Energy and Nutrient Flow in Ecosystems (PDF is here)

Energy Flow
As you read through this topic, keep in mind the following:

• FIRST LAW OF THERMODYNAMICS - energy is neither created nor destroyed, but it may be converted from one form to another
• SECOND LAW OF THERMODYNAMICS - in any energy conversion, less energy will be present after conversion due to heat loss. Useable energy is lost during energy conversion.
• ENERGY IS NEEDED TO MAINTAIN ORDER.
• WHILE ENERGY CANNOT BE RECYCLED WITHIN AN ECOSYSTEM, NUTRIENTS AND OTHER MATERIALS CAN BE RECYCLED.

Figure 1 shows the energy and nutrient flow in a typical ecosystem. Black arrows are energy flow, blue arrows are material flow. Note that, although materials (nutrients) can be recycled, energy just pours through the system. In this figure, the energy content of any trophic level is represented by the symbol _i  where i is the tropic level. Thus, ₁   represents the primary producers, ₂  is the primary consumers, ₃  is the secondary consumers, while ₄   is the decomposer trophic level. The rate of flow from one trophic level to another is depicted as _i,j   where i is the receiving level and j is the level losing energy. Subscripts of 0 are external to the system so that ₀₁  are energy losses from the plants to the external environment (through respiration losses and heat). In this model you can consider _i  to be "leaky buckets" where energy is only temporarily held. At equilibrium (under stable conditions) the energy _i  at each level remains constant and the rate of flow into each tropic level is equal to the rate out such that d /dt =0 for all i. In most ecosystems ₁₀   is only about 1% of the available solar energy that will be available to primary producers to put into Gross Annual Production (GAP; measured in cal/m²/yr). GAP = ₁₀ - ₀₁   and represents the total rate of photosynthesis.

Some of the energy (₀₁) is used by the plant for their own respiration and maintenance; therefore only some of the energy (₂₁) is  available to animals and decomposers (₄₁). The net productivity or Net Annual Production (NAP) is equal to ₁ - ₂₁ - ₄₁. NAP is the rate of storage of organic matter and is the energy available to other trophic levels. Because of use of energy within the first tropic level for maintenance and respiration and because of entropy concerns, the Net Annual Production is always much less than the Gross Primary Production. In tropical rain forests, losses through respiration (₀₁) is typically 75-80% of the available energy (50% for deciduous forests, 25-50% for most other communities). Most estimates place the energy available to animals (₂₁)  as 10%, with the rest going to the decomposers (₄₁). Energy transfer from the second to third trophic level is even worse (₃₂ usually runs at about 3% efficiency). Net Community Production is the rate energy is not used by heterotrophs (₁) while Secondary Productivity is the energy tied up in meat (₂ + ₃ + ₄). These quantities are outlined in table 1.

Figure 2 summarizes the relationships between productivity and energy. Note that productivity is a rate measure (energy fixed per unit time). Productivity is not equivalent to biomass (a yield). Productivity can be high while biomass is low.  Table 2 shows the primary productivity for several ecosystems. Table 3 shows the estimated gross annual production adjusted to area. Note that, although the open ocean has relatively low productivity, because of its size, it is the dominant energy system. Note also that the extremely high productivity of the rain forests puts them in second place for productivity, despite their small area. Finally, notice the difference energy subsidies make to agriculture. In general, productivity is high when:

• Conditions are favorable.
• Energy subsidies from outside the system reduce maintenance costs (wind, nutrients, fertilizers, planting in rows, etc.).
• When energy drains are low (disease, etc.).

The efficiency of energy transfer from one trophic level to another is _i,ji/_i,h   where j = i + 1 and h = i -1. Thus, ₂₁/₁₀ is the efficiency of the primary producers. Energy studies show the minor energetic importance of carnivores and the major role of decomposers (as high as 90% of the net annual production).

What happens to the energy and how do systems control the flow of energy through the system? A good proportion of the energy lost at each level goes to antithermal maintenance (used to pump disorder out of the system). The total community respiration R = ₀₁ + ₀₂ + ₀₃ + ₀₄. Under these conditions, R/B is the maintenance to structure ratio, or Schrödinger ratio, where B is the total community biomass.

Figure 3 shows the relationship between the grazing and detritus food changes and how energy and materials flow. Figure 4 depicts the loss of energy as we move from lower to higher trophic levels. Because of these losses, you rarely see systems with more than four trophic levels (see also figure 5). The net effect of the energy losses as we move up the trophic levels is an ecological pyramid (Figure 6). This could be either a pyramid of biomass or a pyramid of energy since biomass can be converted to energy. Note that those at the top of the food chain take a double-hit; one related to entropy-related losses, the other because of decreased ecological efficiency. Figure 7 shows two pyramids of numbers. Note that in some systems the pyramid can be inverted..

Figure 8 shows the relationship between primary productivity and respiration. Note that most major ecosystems arrange themselves along the continuum where Productivity/respiration =1.0. This implies that communities adjust their productivity and respiration to fit an optimum and that this holds for vastly different systems ("pond" is a pond anywhere in the world, with different species assemblages). Systems outside the P/R=1.0 line are either disturbed systems, or those with an energy subsidy. These data argue strongly for a "natural" equilibrium in energy flow.

BIOGEOCHEMICAL (NUTRIENT) CYCLES

While energy cannot be recycled and can only flow through an ecosystem, nutrients (chemicals such as K, N, S, Fe, etc.) can be recycled (Figure 9). Typically, these nutrients are shuffled between two phases:

• The organismic phase: These are nutrients tied up in living biomass, dead biomass, and organism waste.
• Environmental phase: Nutrients held in non-living components of the biosphere. Depending on the cycle, nutrients may be held in rock, other deposits, the atmosphere, or some combination of the above.

Figure 10 shows the relationship between the two phases. Note that, for any particular community, nutrients may also be imported or exported. Figure 11 shows the nutrient cycle for carbon. In this case, most of the recycling is done through the atmosphere (environmental phase) and is returned to the organismic phase through photosynthesis by terrestrial and aquatic plants. A less important environmental phase for carbon, not shown in this diagram occurs when carbon compounds sink to the bottom of oceans, and fresh water lakes where it can combine with calcium to form Calcium Carbonate, part of the lithosphere.

Figure 11 depicts a much more complicated biogeochemical cycle; this time for nitrogen. As before, the main environmental sink for nitrogen is the atmosphere, but the process is complicated by numerous steps required mainly in the decomposing food chain (although producers such as legumes also contribute). The main processes involve fixation, nitrification, denitrification, and ammonification. Not shown is the contribution of lightning in releasing free nitrogen from the air. Other cycles may be more or less complex, depending on the nutrient. Calcium recycling, for example, is relatively simple, with the main environmental sink being calcium-containing rocks. Study figure 12 for an overview of the relationships between the various environmental and organismic phases in nutrient recycling.

Energy Flow Activity

For a quick overview of the energy flow simulation, click here.  A stand-alone version is here.

1. Run the simulation using the default values. Then set the sun check-box to simulate two years of seasonal solar radiation. Note that, although the populations fluctuate in both cases, they do so about a mean (stability of the first type). From the energy diagram, what can you say about the relative energetic importance of each trophic level?
2. Turn off the solar check box and re-run the simulation to get base values. Pay attention to the population sizes and amount of energy flow. Then change the loss rate for the producers (₀₁) first to 0.5 then 0.9 and run the simulation. What effect did these changes have on the overall community (explain)? Higher respiration rates are typical of communities such as tropical rain forests. What do the energy relationships suggest about stability in these systems?
3. Reset the base values and run the simulation. Now change the efficiency of the primary consumers (₀₂) from 0.46 to 0.2, then 0.8. What effect did this have on the community (explain)?
4. Reset the base values and re-run the default simulation. Next change the feeding rate of the primary consumer (₂₁) from 1.15 to 2.0 (making the herbivores feed faster). What effect did this have on overall community stability (both first and second type). What has it done to energy flow in the system. What kind of organisms are we modeling under these conditions? Explain how these results mirror those of our other models.
5. Now we'll specifically model stability of the second type as it relates to energy flow. Reset the model to the default conditions and run it. Then, under stochastic effects, change the loss rate for the producers (₀₁) to 0.025 (1/4 of the loss rate at the top). Run the simulation five times using these values. What we have done here is model a producing population under stress and/or with energy supplements. What have these changes done to the overall stability of the system?
6. Return to the default values, clear the stochastic effects for ₀₁ to zero and change the stochastic effect for the primary consumers (₀₂) to approximately 1/4 the input value (0.12 is close enough) and do several runs. Then clear the stochastic data for the primary consumers and make similar changes to the secondary, then tertiary consumers. Discuss the relative importance of each trophic level on overall community stability.
7. If I modeled the deutritovore feeding level, where do you think that would fit in as to importance? At what trophic levels do people worry most? Where should we worry?
8. Conceptually pull together the results from the food web simulation and this simulation. What combination of factors leads to instability? How is this related to the concepts of r- and K-selected species? Why don't we see abrupt changes in the biosphere in Kentucky? Where should we look to see damage? Why will that damage matter from an energetic point of view? We live in interesting times.
9. Explore the effects of changing other parameters on community stability. What have you learned?

Do Ecosystems Evolve?

• Examine figure 13. A desert in southwest North America looks pretty much like a desert in Australia. Similarly, a tropical rain forest in Brazil looks very much like one in India or Central Africa. The overall structure of these communities is similar despite their being composed of different species assemblages (the cacti in Australia are not the same as those in the South West U.S.).
• Figure 14 depicts the global biodiversity for vascular plants. Note that this diversity, which mirrors the complexity of the food webs, chains, and energy flow closely follows the biome boundries. Again, these relationships have come about despite different species assemblages for the communities.
• Primary succession is the sequential development of biotic communities where none have existed before. Where glaciers have retreated or volcanoes have left nothing but bare rock, lichens are among the first organisms to move in. The lichens secrete carbonic acid that dissolves the rocks and begins to make a primitive soil. Eventually, the lichens are replaced by mosses as the soil thickens. The mosses are more efficient than the lichens and out-compete them. This attracts additional insects and spider communities whose bodies add to the soil thickness. In addition, the mosses can catch wind-blown dust to add to the soil. As the soil thickens small shrubs and herbs move in, again changing the soil and attracting new animals. The shrubs are eventually replaced with a heath mat, followed by Jack Pine, Black Spruce, and Aspen, which are replaced by Balsam Fir, Paper Birch, and White Spruce (for a Northern Temperate Climate in North America).  Succession takes place since each biotic community alters its habitat so much that it can no longer survive since more efficient organisms can move in. The first inhabitants in primary succession are called the pioneer community. Pioneer communities are followed by intermediate communities and eventually by a climax community. What do you think the energetic efficiency of the climax community would be compared to the intermediate and pioneer communities. Depending on the climate, primary succession can take hundreds, or even thousands of years since soil has to be re-built.
• Secondary Succession is the sequential development of biotic communities after partial destruction of an existing community. Secondary succession is much faster than primary, since the slow development of soil from rocks is not needed. A chart showing the sequence of secondary succession for an abandoned farm field in the mid-west is shown in Table 4.
• Figure 15 compares plant types living in a Mediterranean climate in three different geographical areas. Note that, although the species are different, the communities have developed similar structure. From what you now know about cybernetic systems and energy flow, why should this be? Do ecosystems evolve? What factors lead to similarities in various parts of the world?
• If you landed on a class-M  (earth-like) planet with Captain Kirk, would you feel at home? Explain. While you're at it, punch Captain Kirk here