting in a pyramid of biomass. The trophic pyramid is a thermo-dynamically satisfying view of interactions since according to the second law energy must be lost during each transformation step; in addition, energy is used at each level for the maintenance of that level. Under this paradigm, the trophic levels apparently cap out around five or six levels. Fractional trophic levels have been employed to account for organisms feeding at multiple levels, but even these do not usually account for the role of detritus and decomposition, which extend the feeding pathways to higher numbers. However, instead of linking detritus as a source compartment in the ecosystem conceptual model, the standard paradigm is to envision two parallel food webs one with primary producers as the base and the other with detritus as the base without any input from the rest of the web. If detritus were properly linked as both a source and sink in the ecosystem, then it would be clear that higher-order trophic levels are possible, if not common. The higher observed trophic levels observed in some studies are not in conflict with the laws of thermodynamics, but they show that ecosystems are more thorough at utilizing the energy within the system, mostly by decomposers, before it is lost as degraded, unavailable energy.

Energy resources flowing through the ecosystem are necessary to maintain all growth and development activities. Organisms follow a clear life-history pattern, and while the timescales differ depending on the species, early stage energy availability is generally used for growth, while later energy surplus is used for maintenance or reproduction. A similar pattern is visible in ecosystem-level growth and development. Net primary production is used to build biomass and physical structure of the ecosystem. The additional structure of photosynthetic material allows for the additional import of solar energy until saturation is reached at about 80% of the available solar radiation. At this point the overall growth of the ecosystem begins to level off because although gross primary production is high, the overall system supports more and more nonphotosynthetic biomass both in terms of nonphotosynthetic plant material and heterotrophs. When the average gross production is entirely utilized to support and maintain the existing structure, net production is zero and the system has reached a steady state regarding biomass growth. However, the ecosystem continues to develop both in terms of the network organization and in the information capacity. In addition to being a dynamic steady state, it does not persist indefinitely because disturbances afflict the system setting it back to earlier

Table 3 Ecosystem energetics defined by net and gross production

Net primary production = gross primary production - respiration (autotrophs)

Net secondary production = gross secondary production - respiration (heterotrophs)

Net ecosystem production = gross primary production - ecosystem respiration (autotrophs + heterotrophs)

Net production = biomass (now) - biomass (before)

successional stages in which the growth and development processes begin anew, possibly with different results. In this manner, the disturbance acts according to Holling's creative destruction (see Adaptive Cycle) providing the system the opportunity to develop along a different pathway. Recent work on ecosystem growth and development has focused on the orientation of thermodynamic indicators such as energy throughflow, energy degradation, exergy storage, and specific entropy. These orientors provide good system-level indicators of development during succession or restoration of impaired ecosystems.

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