BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.3. Ecosystems and Energy Flow

10.3.4. Ecological Pyramids

The first ecological diagrams in the form of pyramids were constructed in the 1920s by Charles Elton. They were based on field observations of a range of animals from different size classes. Elton did not include primary producers in them, nor did he make any distinctions between detritivores and decomposers. However, he noted that predators are generally larger than their prey and realized that such a ratio is highly specific only to certain animal size classes. In the 1940s, the American ecologist Raymond Lindeman applied Elton's idea to trophic levels, abstracting away from the specific organisms that constitute them. Yet, while classifying animals by size classes is straightforward, determining which trophic level they belong to is far more complex. In any case, this can only be done in a highly simplified and generalized manner.

10.4. Using the information presented in Fig. 10.5 and Section 10.3.2 regarding consumers, identify and list the food chains with a hawk at the third, fourth, fifth, and sixth trophic levels.

Feeding relationships and Energy Transfer Efficiency within the biotic component of an ecosystem are traditionally depicted as stepped pyramids. This provides a clear visual basis for comparing: 1) different ecosystems; 2) seasonal states of the same ecosystem; 3) different phases of ecosystem succession.

There are Three types of pyramids:

1) pyramids of numbers, based on counting the organisms at each trophic level;

2) pyramids of biomass, which use the total mass (usually dry mass) of organisms at each trophic level;

3) pyramids of energy, which account for the energy content of organisms at each trophic level.

Pyramids of energy are considered the most important because they directly address the foundation of feeding relationships—the flow of energy required for the metabolic activities of all organisms.

Pyramids of Numbers

To construct a pyramid of numbers, one must first count the individuals of different species in a given habitat and then attempt to assign these species to trophic levels. Typically, this results in a gradual decrease in the number of organisms at each trophic level when moving from the lowest level to the highest. The number of individuals at different levels is represented by stacked rectangles whose length is proportional to the number of organisms per unit area of the habitat or per unit volume (if the aquatic ecosystem is considered). An idealized pyramid of numbers is shown in Fig. 10.6, A.

While data for constructing such a pyramid are relatively straightforward to obtain using standard census Methods, their application is fraught with several problems. The three most significant are as follows.

1. Producers vary greatly in size, yet any individual herbaceous plant, alga, or tree must be assigned the same status. This explains why a true "pyramid" shape is not always obtained (see Fig. 10.6, B). Parasitic food chains can also yield an inverted pyramid.

2. The range of numbers involved can be so broad that drawing the pyramid on a single linear scale is often difficult. A logarithmic scale can be used, but the resulting picture must be interpreted with extreme caution.

3. The trophic level of a given species can be difficult to determine.

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Fig. 10.6. A. A typical pyramid of numbers. The length of each rectangle is proportional to the Abundance of organisms at each trophic level. A logarithmic horizontal scale is most commonly used. Higher-order consumers are conventionally referred to as apex predators. B. Inverted pyramid of numbers for Wytham Woods in Oxfordshire. The horizontal scale is logarithmic. The numbers on the left reflect Organism abundance per hectare. Only oak trees were counted as producers. (Data from: Varley, 1970.)

10.5. What changes would occur in the pyramid of numbers shown in Fig. 10.6, B, for instance, by mid-winter?

Pyramids of Biomass

The inconveniences associated with pyramids of numbers can be avoided by constructing pyramids of biomass, which account for the total mass of organisms (biomass) at each trophic level. Such assessments require weighing typical representatives of each species, which is naturally labor-intensive and may require expensive equipment. Ideally, dry biomasses are compared. These are either estimated approximately from "fresh" weight data or determined destructively (Experiment 11.2). The rectangles forming the pyramid now correspond to the mass of organisms per unit area or unit volume of the habitat. Fig. 10.7 shows the biomass pyramid of a certain aquatic ecosystem. Note that the biomass of producers (phytoplankton) is less than the biomass of primary consumers (zooplankton), and only above this level does the pyramid acquire its characteristic shape. When samples are taken—in other words, at any given moment in time—what is always measured is the standing crop or standing biomass. It is important to understand that this value contains no information about The rate of biomass formation (productivity) or its consumption. Otherwise, errors may arise for two reasons.

1. If the rate of biomass consumption (grazing/predation) is roughly equal to the rate of its formation, the standing crop will not reflect productivity—that is, The amount of matter (and energy) transferred from one trophic level to another over a specific period, say a year. For example, in a good but intensively grazed pasture, the grass biomass at any given moment may be lower, yet its productivity higher, than in a fertile but lightly grazed pasture.

2. If the producers are small, such as planktonic Algae, they have high turnover rates; their rapid GROWTH AND REPRODUCTION compensate for heavy consumption and mortality. As a result, their standing crop will be smaller than that of large producers, such as trees, but their productivity over a given period may actually be higher. In other words, given the same productivity, plankton—which is much lighter than a tree—can support an amount of animal biomass equivalent to that supported by a tree. In general, if individuals are large and long-lived, the species' turnover rate is lower than when they are small and short-lived; that is, matter and energy accumulate more slowly in the former case. One of the potential consequences of this is illustrated in Fig. 10.7, where the biomass pyramid for the two lowest trophic levels is inverted. The mass of zooplankton exceeds the mass of the phytoplankton upon which they feed. This is characteristic of oceanic and lacustrine plankton communities during certain times of the year. Phytoplankton biomass exceeds zooplankton biomass during the spring "algal bloom," while the ratio reverses in other seasons. Such an apparent anomaly can be avoided by using the energy pyramid described below.

Fig. 10.7. Biomass pyramid for an aquatic ecosystem. The width of the tiers is proportional to the biomass at each trophic level. This pyramid is inverted at the base, which is frequently observed in food chains starting with phytoplankton. The organisms making up the phytoplankton are very small and the turnover rate of the zooplankton feeding on them is high.

However, once these nuances are understood, a comparative analysis of biomass pyramids yields valuable insights. For example, in an aquatic ecosystem, a stable pyramidal shape with a broad base suggests a persistent "algal bloom," i.e., an advanced stage of eutrophication (Sec. 10.8.2). Similarly, frequent inversions of marine pyramids suggest that exploiting the autotrophic (phytoplankton) base of aquatic ecosystems carries more risk than doing so in terrestrial ones.

10.6. Fig. 10.8 shows the annual fluctuations in the standing crop biomass of producers and primary consumers in a lake, along with certain environmental parameters.

a) In which months is the biomass pyramid inverted?

b) What factors account for: 1) the spring increase in phytoplankton biomass; 2) the rapid decline in its biomass during the summer; 3) its autumn increase; 4) its winter decrease?

Fig. 10.8. Fluctuations in the biomass of producers and primary consumers, as well as certain abiotic parameters of a lake over the course of a year. (M. A. Tribe, M. R. Erant, R. K. Snook (1974) Ecological principles, Basic Biology Course 4, CUP.)

Energy pyramids

Energy pyramids provide the clearest representation of the relationships between organisms at different trophic levels. These pyramids offer several key advantages.

1. Unlike pyramids of numbers and biomass, which describe the instantaneous state of an ecosystem, energy pyramids account for productivity, i.e., the rate of biomass formation. Each bar in an energy pyramid corresponds to the amount of energy (per unit volume or area) flowing through a given trophic level over a specified period. Fig. 10.9 illustrates such a pyramid for an aquatic ecosystem.

2. As shown in Table 10.1, the energy content per unit mass varies among different organisms. Therefore, comparisons based solely on biomass can lead to misleading Conclusions.

3. They allow comparisons not only between different ecosystems but also regarding the relative roles of populations within a single ecosystem, and inverted pyramids never occur.

4. A tier corresponding to the input of solar energy into the ecosystem can be added to the Base of the pyramid.

Although energy pyramids are generally considered the most useful of the three types discussed, the data required to construct them are the hardest to obtain. This approach requires additional information, such as the specific energy content (per unit of biomass) of various organisms, which entails burning representative samples of organisms. In practice, energy pyramids are constructed with reasonable accuracy using biomass data combined with previously accumulated energy content values.

Fig. 10.9. Energy pyramid for Silver Springs, Florida. The bars represent the flow of energy through the respective trophic levels (kJ · m^-2 · year^-1). (H. T. Odum (1971) Fundamentals of ecology, 3rd edition, W. B. Saunders)

Problems associated with The Use of ecological pyramids

1. The primary challenge lies in assigning organisms to specific trophic levels. As noted above, many consumers obtain food from multiple trophic levels simultaneously.

2. Some ecologists also argue that it is not entirely accurate to classify all plant material under the producer level. Many plant structures contain no chlorophyll, such as tubers, fruits, seeds, etc. It is more appropriate to classify these parts as consumed material rather than primary production. At the same time, many herbivores can digest chlorophyll, while others are highly selective in their diet, feeding solely on seeds, pollen, or nectar. It would be logical to modify ecological pyramids to account for these considerations.

3. Another issue is the frequent omission of dead organic matter (DOM) from pyramids. However, as previously mentioned, up to 80% of the energy assimilated by producers may be channeled not to consumers, but directly to detritivores and decomposers.