The relative roles of decomposers and detritivores

The roles of the decomposers and detritivores in decomposing dead organic matter can be compared in a variety of ways. A comparison of numbers will reveal a predominance of bacteria. This is almost inevitable because we are counting individual cells. A comparison of biomass gives a quite different picture. Figure 11.7 shows the relative amounts of biomass represented in different groups involved in the decomposition of litter on a forest floor (expressed as the relative amounts of nitrogen present). For most of the year, decomposers (microorganisms) accounted for five to 10 times as much of the biomass as the detri-tivores. The biomass of detritivores varied less through the year because they are less sensitive to climatic change, and they were actually predominant during a period in the winter.

Unfortunately, the biomass present in different groups of decomposers is itself a poor measure of their relative importance in the process of decomposition. Populations of organisms with short lives and high activity may contribute more to the activities in the community than larger, long-lived, sluggish species (e.g. slugs!) that make a greater contribution to biomass.

Lillebo et al. (1999) attempted to distinguish the relative roles, in the detritivore-dominated communities assessing the relative importance of decomposers and detritivores . . .

... in the decomposition of a salt marsh plant,...

Litter Decomposition Forest

Figure 11.7 The relative importance in forest litter decomposition of microflora in comparison with arthropods, earthworms and nematodes, expressed in terms of their relative content of nitrogen - a measure of their biomass. Microbial activity is much greater than that of detritivores but the latter is more constant through the year. (After Ausmus et al., 1976.)

Time (month)

Figure 11.7 The relative importance in forest litter decomposition of microflora in comparison with arthropods, earthworms and nematodes, expressed in terms of their relative content of nitrogen - a measure of their biomass. Microbial activity is much greater than that of detritivores but the latter is more constant through the year. (After Ausmus et al., 1976.)

decomposition of the salt marsh plant Spartina maritima, of bacteria, microfauna (e.g. flagellates) and macrofauna (e.g. the snail Hydrobia ulvae) by creating artificial communities in laboratory microcosms. At the end of the 99-day study, 32% of the biomass of Spartina leaves remained in the bacteria treatment, whereas only 8% remained when the microfauna and macrofauna were also present (Figure 11.8a). Separate analyses of the mineralization of the carbon, nitrogen and phosphorus content of the leaves also revealed that bacteria were responsible for the majority of the mineralization, but that microfauna and particularly macro-fauna enhanced the mineralization rates in the case of carbon and nitrogen (Figure 11.8b).

The decomposition of dead material is not simply due to the sum of the activities of microbes and detritivores: it is largely the result of interaction between the two. The shredding action of detritivores, such as the snail Hydrobia ulvae in the experiment of Lillebo et al. (1999), usually produces smaller particles with a larger surface area (per unit volume of litter) and thus increases the area of substrate available for microorganism growth. In addition, the activity of fungi may be stimulated by the disruption, through grazing, of competing hyphal networks. Moreover, the activity of both fungi and bacteria may be enhanced by the addition of mineral nutrients in urine and feces (Lussenhop, 1992).

The ways in which the decomposers and detritivores interact might be studied by following a leaf fragment through the process of decomposition, focusing attention on a part of the wall of a single cell. Initially, when the leaf falls to the ground, the piece of cell wall is protected from microbial attack because it lies within the plant tissue. The leaf is now chewed and the fragment enters the gut of, say, an isopod. Here it meets a new microbial flora in the gut and is acted on by the digestive enzymes of the isopod. The fragment emerges, changed by its passage through the gut. It is now part of the isopod's feces and is much more easily attacked by microorganisms, because it has been fragmented and partially digested. While microorganisms are colonizing, it may again be

Spartina Carbon

Figure 11.8 (a) Weight loss of Spartina maritima leaves during 99 days in the presence of: (i) macrofauna + microfauna + bacteria, (ii) microfauna + bacteria, or (iii) bacteria alone (mean ± SD). (b) Percentage of initial carbon, nitrogen and phosphorus content that was mineralized during 99 days in the three treatments. (After Lillebo et al., 1999.)

Figure 11.8 (a) Weight loss of Spartina maritima leaves during 99 days in the presence of: (i) macrofauna + microfauna + bacteria, (ii) microfauna + bacteria, or (iii) bacteria alone (mean ± SD). (b) Percentage of initial carbon, nitrogen and phosphorus content that was mineralized during 99 days in the three treatments. (After Lillebo et al., 1999.)

Figure 11.9 Relationships between (a) the decay of standard pieces of dead spruce wood over a 2.5-year period in Finland and the number of insect marks, and (b) the fungal infection rate (number of fungal isolates per standard piece of wood) and number of marks made by the beetle Tripodendron lineatum. Dry weight loss and number of insect marks in (a) were obtained by subtracting the values for each wood sample held in a permanently closed net cage from the corresponding value for its counterpart in a control cage that permitted insect entry. In some cases, the dry weight loss of the counterpart wood sample was lower, so the percentage weight loss was negative. This is possible because the number of insect visits does not explain all the variation in dry weight loss. (After Muller et al., 2002.)

Figure 11.9 Relationships between (a) the decay of standard pieces of dead spruce wood over a 2.5-year period in Finland and the number of insect marks, and (b) the fungal infection rate (number of fungal isolates per standard piece of wood) and number of marks made by the beetle Tripodendron lineatum. Dry weight loss and number of insect marks in (a) were obtained by subtracting the values for each wood sample held in a permanently closed net cage from the corresponding value for its counterpart in a control cage that permitted insect entry. In some cases, the dry weight loss of the counterpart wood sample was lower, so the percentage weight loss was negative. This is possible because the number of insect visits does not explain all the variation in dry weight loss. (After Muller et al., 2002.)

eaten, perhaps by a coprophagous springtail, and pass through the new environment of the springtail's gut. Incompletely digested fragments may again appear, this time in springtail feces, yet more easily accessible to microorganisms. The fragment may pass through several other guts in its progress from being a piece of dead tissue to its inevitable fate of becoming carbon dioxide and minerals.

Fragmentation by detritivores plays a key role in terrestrial situations because of the tough cell walls characteristic of vascular plant detritus. The same is true in many freshwater environments where terrestrial litter makes up most of the available detritus. In contrast, detritus at the lowest trophic level in marine environments consists of phytoplankton cells and seaweeds; the former present a high surface area without the need for physical disruption and the latter, lacking the structural polymers of vascular plant cell walls, are prone to fragmentation by physical factors. Rapid decomposition of marine detritus is probably less dependent on fragmentation by invertebrates; shredders are rare in the marine environment compared to its terrestrial and freshwater counterparts (Plante et al., 1990).

Dead wood provides particular challenges to colonization by microorganisms because of its patchy distribution and tough exterior. Insects can enhance fungal colonization of dead wood by carrying fungi to their 'target' or by enhancing access of air-disseminated fungal propagules by making holes in the outer bark into the phloem and xylem. Muller et al. (2002)

distributed standard pieces of spruce wood (Picea abies) on a forest floor in Finland. After 2.5 years, the numbers of insect 'marks' (boring and gnawing) were recorded and were found to be correlated with dry weight loss of the wood (Figure 11.9a). This relationship comes about because of biomass consumption by the insects but also, to an unknown extent, by fungal action that has been enhanced by insect activity. Thus, fungal infection rate was always high when there were more than 400 marks per piece of wood made by the common ambrosia beetle Tripodendron lineatum (Figure 11.9b). This species burrows deeply into the sapwood and produces galleries about 1 mm in diameter. Some of the fungal species involved are likely to have been transmitted by the beetle (e.g. Ceratocystis piceae) but the invasion of other, air-disseminated types is likely to have been promoted by the galleries left by the beetle.

The enhancement of microbial respiration by the action of detritivores has also been reported in the decomposition of small mammal carcasses. Two sets of insect-free rodent carcasses weighing 25 g were exposed under experimental conditions in an English grassland in the fall. In one set the carcasses were left intact. In the other, the bodies were artificially riddled with tunnels by repeated piercing of the material with a dissecting needle to simulate the action of blowfly larvae in the carcass. The results of this experiment paralleled those of the wood decomposition study above; here, the tunnels enhanced microbial activity (Figure 11.10) by disseminating the microflora as well as increasing the aeration of the carcass.

... and in small mammal carcasses

150

Artificially tunneled

of CO2 per hour per gram litial dry weight of carcass 50

o o

carcass

/ / / /

Intact carcass

1 1

i

0

5 10

15

Time (days)

Figure 11.10 The evolution of carbon dioxide (CO2), a measure of microbial activity, from carcasses of small mammals placed in 'respiration' cylinders and screened from insect attack. One set of carcasses was left intact, while the second set was pierced repeatedly with a dissecting needle to simulate the action of tunneling by blowfly larvae. (After Putman, 1978a.)

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Responses

  • Marco Wexler
    What are macro decomposers?
    6 years ago
  • ERNO
    Why spartina weight loss around decomposers?
    5 years ago
  • JUSTIN
    Which attacks organic matter first decomosers or detrivores?
    3 years ago

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