Chapter 18 Primary Production and Energy Flow
Suntight shines down on the canopy of a forest-- some
is reflected, some is converted to heat energy, and some is absorbed by chlorophyll.
Infrared radiation is absorbed by the molecules in organisms, soil, and water,
increasing their kinetic state and raising the temperature of the forest.
A portion of the energy fixed by forest vegetation is consumed by herbivores, some is consumed by detritivores, and some ends up as soil organic matter. Energy fixed by forest vegetation powers bird flight through the forest canopy and fuels the muscle contractions of earthworms as they burrow through the forest soil. The forest vegetation is sunlight transformed, as are all the associated bacteria, fungi, and animals and all their activities (fig. 18.1).
We can view a forest as a system that absorbs, transforms, and stores energy. In this view, physical, chemical, and biological structures and processes are inseparable. When we look at a forest (or stream or coral reef) in this way we view it as an ecosystem. A0 ecosystem is a biological community plus all of the abiotic factors influencing that community. The term ecosystem and its deflnitionwere first proposed in I935 by the British ecologist Arthur Tansley. We first encountered Tansley in chapter 13, where we discussed his early work on the soil requirements of Galium species. Sometime during bis exploration of nature, he realized the importance of considering organisms and their environment as an integrated system. Tansley wrote: "Though the organisms may claim our primary interest .... we cannot separate them from their special environment, with which they form one physical system. It is the [eco]systems so formed which, from the point of view of the ecologist, are the basic units of nature on the face of the earth."
Ecosystem ecologists study the flows of energy, water, and nutrients in ecosystems and, as suggested by Tansley, pay as much attention to physical and chemical processes as they do to biological one.Some fundamental areas of interest for ecosystem ecologists are primary production, energy flow, and nutrient cycling. We will discuss the first two topics in this chapter and nutrient cycling in chapter 19.
We saw in chapter 6 how the photosynthetic machinery of plants uses solar energy to synthesize sugars. In that chapter we considered photosynthesis from the perspective of the individual grass, tree, or cactus. Here we step back from the biochemical and physiological details of photosynthesis and back even from the individual organism to look at photosynthesis at the level of the whole ecosystem.
Primary production is the fixation of energy by autotrophs in an ecosystem. The rate of primary production is the amount of energy fixed over some interval of time. Ecosystem ecologists distinguish between gross and net primary production. Gross primary production is the total amount of energy fixed by all the autotrophs in the ecosystem. Net primary production is the amount of energy left over after autotrophs have met their own energetic needs.Net primary production is gross primary production minus respiration by primary producers; it is the amount of energy available to the consumers in an ecosystem. Ecologists have measured primary production in a variety of ways but mainly as the rate of carbon uptake by primary producers or by the amount of biomass or oxygen produced.
We discussed feeding biology from a variety of perspectives in previous chapters. In chapter 6, we examined the biology of herbivores, detritivores, and carnivores. In chapter 14, we discussed the ecology of exploitation, and in chapter 17, we used food webs as a means of representing the trophic structure of communities. Ecosystem ecologists are also concerned with trophic structure but have taken a different approach than population and community ecologists.
Ecosystem ecologists have simplified the trophic structure of ecosystems by arranging species into trophic levels based on the predominant source of their nutrition. A trophic level is a position in a food web and is determined by the number of transfers of energy from primary producers to that level. Primary producers occupy the first trophic level in ecosystems since they convert inorganic forms of energy, principally light, into biomass. Herbivores and detritivores are often called primary consumers and occupy the second trophic level. Carnivores feeding on herbivores and detrltivores are called secondary consumers and occupy the third trophic level. Predators that feed on carnivores occupy a fourth trophic level. Since each trophic level may contain several species, in some cases hundreds, an ecosystem perspective simplifies trophic structure.
Primary production, the conversion of inorganic forms of energy into organic forms, is a key ecosystem process. All consumer organisms, including humans, depend upon primary production for their existence. Because of its importance and because rates of primary production vary substantially from one ecosystem to another, ecosystem ecologists study the factots controlling rates of primary production in ecosystems.
Patterns of natural variation in primary production provide clues to the environmental factors that control this key ecosystem process. Experiments test the importance of those controls. In this chapter, we discuss the major patterns of variation in primary production in terrestrial and aquatic ecosystems and key experiments designed to determine the mechanisms producing those patterns. In the last sections of the chapter, we examine patterns of energy flow through ecosystems.
l Terrestrial primary production is generally limited by temperature and moisture.
l Aquatic primary production is generally limited by nutrient availability.
l Consumers can influence rates of primary production in aquatic and terrestrial ecosystems.
l Energy losses limit the number of trophic levels in ecosystems.
patterns of terrestrial primary production
Terrestrial primary production is generally limited by temperature and moisture.
As we surveyed the major terrestrial biomes in chapter 2, you probably got a sense of the geographic variation in rates of primary production. Perhaps you also developed a feeling for the major environmental correlates with that variation. The variables most highly correlated with variation in terrestrial primary production are temperature and moisture. Highest rates of terrestrial primary production occur under warm, moist conditions.
Actual Evapotranspiration and Terrestrial Primary Production
Michael Rosenzweig (1968) estimated the influence of moisture and temperature on rates of primary production by plotting the relationship between annual net primary production and annual actual evapotranspiratioo. Annual actual evapotranspiration (AET) is the total amount of water that evaporates and transpires off a landscape during the course of a year and is measured in millimeters of water per year. The AET process is affected by both temperature and precipitation. The ecosystems showing the highest levels of primary production are those that are warm and receive large amounts of precipitation. Conversely, ecosystems show low levels of AET either because they receive little precipitation, are very cold, or both. For instance, both hot deserts and tundra exhibit low levels of AET.
Figure 18.2 shows Rosenzweig's plot of the positive relationship between net primary production and AET. Tropical forests show the highest levels of net primary production and AET. At the other end of the spectrum, hot, dry deserts and cold, dry tundra show the lowest levels. Intermediate levels occur in temperate forests, temperate grasslands, Woodlands, and high-elevation forests. Figure 18.2 shows that AET accounts for a significant proportion of the variation in annual net primary production among terrestrial ecosystems.
FIGURE 18.2 Relationship between actual evapotranspiration and net aboveground primary production in a series of terrestrial ecosystems (data from Rosenzweig 1968).
Rosenzweig's analysis attempts to explain variation in primary
production across the whole spectrum of terrestrial ecosystems. What controls
variation in primary production within similar ecosystems? O. E. Sala and his colleagues (1988) at
The study areas extended from
Influence of annual precipitation on net aboveground primary production in
grasslands of central
Compare the plot by Sala and his colleagues (fig. 18.3) with the one constructed by Rosenzweig (fig.18.2). How are they similar? How are they different? Both graphs have primary production plotted on the vertical axis as a dependent variable. However, while the Rosenzweig plot includes ecosystems ranging from tundra to tropical rain forest, the plot by Sala and his colleagues includes grasslands only. In addition, different variables are plotted on the horizontal axes of the two graphs. While Rosenzweig plotted actual evapotranspiration, which depends upon temperature and precipitation, Sala and his colleagues plotted precipitation only. They found that including temperature in their analysis did not improve their ability to predict net primary production. Why do you think precipitation alone was sufficient to account for most of the variation in grassland production? A likely reason is that warm temperatures occur during the growing season at all of the study areas included by Sala and his colleagues. In contrast, Rosenzweig's study areas vary widely in growing season temperature.
These researchers found strong correlations
between AET or precipitation and rates of terrestrial primary production.
However, their models did not completely explain the variation in primary
production among the study ecosystems. For instance, in figure 18.2 ecosystems
with annual AET levels of 500 to
Soil Fertility and Terrestrial Primary Production
Significant variation in terrestrial primary production can be explained by differences in soil fertility. Farmers have long known that adding fertilizers to soil can increase agricultural production. However, it was not until the nineteenth century that scientists began to quantify the influence of specific nutrients, such as nitrogen (N) or phosphorus (P), on rates of primary production. Justus Liebig (1840) pointed out that nutrient supplies often limit plant growth. He also suggested that nutrient limitation to plant growth could be traced to a single limiting nutrient. This hypothetical control of primary production by a single nutrient was later called "Liebig's Law of the Minimum." We now know that Liebig's perspective was too simplistic. Usually several factors, including a number of nutrients, simultaneously affect levels of terrestrial primary production. However, his work led the way to a concept that remains true today; variation in soil fertility can significantly affect rates of terrestrial primary production.
Liebig's work, and most practical experience prior to Liebig, concerned the productivity of agricultural ecosystems. Do nutrients influence rates of primary production in other ecosystems, such as the tundra or deserts, where human manipulation has been less prominent? Ecologists have demonstrated the significant influence of nutrients on terrestrial primary production through numerous experiments involving addition of nutrients to natural ecosystems.
Ecologists have increased primary production
by adding nutrients to a wide variety of terrestrial ecosystems, including
arctic tundra, alpine tundra, grasslands, deserts, and forests. For instance, Gaius Shaver and Stuart Chapin (1988) studied the potential
for nutrient limitation in arctic tundra. They added commercial fertilizer
containing nitrogen, pbosphorus, and potassium to
several tundra ecosystems in
Shaver and Chapin measured net primary production at their control and experimental sites 2 to 4 years after the first nutrient additions. Nutrient additions increased net primary production (by 23%-300%) at all of the study sites. The response to fertilization was substantial and clear at most study sites. Four years after the initial application of fertilizer, net primary production on Kuparuk Ridge was twice as high on the fertilized plots compared to the unfertilized control plots (fig. 18.4).
FIGURE 18.4 Effect of addition of nitrogen, phosphorus, and potassium on net aboveground primary production in Arctic tundra (data from Shaver and Chapin 1986).
Nutrient additions to alpine tundra
indicate that the response of ecosystems to nutrient addition is affected by prior
nutrient availability. William Bowman and his colleagues (1993) added nutrients
to the alpine tundra on
Initial concentration of both nitrogen and phosphorus were higher in the wet meadow ecosystem~ And while fertilizing raised the concentrations of both nitrogen and phosphorus in the dry meadow, fertilizing the wet meadow raised the concentration of nitrogen but not phosphorus.
Fertilizing produced greater increases in primary production in the dry meadow than in the wet meadow. Adding nitrogen to the dry meadow increased primary production by 63%. Adding nitrogen and phosphorus increased primary production by 178%. In contrast, the wet meadow only showed relatively small but statistically significant responses to the additions of both nitrogen and phosphorus (fig. 18.5).
FIGURE 18.5 Effect of adding phosphorus (P) and/or nitrogen (N) on aboveground primary production in two environments in alpine tundra (clara from Bowman et al. 1993).
Bowman and bis colleagues suggest that these results show that nitrogen is the main nutrient limiting net primary production in the dry meadow and that nitrogen and phosphorus jointly limit net primary production in the wet meadow. They also suggest that light, not nutrients, may limit net primary production in the wet meadow. In other words, the higher biomass in the wet meadow may have produced enough shading to inhibit the growth response of some species to nutrient additions.
Experiments such as these have shown that despite the major influence of temperature and moisture on rates of primary production in terrestrial ecosystems, variation in nutrient availability can also have measurable influence. As we shall see in the next Case Histories section, nutrient availability is the main factor limiting primary production in aquatic ecosystems.
patterns of aquatic primary production
Aquatic primary production is generally limited by nutrient availability.
Limnologists and oceanographers have measured rates of primary production and nutrient concentrations in many lakes and at many coastal and oceanic study sites. These studies have produced one of the best documented patterns in the biosphere: the positive relationship between nutrient availability and rate of primary production in aquatic ecosystems.
Patterns and Models
A quantitative relationship between
phosphorus, an essential plant nutrient, and phytoplankton biomass was first
described for a series of lakes in
Later, Dillon and Rigler (1974) described a similar positive relationship between phosphorus and phytoplankton biomass for lake ecosystems throughout the Northern Hemisphere. Remarkably, the slopes of the lines describing the relationship between phosphorus and phytoplankton biomass for the Japanese and Canadian lakes were nearly identical (fig. 18.6).
FIGURE 18.6 Relationship between phosphorus concentration and algal biomas$ in north temperate lakes (data from Dillon and Rigler 1974).
The data from
FIGURE 18.7 Relationship between algal biomass and rate of primary production in temperate zone lakes (data from Smith 1979).
The Experimental Lakes Area was founded in
Both sides of
In conclusion, both correlations--between phosphorus concentration and rate of primary production, and whole lake experiments, involving nutrient additions---support the generalization that nutrient availability controls rates of primary production in freshwater ecosystems. Now, let's examine the evidence for this relationship in marine ecosystems.
Global Patterns of Marine
The geographic distribution of net primary
production in the sea indicates a positive influence of nuUient
availability on rates of primary production. Oceanographers have observed that
the highest rates of primary production by marine phytoplankton are generally
concentrated in areas with higher levels of nutrient availability (fig. 18.9).
The highest rates of primary production are concentrated along the margins of
continents over continental shelves and in areas of upwelling. Along
continental margins nutrients are renewed by runoff from the land and by
biological or physical disturbance of bottom sediments. As we saw in chapter 3,
the upwelling that brings nutrient-laden water from the depths to the surface
is concentrated along the west coasts of continents and around the continent of
FIGURE 18.9 Geographic variation in marine primary production (data from F.A.O. 1972).
Meanwhile, the central portions of the major oceans show low levels of nutrient availability and low rates of primary production. The main source of nutrient renewal in the surface waters of the open ocean is vertical mixing. Vertical mixing is generally blocked in open tropical oceans by a permanent thermocline. Consequently, the surface waters of open tropical oceans contain very low concentrations of nutrients and show some of the lowest rates of marine primary production.
What is the experimental evidence for
nutrient limitation of marine primary production? Some of the most thorough
studies have been conducted in the
In a test using a single algal species, Graneli added nutrients to filtered seawater from a series
of study sites. She added nitrate to one experimental group, phosphates to
another, and nothing to a third group of flasks (fig. 18.10). Notice that the flasks
with additional nitrate showed increased chlorophyll a concentrations at all
sites, while the flasks with additional phosphate had chlorophyll a
concentrations very similar to the control flasks. What do these results
indicate? They suggest that the rate of primary production in the
18.10 Nitrate control of primary production in the
Graneli did similar enrichment studies along a
series of stations in the Kattegat, the
There have been no experiments done in the
marine environment that are equivalent to the whole lake manipulations at the
Experimental Lakes Area (e.g., Schindler 1990). However, in one experiment,
researchers were able to alter the nutrient inputs and concentrations in
Dillon and Rigler suggested that limnologists pay attention to the scatter of points around lines showing a relationship between nutrient concentrations and phytoplankton biomass (F.A.O. 1972). We call that scatter of points residual variation. Residual variation is that proportion of variation not explained by the independent variable, in this case, by nutrient concentration. Dillon and Rigler suggested that environmental factors besides nutrient availability significantly influence phytoplankton biomass. One of those factors is the intensity of predation on the zooplankton that feed on phytoplankton. As we shall see in the next Case Histories section, consumers can influence rates of primary production in both terrestrial and aquatic ecosystems.
Consumers can influence rates of primary production in aquatic and terrestrial ecosystems.
In the first section of this chapter, we emphasized the effects of physical and chemical factors on rates of primary production. More recently, ecologists have discovered that primary production is also affected by consumers. Ecologists refer to the influences of physical and chemical factors, such as temperature and nutrients, on ecosystems as bottom-up controls. The influences of consumers on ecosystems are known as top-down controls. In the previous two sections we discussed bottom-up controls on rates of primary production. Here we discuss top-down control.
Stephen Carpenter, James Kitchell, and James Hodgson (1985) proposed that while nutrient inputs determine the potential rate of primary production in a lake, piscivorous and planktivorous fish can cause significant deviations from potential primary production. In support of their hypothesis, Carpenter and his colleagues (1991) cited a negative correlation between zooplankton size, an indication of grazing intensity, and primary production.
Carpenter and Kitchell (1988) proposed that the influences of consumers on lake primary production propagate through food webs. Since they visualized the effects of consumers coming from the top of food webs to the base, they called these effects on ecosystem properties "trophic cascades.'' The trophic cascade hypothesis (fig. 18.11) is very similar to the keystone species hypothesis (see chapter 17). However, notice that the trophic cascade model is focused on the effects of consumers on ecosystem processes, such as primary production, and not on their effects on species diversity.
FIGURE 18.11 The trophic cascade hypothesis.
Carpenter and Kitchell (1993) interpreted the trophic cascade in their study lakes as follows: Piscivores, such as largemouth bass, feed on planktivorous fish and invertebrates. Because of their influence on planktivorous fish, large-mouth bass indirectly affect populations of zooplankton. By reducing populations of planktivorous fish, largemouth bass reduce feeding pressure on zooplankton and zooplankton populations. Large-bodied zooplankton, the preferred prey of size-selective planktivorous fish (see chapter 6), soon dominate the zooplankton community. A dense population of large zooplankton reduces phytoplankton biomass and the rate of primary production. This interpretation of the trophic cascade is consistent with the negative correlation between zooplankton body size and primary production reported by Carpenter and his research team. This hypothesis is summarized in figure 18.12.
FIGURE 18.12 Predicted effects of piscivores on planktivore, herbivore, and phytoplankton biomass and production (data from Carpenter: Kitchell, and Hodgson 1985).
Carpenter and Kitchell tested their trophic cascade model by manipulating the fish communities in two lakes and using a third lake as a control. Figure 18.13 shows the overall design of their experiment. Two of the lakes contained substantial populations of largemouth bass. A third lake had no bass, due to occasional winterkill, but contained an abundance of planktivorous minnows. The researchers removed 90% of the large-mouth bass from one experimental lake and put them into the other. They simultaneously removed 90% of the planktivorous minnows from the second lake and introduced them to the first.They left a reference lake unmanipulated as a control.
The responses of the study lakes to the experimental manipulations support the trophic cascade hypothesis (fig. 18.13). Reducing the planktivorous fish population led to reduced rates of primary production. In the absence of planktivorous minnows, the predaceous invertebrate Chaoborus became more numerous. Chaoborus fed heavily upon the smaller herbivorous zooplankton, and the herbivorous zooplankton assemblage shifted in dominance from small to large species. In the presence of abundant, large herbivorous zooplankton, phytoplankton biomass and rate of primary production declined.
FIGURE 18.13 Experimental manipulations of ponds and responses.
Adding planktivorous minnows produced a complex ecological response. Increasing the planktivorous fish population led to increased rates of primary production. However, though the researchers increased the population of planktivorous fish in this experimental lake, they did so in an unintended way. Despite the best efforts of the researchers, a few bass remained. So, by introducing a large number of minnows they basically fed the remaining bass. An increased food supply combined with reduced population density induced a strong numerical response by the bass population (see chapter 10). The manipulation increased the reproductive rate of the remaining largemouth bass 50-fold, producing an abundance of young largemouth bass that feed voraciously on zooplankton.
The lake ecosystem responded to the increased biomass of planktivorous fish (young largemouth bass) as predicted at the outset of the experiment. The biomass of zooplankton decreased sharply, the average size of herbivorous zooplankton decreased, and phytoplankton biomass and primary production increased.
The results of these whole lake experiments show that the trophic activities of a few species can have large effects on ecosystem processes. However, the majority of trophic cascades described by ecologists have been in aquatic ecosystems with algae as primary producers. This pattern prompted Donald Strong (1992) to ask, "Are trophic cascades all wet?'' Strong suggested that trophic cascades most likely occur in ecosystems of lower species diversity and reduced spatial and temporal complexity. These are characteristics of many aquatic ecosystems. Despite these restrictions, consumers have significant effects on rates of primary production in some terrestrial ecosystems; one of those is the Serengeti grassland ecosystem.
Grazing by Large Mammals and Primary Production on the Serengeti
The Serengeti-Mara a
Over two decades of research on the
Serengeti ecosystem in
As you might predict, the rate of primary production on the Serengeti is positively correlated with the quantity of rainfall. However, McNaughton (1976) also found that grazing can increase primary production. He fenced in some areas in the western Serengeti to explore the influence of herbivores on production. The migrating wildebeest that flooded into the study site grazed intensively for 4 days, consuming approxi mately 85% of plant biomass.
During the month after the wildebeest left the study area, biomass within the enclosures decreased, while the biomass of vegetation outside the enclosures increased (fig. 18.14). Grazing increases the growth rate of many grass species, a response to grazing called compensatory growth. The mechanisms underlying compensatory growth include lower rates of respiration due to lower plant biomass reduced self-shading. and improved water baLance due to reduce leaf area.
FIGURE 18.14 Growth response by grasses grazed by wildebeest (data from McNaughton 1976).
The compensatory growth observed by McNanghton was highest at intermediate grazing intensities (fig. 18.15). Apparently, light grazing is insufficient to produce compensatory growth and very heavy grazing reduces the capacity of the plant to recover. The large grazing mammals of the Serengeti have substantial influences on its rate of primary production. As McNanghton put it, "African ecosystems cannot be understood without close consideration of the large mammals. These animals interact with their habitats in complex and powerful patterns influencing ecosystems for long periods."
FIGURE 18.15 Grazing intensity and primary production of Serengeti grassland (data from McNaughton 1985).
and his colleagues described is essentially atrophic cascade in a terrestrial
environment where the feeding activities of consumers have a major influence on
ecosystem properties. The Serengeti is now an exceptional terrestrial ecosystem
but it was not always so. As we saw in chapter 2, the extensive grasslands of
North America and
In the Serengeti, lions are the top predators. Though they are occasionally killed by hyenas, there are no predators that depend principally upon hunting lions as a source of energy. In the ponds studied by Carpenter and Kitchell, largemouth bass were the top carnivores. The number of trophic levels in ecosystems ranges from two to five or six, perhaps seven or eight in exceptional ecosystems. In any case, ecosystems have a limited number of trophic levels. What limits the number of trophic levels? We will consider the factors that limit the number of trophic levels in ecosystems in the next section.
CASE HISTORIES: trophic levels
Energy losses limit the number of trophic levels in ecosystems.
We began this chapter with a partial and highly qualitative energy budget for a forest: Sunlight shines down on the canopy of a forest--some is reflected, some is converted to heat energy, and some is absorbed by chlorophyll. The energy budgets of ecosystems reveal that with each transfer or conversion of energy, some energy is lost. To verify that these losses have the potential to limit the number of tropbic levels in ecosystems, we need to quantify the flow of energy through ecosystems. One of the very first ecologists to quantify the flux of energy through ecosystems was Raymond Lindeman.
ATrophic Dynamic View of Ecosystems
(1942) received his Ph.D. from the
Lindeman suggested grouping organisms within an ecosystem into trophic levels: primary producers, primary consumers, secondary consumers, tertiary consumers, and so forth. In this scheme, each trophic level feeds on the one immediately below it. Energy enters the ecosystem as primary producers engage in photosynthesis and convert solar energy into biomass. As energy is transferred from one trophic level to another, energy is lost due to limited assimilation, respiration by consumers, and heat production. As a result of these losses, the quantity of energy in an ecosystem decreases with each successive trophic level, forming a pyramid-shaped distribution of energy among trophic levels. Lindeman called these trophic pyramids "Eltonian pyramids," since Charles Elton (1927) was the first to propose that the distribution of energy among trophic levels is shaped like a pyramid.
Figure 18.16 shows the distribution of
annual primary production among trophic levels in
FIGURE 18.16 Annual production by trophic level in two lakes (data from Lindeman 1942).
pioneering work, many other ecologists studied energy flow within ecosystems.
One of the most comprehensive of these later studies focused on the
Flow in a Temperate Deciduous
James Gosz and
his colleagues (1978) studied energy flow in the
FIGURE 18.17 Energy budget for a temperate deciduous forest (data from Gosz et al. 1978).
First let's examine the distribution of
organic matter among the major components of the Hubbard Brook ecosystem. The
largest single pool of energy in the forest, 122,442 kcal/m2,
occurred as dead organic matter. Most of the dead organic matter, 88,120 kcal/m2, was organic matter in the upper
The total standing stock of energy
occurring as dead organic matter and living plant biomass was 193,862 kcal/m2. This estimate by Gosz and his colleagues dwarfs the energy stored in all
other portions of the ecosystem. For instance, the energetic content of a
caterpillar population during a severe population outbreak amounted to only 160 kcal/m2. However, even this amount
far exceeds the total energetic content of all vertebrate biomass. The
researchers estimated that the total energetic content of the most numerous
vertebrates, including chipmunks, mice, shrews, salamanders, and birds,
amounted to less than 1 kcal/m2. Now that we have inventoried the major
standing stocks of energy, let's look at energy flow through the
The main source of energy for the ecosystem is solar radiation. The total input of solar energy to the study area during the growing season was estimated to be 480,000 kcal/m2 (expressed as 100% in fig. 18.17). Of this total energy input, 15% was reflected, 41% was converted to heat, and 42% was absorbed during evapotranspiration. About 2.2% of the solar input was fixed by plants as gross primary production. Plant respiration accounted for 1.2%, leaving about 1% as net primary production. In other words, only about 1% of the solar input to the Hubbard Brook ecosystem was available to the herbivores and detritivores that made up the second trophic level.
About 1,199 kcal/m2 of net
primary production in the
Now let's go back to the concept that
started this section: Energy losses limit the number of trophic
levels in ecosystems. The energy budget carefully constructed by Gosz and his colleagues gives us a basis for understanding
this concept. Net primary production in the
The top predator on the African savanna is the lion. We might imagine predators fierce enough to prey on lions, but the energetics of energy conversion and transfer within ecosystems would preclude such a predator.
We can see from the studies of Gosz and his colleagues and others that ecosystems depend upon an outside input of energy. Ecosystems store some energy in the form of dead organic matter and biomass, but most energy flows through. As we shall see in chapter 19, however, ecosystems recycle elements such as nitrogen and sulfur. In the next Applications and Tools section we review how forms of these and other elements can be used as a tool to determine the trophic structure of ecosystems.
APPLICATIONS AND TOOLS:
using stable isotope analysis to trace energy flow through ecosystems
How do ecologists study the flow of energy through ecosystems? First, they identify the organisms that make up the biological part of the ecosystem. Then, they determine the feeding habits of consumers. They may identify consumers down to species or assign them broader taxonomic categories. Next, they assign organisms to trophic levels and determine (1) the biomass of each trophic level, (2) the rate of energy or food intake by each trophic level, (3) the rate of energy assimilation, (4) the rate of respiration, and (5) rates of loss of energy to predators, parasites, etc. Finally, ecologists combine their information on individual trophic levels to construct atrophic pyramid such as that constructed by Lindeman (see fig. 18.16) or an energy flow diagram such as that by Gosz and his colleagues (see fig. 18.17).
One of the fundamental steps in constructing a trophic pyramid or energy flow diagram is assigning organisms to trophic levels. While this task may sound easy, for most organisms, it is not. Most assignments are based on studies of feeding habits. If food items are easily identified and feeding habits are well studied and do not change significantly over time or from place to place, you may accurately identify feeding relations and assign organisms to trophic levels. However, if feeding habits are variable or if food items are difficult to identify, it may be difficult to assign organisms accurately to a particular trophic level.
Stable Isotope Analysis
A relatively new tool for studying trophic structure, stable isotope analysis, demonstrates that many species have highly variable feeding habits. This result suggests that conventional analyses of trophic structure may be inadequate to accurately estimate the trophic level of many species. However, stable isotope analysis also offers a potential solution to the problem it identifies. To understand the applications of this analytical tool, we need to know a little about the isotopes themselves and about their behavior in ecosystems.
Most chemical elements include several
stable isotopes, which occur in different concentrations in different environments
or differ in concentration from one organism to anothen
Stable isotopes of carbon, for example, include
Different organisms contain different
ratios of light and heavy stable isotopes because they use different sources of
these elements, because they preferentially use (fractionate) different stable
isotopes, or because they use different sources and fractionate. For instance,
the lighter isotope of nitrogen, 14N, is preferentially excreted by
organisms during protein synthesis. As a consequence of this preferential excretion
of 14N, an organism becomes relatively enriched in 15N compared to its food.
Therefore, as materials pass from one trophic level
to the next, tissues become richer in 15N. The highest trophic levels within an ecosystem contain the highest relative
concentrations of 15N, while the lowest trophic
levels contain the lowest concentrations. Stable isotope analysis can also
measure the relative contribution of C3 and C4 plants to
a species' diet. This is possible because C4 plants are relatively richer
The concentrations of stable isotopes are generally expressed as differences in the concentration of the heavier isotope relative to some standard. The units of measurement are differences (±) in parts per thousand (±‰). These differences are calculated as:
X = the relative concentration of the heavier isotope,
Rsample = the isotopic ratio in the sample, for example,
Rstandard = the isotopic ratio in the standard, for example,
The reference materials used as standards
in the isotopic analyses of nitrogen, carbon, and sulfur are the 1 5N:
14N ratio in atmospheric nitrogen, the
The ecologist measures the ratio of stable isotopes in a sample and then expresses that ratio as a difference relative to some standard. IfδX = 0, then the ratios of the isotopes in the sample and the standard are the same; ifδX = -X ‰, the con- centration of the heavier isotope is lower (e.g., 15N) in the sample compared to the standard, and ifδ = +X ‰, the concentration of the heavier isotope is higher in the sample eompared to the standard. The important point here is that these isotopic ratios are generally different in different parts of ecosystems. Therefore, ecologists can use isotopic ratios to study the structure and processes in ecosystems. Here are some examples.
Using Stable Isotopes to Identify Sources of Energy in a Salt Marsh
The main energy source in a salt marsh in eastern North America is primary production by the salt marsh grass Spartina, most of which is consumed as detritus. The detritus of Spartina is carried into tidal creeks at high tide, where it is consumed by a variety of organisms, including crabs, oysters, and mussels. However, Spartina is not the only potential source of food for these organisms. The waters of the salt marsh also contain organic matter from upland plants and carry phytoplankton. How much might these other food sources contribute to energy flow through the salt marsh ecosystem?
Bruce Peterson. Robert Howarth, and Robert Garritt (1985) used stable isotopes to determine the relative
contributions of Spartina, phytoplankton, and upland
plants to the nutriation of the ribbed mussel, Geukinsia demissa, a dominant filter-feeding
As a solution for these problems, Peterson
and his colleagues used the ratios of stable isotopes of carbon, nitrogen, and
sulfur to assess the relative contributions of alternative food sources to the
nutrition of the mussel. They used the stable isotopes of these three elements
because their ratios are different in phytoplankton, upland C3
plants (see chapter 6), and Spartina, a C4
grass (fig. 18.18). Upland plants, with a δ
Isotopic content of potential food sources for the ribbed mussel, Geukinsia demissa, in a
Because of these differences in isotopic concentrations, the researchers were abte to identify the relative contributions of potential food sources to the diet of the mussel (fig.18.19). Their analyses showed that Geukinsia gets most of its energy from plankton and Spartina but that the relative contributions of these two food sources depends upon location. In the interior of the marsh, the mussel feeds mainly on Spartina, while near the mouth of the marsh it depends mainly on plankton. This is an example of how analyses of stable isotopes can provide us with a window to the otherwise hidden biology of species.
Variation inisotopiccompositionofribbedmussels. Geukinsia demissa, by distance
inland in a
Food Habits of Prehistoric Human Populations
Stable isotope analysis is also helping
archeologists to reconstruct the history of our own species. For instance,
stable isotopes have provided insights into the trophic
position of humans in prehistoric ecosystems. The people of CentraI
and South America began to cultivate corn, Zea mays, about 6,000 to 7,000 years ago, and the farming of
corn eventually spread into
Corn appears in the archaeological record
of the forested regions of eastern
Concentration of I
Stable isotope analyses have been used to analyze the diets of other prehistoric human populations. For instance, stable isotope analysis has shown that about 6,000 years ago the diets of human populations in what is now Denmark shifted from a predominately marine diet to one dominated by terrestrial foods. Without the tool of stable isotope analysis, it would be much more difficult to accurately estimate timing of these significant shifts in the trophic ecology of prehistoric human populations.
Stable isotope analyses continue to improve our understanding of energy flow through ecosystems. While energy flows through ecosystems in a one-way path, the elements, or nutrients, upon which organisms depend are recycled and may be used over and over again. The cycling of these nutrients is the subject of chapter 19.
We can view a forest, a stream, or an ocean as a system that absorbs, transforms, and stores energy. In this view, physical, chemical, and biological structures and processes are inseparable. When we look at natural systems in this way we view them as ecosystems. An ecosystem is a biological community plus all of the abiotic factors influencing that community.
Primary production, the fixation of energy by autotrophs, is one of the most important ecosystem processes. The rate of primary production is the amount of energy fixed over some interval of time. Gross primary production is the total amount of energy fixed by all the autotrophs in the ecosystem. Net primary production is the amount of energy left over after autotrophs have met their own energetic needs.
Terrestrial primary production is generally limited by temperature and moisture. The variables most highly correlated with variation in terrestrial primary production are temperature and moisture. Highest rates of terrestrial primary production occur under warm, moist conditions. Temperature and moisture conditions can be combined in a single measure called annual actual evapotranspiration, or AET, which is the total amount of water that evaporates and transpires off a landscape during the course of a year. Annual AET is positively correlated with net primary production in terrestrial ecosystems. However, significant variation in terrestrial primary production results from differences in soil fertility.
Aquatic primary production is generally limited by nutrient availability. One of the best documented patterns in the biosphere is the positive relationship between nutrient availability and rate of primary production in aquatic ecosystems. Phosphorus concentration usually limits rates of primary production in freshwater ecosystems, while nitrogen concentration usually limits rates of marine primary production.
Consumers can influence rates of primary production in aquatic and terrestrial ecosystems. Piscivorous fish can indirectly reduce rates of primary production in lakes by reducing the density of plankton-feeding fish. Reduced density of planktivorous fish can lead to increased density of herbivorous zooplankton, which can reduce the densities of phytoplankton and rates of primary production. Intense grazing by large mammalian herbivores on the Serengeti increases annual net primary production by inducing compensatory growth in grasses.
Energy losses limit the number of tropbie levels in ecosystems. Ecosystem ecologists have simplified the trophic structure of ecosystems by arranging species into trophic levels based upon the predominant source of their nutrition. A trophic level is determined by the number of transfers of energy from primary producers to that level. As energy is transferred from one trophic level to another, energy is lost due to limited assimilation, respiration by consumers, and heat production. As a result of these losses, the quantity of energy in an ecosystem decreases with each successive trophic level, forming a pyramid-shaped distribution of energy among trophic levels. As losses between trophic levels accumulate, eventually there is insufficient energy to support a viable population at a higher trophic level.
Stable isotope analysis can be used to trace the flow of energy through ecosystems. The ratios of different stable isotopes of important elements such as nitrogen and carbon are generally different in different parts of ecosystems. As a consequence, ecologists can use isotopic ratios to study the trophic structure and energy flow through ecosystems. Stable isotope analysis has helped quantify dietary composition of wild populations and the major sources of energy used by prehistoric human populations.
1. Population, community, and ecosystem ecologists study structure and process. However, they focus on different natural characteristics. Contrast the important structures and processes in a forest from the perspectives of population, community, and ecosystem ecologists.
2. M. Huston (1994b) pointed out that the
well-documented pattern of increasing annual primary production from the poles
to the equator is strongly influenced by the longer growing season at low
latitudes. The following data are from table
Complete the missing data to compare the monthly production of boreal, temperate, and tropical forests. How does this shortterm perspective of primary production in high-, middle-, and low-latitude forests compare to an annual perspective? How does the short-term perspective change our perception of tropical versus high-latitude forests?
3. Many migratory birds spend approximately half the year in temperate forests during the warm breeding season and the other half of the year in tropical forest. Given the analyses you made in the previous question, which forest appears to be more productive from the perspective of these migratory birds?
4. Field experiments demonstrate that variation in soil fertility influences terrestrial primary production. However, we cannot say that nutrients exert primary control. That role is still attrib uted to temperature and moisture. Why do ecologists still attribute the main control of terrestrial primary production to temperature and moisture? Consider the difference in primary production between arctic tundra and tropical rain forest (see fig. 18.2) and the extent to which nutrient additions (Shaver and Chapin 1988) changed primary production in tundra.
5. Shaver and Chapin (1988) pointed out that though the tundra ecosystems they studied consistently increased primary production in response to fertilization, individual species and growth forms showed more variation in response. Some species and growth forms showed no response, while others decreased production on the fertilized plots. What do these dif ferences in response say about using the responses of individual species to predict responses at the ecosystem level? What about the reverse----can we predict the responses of individual species or growth forms from ecosystem-level responses?
6. Compare the pictures of trophic structure that emerged from our discussions of food webs in chapter 17 with those in this chapter. What are the strengths of each perspective? What are their limitations?
7. Suppose you are studying a community of small mammals that live on the boundary between a riverside forest and a semidesert grassland. One of your concerns is to discover the relative contributions of the grassland and the forest to the nutrition of small mammals living between the two ecosystems. Design a research program to find out. (Hint: The grassland is dominated by C4 grasses and the forest by C3 plants).
8. Most of the energy that flows through a forest ecosystem flows through detritus-based food chains, and the detritus consists mainly of dead plant tissues (e.g., leaves and wood). In contrast, most of the energy flowing through a pelagic marine or freshwater ecosystem flows through grazing food chains with phytoplankton constituting the major primary producers. Ecologists have determined that on average, a calorie or joule of energy takes only several days to pass through the pelagic ecosystem but on the order of a quarter of a century to pass through the forest ecosystem. Explain.
9. In chapter 17, we examined the influences of keystone species on the structure of communities. In this chapter we reviewed trophic cascades. Discuss the similarities and differences between these two concepts. Compare the measurements and methods of ecologists studying keystone species remus those studying trophic cascades.
10. The studies of nutrient limitation of aquatic primary production that we reviewed focused almost entirely on lakes within the temperate zone. Suppose you are an ecologist interested in determining whether primary production in tropical lakes is subject to similar control by nutrient availability. Design a study to find out what controls rates of primary production in tropical lakes. Use all the sources of information at your disposal, including published research, surveys of natural variation, and large- and small-scale experiments.