Ecological Example Four Compartment Food

Figure 13 shows steady-state energy flows and stocks in a bog in Russia. The analysts disaggregated this ecosystem into four compartments: plants, animals, decomposers, and detritus. Detritus consists of undifferentiated dead material and therefore has no metabolic losses. All other compartments contribute to detritus. Additionally, animals and decomposers also eat detritus, resulting in two feedback flows and a web structure. All indicators can be calculated from the equations given in Table 2. The results are given in Table 4. Because plants have only a solar input, their energy intensity is quite low and TP = 1. For the other compartments, however, the energy intensities are higher and the TPs are high. Decomposers have a TP = 4.9, higher than the value of 4 which one would expect for a food chain instead of this web. Decomposers come out on top in both energy intensity and TP. Because this system has only one input, PL is just TP — 1.

Table 4 shows that residence times are affected dramatically by web structure. Isolated compartment residence times (=stock/throughflow) are long for plants and detritus, and short for animals and decomposers. The longest, detritus, is 760 times the shortest, animals. In contrast, in-system residence times differ by only a factor of 4. Both animals and decomposers, which in isolation

Figure 13 Energy flows (g fixed carbon per m2 per year) in a bog in Russia. Detritus is undifferentiated dead material, and therefore has no metabolic loss. Numbers in compartments are stocks (g fixed carbon per m2). From Logofet DO and Alexandrov GA (1984) Modelling of matter cycle in a mesotrophic bog ecosystem. Part 1: Linear analysis of carbon environs. Ecological Modelling 21: 247-258.

Figure 13 Energy flows (g fixed carbon per m2 per year) in a bog in Russia. Detritus is undifferentiated dead material, and therefore has no metabolic loss. Numbers in compartments are stocks (g fixed carbon per m2). From Logofet DO and Alexandrov GA (1984) Modelling of matter cycle in a mesotrophic bog ecosystem. Part 1: Linear analysis of carbon environs. Ecological Modelling 21: 247-258.

Table 4 Energy intensities, trophic positions, path lengths, and residence times for the Russian bog food web of Figure 13

Energy

Trophic

In-system residence

intensity, e

position

Path length

Isolated-compartment residence

time, t

Compartment

(cal GPP/cal)

(TP)

(PL)

time, t (years)

(years)

Plants

2.60

1.00

0.00

8.60

8.60

Animals

9.56

3.43

2.43

0.017

20.5

Detritus

12.4

3.90

2.90

12.6

32.7

Decomposers

23.8

4.90

3.90

0.060

32.8

would be fast, have large input flows from detritus (which in isolation is slow). The consequence is that all three are comparably slow. This is one aspect of the notion that detritus links tend to slow down the response of ecosystems to perturbations.

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