The Flow of Energy: Higher Trophic Levels

In this lesson, we will answer the following questions:

• What is the efficiency with which energy is converted from trophic level to trophic level? 
• What are the differences between assimilation efficiency, net production efficiency, and ecological efficiency? 
• How do ecosystems differ in the amount of biomass or number of organisms present at any point in time, and generated over time, at each trophic level?  
• How much energy is available to humans, and how much do we use? 
• What are the main controls on ecosystem function? 

Jump to: [Introduction] [Energy Transfer] [Fox and Hare Example] [Pyramids Models] [Human Energy Consumption] [Summary]

Introduction

Most of you are now familiar with the concept of the trophic level (see Figure 1). It is simply a feeding level, as often represented in a food chain or food web. Primary producers comprise the bottom trophic level, followed by primary consumers (herbivores), then secondary consumers (carnivores feeding on herbivores), and so on. When we talk of moving "up" the food chain, we are speaking figuratively and mean that we move from plants to herbivores to carnivores. This does not take into account decomposers and detritivores (organisms that feed on dead organic matter), which make up their own, highly important trophic pathways.

The Transfer of Energy to Higher Trophic Levels

In the process, metabolic work is done and energy in chemical bonds is converted to heat energy. If NPP was not consumed, it would pile up somewhere. Usually this doesn't happen, but during periods of earth history such as the Carboniferous and Pennsylvanian, enormous amounts of NPP in excess of consumption accumulated in swamps. It was buried and compressed to form the coal and oil deposits that we mine today. When we burn these deposits (same chemical reaction as above except that there is greater energy produced) we release the energy to drive the machines of industry, and of course the CO₂ goes into the atmosphere as a greenhouse gas. This is the situation that we have today, where the excess CO₂ from burning these deposits (past excess NPP) is going into the atmosphere and building up over time.

But let's get back to an ecosystem that is balanced, or in "steady state" ("equilibrium"), where annual total respiration balances annual total GPP. As energy passes from trophic level to trophic level, the following rules apply:

• Only a fraction of the energy available at one trophic level is transferred to the next trophic level. The rule of thumb is 10%, but this is very approximate. 
• Typically the numbers and biomass of organisms decrease as one ascends the food chain.

 An Example: The Fox and the Hare

  
A hare (or a population of hares) ingests plant matter; we'll call this ingestion. Part of this material is processed by the digestive system and used to make new cells or tissues, and this part is called assimilation. What cannot be assimilated, for example maybe some parts of the plant stems or roots, exits the hare's body and this is called excretion. Thus we can make the following definition: Assimilation = (Ingestion - Excretion). The efficiency of this process of assimilation varies in animals, ranging from 15-50% if the food is plant material, and from 60-90% if the food is animal material. 

The hare uses a significant fraction of the assimilated energy just being a hare -- maintaining a high, constant body temperature, synthesizing proteins, and hopping about. This energy used (lost) is attributed to cellular respiration. The remainder goes into making more hare biomass by growth and reproduction. The conversion of assimilated energy into new tissue is termed secondary production in consumers, and it is conceptually the same as the primary production or NPP of plants. In our example, the secondary production of the hare is the energy available to foxes who eat the hares for their needs. Clearly, because of all of the energy costs of hares engaged in normal metabolic activities, the energy available to foxes is much less than the energy available to hares. 

Just as we calculated the assimilation efficiency above, we can also calculate the net production efficiency for any organism. This efficiency is equal to the production divided by the assimilation for animals, or the NPP divided by the GPP for plants. The "production" here refers to growth plus reproduction. In equation form, we have net production efficiency = (production / assimilation), or for plants = (NPP / GPP). These ratios measure the efficiency with which an organism converts assimilated energy into primary or secondary production. 

These efficiencies vary among organisms, largely due to widely differing metabolic requirements. For instance, on average vertebrates use about 98% of assimilated energy for metabolism, leaving only 2% for growth and reproduction. On average, invertebrates use only ~80% of assimilated energy for metabolism, and thus exhibit greater net production efficiency (~20%) than do vertebrates. Plants have the greatest net production efficiencies, which range from 30-85%. The reason that some organisms have such low net production efficiencies is that they are homeotherms, or animals that maintain a constant internal body temperature. This requires much more energy than is used by poikilotherms, which are organisms that do not regulate their temperatures internally. 

Just as we can build our understanding of a system from the individual to the population to the community, we can now examine whole trophic levels by calculating ecological efficiencies. Ecological efficiency is defined as the energy supply available to trophic level N + 1, divided by the energy consumed by trophic level N. You might think of it as the efficiency of hares at converting plants into fox food. In equation form for our example, the ecological efficiency = (fox production / hare production).

Thinking about ecological efficiency brings us back to our first rule for the transfer of energy through trophic levels and up the food chain. In general, only about 10% of the energy consumed by one level is available to the next. For example, If hares consumed 1000 kcal of plant energy, they might only be able to form 100 kcal of new hare tissue. For the hare population to be in steady state (neither increasing nor decreasing), each year's consumption of hares by foxes should roughly equal each year's production of new hare biomass. So the foxes consume about 100 kcal of hare biomass, and convert perhaps 10 kcal into new fox biomass. In fact, this ecological efficiency is quite variable, with homeotherms averaging 1- 5% and poikilotherms averaging 5-15%. The overall loss of energy from lower to higher trophic levels is important in setting the absolute number of trophic levels that any ecosystem can contain. 

From this understanding, it should be obvious that the mass of foxes should be less than the mass of hares, and the mass of hares less than the mass of plants. Generally this is true, and we can represent this concept visually by constructing a pyramid of biomass for any ecosystem (see Figure 3).

Pyramids of Biomass, Energy, and Numbers

We could also construct a pyramid of numbers, which as its name implies represents the number of organisms in each trophic level (see Figure 4a). For the oceans as shown in Figure 4, the bottom level would be quite large, due to the enormous number of small algae. For other ecosystems, the pyramid of numbers might be inverted: for instance, if a forest's plant community was composed of only a handful of very large trees, and yet there were many millions of insect grazers which ate the plant material.

Just as with the inverted pyramid of numbers, in some rare exceptions, there could be an inverted pyramid of biomass, where the biomass of the lower trophic level is less than the biomass of the next higher trophic level. The oceans are such an exception because at any point in time the total amount of biomass in microscopic algae is small. Thus a pyramid of biomass for the oceans can appear inverted (see Figure 4b). You should now ask "how can that be?" If the amount of energy in biomass at one level sets the limit of energy in biomass at the next level, as was the case with the hares and foxes, how can you have less energy at the lower trophic level? This is a good question, and can be answered by considering, as we discussed in the last lecture, the all important aspect of "time". Even though the biomass may be small, the RATE at which new biomass is produced may be very large. Thus over time it is the amount of new biomass that is produced, from whatever the standing stock of biomass might be, that is important for the next trophic level.

We can examine this further by constructing a pyramid of energy, which shows rates of production rather than standing crop. Once done, the figure for the ocean would have the characteristic pyramid shape (see Figure 4c). Algal populations can double in a few days, whereas the zooplankton that feed on them reproduce more slowly and might double in numbers in a few months, and the fish feeding on zooplankton might only reproduce once a year. Thus, a pyramid of energy takes into account the turnover rate of the organisms, and can never be inverted.
 

We see that thinking about pyramids of energy and turnover time is similar to our discussions of residence time of elements. But here we are talking about the residence time of "energy". The residence time of energy is equal to the energy in biomass divided by the net productivity, R_t = (energy in biomass / net productivity). If we calculate the residence time of energy in the primary producers of various ecosystems, we find that the residence times range from about 20-25 years for forests (both tropical rainforests and boreal forests), down to ~3-5 years for grasslands, and finally down to only 10-15 days for lakes and oceans. This difference in residence time between aquatic and terrestrial ecosystems is reflected in the pyramids of biomass, as discussed above, and is also very important to consider in analyzing how these different ecosystems would respond to a disturbance or what scheme might best be used to manage the resources of the ecosystem.

Humans and Energy Consumption

We can start by looking at the Inputs and Outputs:

Inputs: NPP, calculated as annual harvest. In a cropland NPP and annual harvest occur in the same year. In forests, annual harvest can exceed annual NPP (for example, when a forest is cut down the harvest is of many years of growth), but we can still compute annual averages. 

Outputs: 3 Scenarios 

1. How much NPP humans use directly, as food, fuel, fiber, timber. This gives a low estimate of human appropriation of NPP. 
2. Total productivity of lands devoted entirely to human activities. This includes total cropland NPP, and also energy consumed in setting fires to clear land. This gives a middle estimate. 
3. A high estimate is obtained by including lost productive capacity resulting from converting open land to cities, forests to pastures, and due to desertification and other overuse of land. This is an estimate of the total human impact on terrestrial productivity.

Units: We will use the Pg or Pedagram of organic matter (= 10¹⁵ g, = 10⁹ metric tons, = 1 "gigaton") (1 metric ton = 1,000 kg).

Table 1 provides estimates of total NPP of the world. There is some possibility that below-ground NPP is under-estimated, and likewise marine NPP may be underestimated because the contribution of the smallest plankton cells is not well known. Total = 224.5 Pg

1. The Low Calculation: (See Table 2) 

(a) Plant material directly consumed = 5 billion people X 2500 kcal/person/day X 0.2 (to convert kcal -- organic matter) = 0.91 Pg organic matter. If we assume that 17% of these calories derive from animal products, humans directly consume 0.76 Pg of plant matter. Estimate of human harvest of grains and other plant crops is 1.15 Pg annually. This implies loss, spoilage, or wastage of 0.39 Pg, or 34% of the total harvest.

(b) Consumption by livestock: estimates range from 2.8 to 5 Pg, and there seems to be some uncertainty here. Our low estimate uses 2.2 Pg.

(c) Forests: harvest of wood for construction and fiber is well known. Amount used for firewood, especially in tropics, is not. The table gives a low estimate. 

(d) Fish harvest: 0.075 Pg wet weight = 0.02 Pg dry wt. If we assume the average fish is two trophic transfers (@ 10% each) above primary producers, the NPP to produce those fish was 2 Pg annually. 

Total: Humans consume 7.2 Pg of organic matter directly each year. This is about 3 % of the biosphere's total annual NPP. 

 2. The Intermediate Calculation: (See Table 3) 

We add to the low calculation the amount of NPP co-opted by humans. This is:

(a) All cropland NPP

(b) All pastureland that was converted from other ecosystem types, NPP consumed by livestock on natural grazing land, and human-set fires

(c) A number of forest land uses

(d) Human occupied areas including lawns, parks, golf courses, etc. 

Total is 42.6 Pg of NPP per year, or 19% of world NPP.

 
3. The High Calculation: (See Table 4)

For the high estimate we now include both co-opted NPP and potential NPP lost as a consequence of human activities: 

(a) Croplands are likely to be less productive than the natural systems they replace. If we use production estimates from savanna-grasslands, it looks like cropland production is less by 9 Pg.

(b) Forest conversion to pasture: the roughly 7 million km² of forest converted to pasture represents a loss of 1.4 Pg.

(c) Overuse: Some 35 million km² of land has been made more arid and less productive as a result of human overuse, some 15 million km² severely so. Using dry savanna estimates of NPP, global NPP has been reduced by 4.5 Pg.

(d) Land conversion: Assuming the 2 million km² of land in cities, highways, etc. had a productivity equivalent to natural forests, 2.6 Pg of NPP is foregone. 

The total for the high estimate is 58.1 Pg of NPP used, co-opted, or lost. We also must add the potential NPP to the world estimated NPP before we compute the fraction appropriated by humans. This gives us 58.1/149.6, or nearly 40% of potential terrestrial production (about 25 % of terrestrial + aquatic production). Caveat: These estimates are based on best available data and are approximate. They probably give the correct order of magnitude.
 

Inferences:

What can we conclude from the above analysis of the fate of net primary production in our world?

(a) Human use of marine productivity is relatively small. Moreover, although major fish stocks are heavily fished, and many coastal areas are severely polluted, human impact on the seas is less than on land.

(b) On land, one species, Homo sapiens, commands about 40% of the total terrestrial NPP. This has probably never occurred before in earth's history. 

(c) There are many consequences of this co-option of NPP by humans. The consequences include environmental degradation, species extinctions, and altered climate.

(d) Human "carrying capacity" on earth is hard to estimate, because it depends upon affluence of a population and the technology supporting that population. But at present levels of affluence and technology, a population 50 to 100% larger than we have today would push our use of terrestrial NPP to well over 50% of the available production, and the attending degradation of ecosystems on earth (e.g., air and water pollution) would be of major concern. Thus the limits to unchecked growth must be very near. Notice that the lower we "feed" on the trophic chain, the more efficient the web of life becomes -- eating animals that eat animals that eat plants is a very inefficient use of solar energy.

Now that we have learned something about how ecosystems are put together and how materials and energy flow through ecosystems, we can better address the question of "what controls ecosystem function"? There are two dominant theories of the control of ecosystems. The first, called bottom-up control, states that it is the nutrient supply to the primary producers that ultimately controls how ecosystems function. If the nutrient supply is increased, the resulting increase in production of autotrophs is propagated through the food web and all of the other trophic levels will respond to the increased availability of food (energy and materials will cycle faster). 

The second theory, called top-down control, states that predation and grazing by higher trophic levels on lower trophic levels ultimately controls ecosystem function. For example, if you have an increase in predators, that increase will result in fewer grazers, and that decrease in grazers will result in turn in more primary producers because fewer of them are being eaten by the grazers. Thus the control of population numbers and overall productivity "cascades" from the top levels of the food chain down to the bottom trophic levels.

So, which theory is correct? Well, as is often the case when there is a clear dichotomy to choose from, the answer lies somewhere in the middle. There is evidence from many ecosystem studies that BOTH controls are operating to some degree, but that NEITHER control is complete. For example, the "top-down" effect is often very strong at trophic levels near to the top predators, but the control weakens as you move further down the food chain. Similarly, the "bottom-up" effect of adding nutrients usually stimulates primary production, but the stimulation of secondary production further up the food chain is less strong or is absent.

Thus we find that both of these controls are operating in any system at any time, and we must understand the relative importance of each control in order to help us to predict how an ecosystem will behave or change under different circumstances, such as in the face of a changing climate.
 

Summary

1. Only a fraction of the energy available at one trophic level is transferred to the next trophic level; the fractions can vary between 1-15%, with an average value of 10%.
2. Typically the numbers and biomass of organisms decreases as one ascends the food chain.
3. We can construct pyramids of biomass, energy, and numbers to represent the relative sizes of trophic levels in ecosystems. Pyramids can often be "inverted" as a consequence of high production rates at lower trophic levels. 
4. The human diet is derived from plant material. Humans may consume, co-opt or make unavailable as much as 40% of the earth's total terrestrial NPP for food, land, and other uses.
5. Ecosystem function is controlled mainly by two processes, "top-down" and "bottom-up" controls.

Review

• Review of main terms and concepts in this lecture.

Suggested Readings

• Wessells, N.K. and J.L. Hopson. 1988. Biology. New York: Random House, ch. 44
  
  
• Townsend, C.R., J.L. Harper, and M. Begon. 2000. Essentials of Ecology. Blackwell Science.