The influence of detritivores on litter decomposition

The fragmentation of leaves by invertebrate feeding and abrasion constitutes another important stage in leaf breakdown. Aquatic insects and crustaceans are the most common consumers of CPOM. During the decomposition of autumn-shed leaves in temperate woodland streams, microbial populations play a central role not only in decomposing the leaf substrate, but also in altering the chemical nature of the leaf material, rendering it more palatable and nutritious to consumers. In turn, the feeding activities of detritivores significantly accelerate the decomposition process. Their contribution to the fragmentation of coarse particles through feeding activities and production of feces significantly accelerates breakdown rates and influences subsequent biological processing of the original CPOM inputs.

Several lines of evidence indicate that shredders accelerate the breakdown of leaves in streams (Webster and Benfield 1986). The finding that leaf packs in mesh bags decomposed more slowly than those tethered to bricks with fishing line indicated that the former method underestimated breakdown rate. Exclusion of detritivores is a likely cause of this difference, and leaf breakdown is more rapid when bags with larger mesh size are used (e.g., Benfield et al. 1979, Wright and Covich 2005), presumably because invertebrate access is greater. Furthermore, breakdown rates are higher where invertebrates are more abundant (Graca 2001, Sponseller and Benfield 2001), and there is a positive relationship between invertebrate preferences and decay rates (Webster and Benfield 1986). Hieber and Gessner (2002) estimated that 64% of mass loss in alder and 51% in willow leaves were due to shredders that colonized leaves early in the experiment (Figure 7.9).

Comparison of decay rates in experiments with and without insect detritivores establishes that as much as half of leaf degradation can be attributed to the presence of animals. Processing rates in two experimental streams, one lacking invertebrates and another stocked with detritivores (Típula, Pycnopsyche, and Pteronarcys) at densities believed to represent natural maxima, indicated that 21-24% of the loss of hickory leaves was due to the influence of detritivores (Petersen and Cummins 1974). The contribution of macroinvertebrates to the breakdown of Phragmítes, a macrophyte, was comparable

FIGURE 7.9 Colonization of alder (*) and willow (O) leaf packs by (a) macroinvertebrates and (b) shredders in a Black Forest stream, Germany. Error bars represent 95% confidence intervals. (Reproduced from Hieber and Gessner 2002.)

(Polunin 1982). In an Appalachian stream, exclusion of macroconsumers, primarily crayfish, using electric fences resulted in lower breakdown rates of rhododendron leaves (Schofield et al. 2001). When present, crayfish were responsible for 33% of leaf breakdown in summer and 54% in autumn. Sponseller and Benfield (2001) observed faster leaf breakdown with higher shredder density and biomass (Figure 7.10). In addition to direct consumption, possible influences of detritivore feeding include release of nutrients and DOM, comminution of litter, and modification of water circulation (Polunin 1984). The freshwater shrimp Xipho-caris elongata, a consumer of large leaves in

Act Science Graph Question
FIGURE 7.10 Correlations between leaf breakdown rates and (a) density and (b) biomass of shredders expressed per gram of leaf AFDM. (Reproduced from Sponseller and Benfield 2001.)

tropical streams, increased the concentration of both total dissolved N and dissolved organic carbon (DOC), and also the concentration and transport of POM (Crowl et al. 2001).

The experimental removal of detritivorous insects from a small mountain stream in North Carolina provides a particularly convincing demonstration that animal consumers regulate rates of litter decomposition. Wallace et al. (1982a, 1991) added the insecticide methoxychlor to one small stream in February, with supplemental treatments in May, August, and November. Massive downstream drift of invertebrates occurred and insect densities subsequently were reduced to <10% of numbers in an adjacent, untreated reference stream, while oligochaetes increased roughly threefold. Leaf breakdown rates were significantly slower in the treated stream, presumably due to the great reduction in insect density, and the magnitude of the effect was greatest for the most refractory leaf species (Table 7.2). Export of suspended fine particu-lates also was reduced in the treated stream, consistent with the finding of reduced leaf processing.

Most research on the role of detritivores in leaf breakdown has been conducted in the temperate zone, and less is known from other latitudes. Studies conducted in Colombia, Costa Rica, Venezuela, Papua New Guinea, and Kenya found that insect shredders were scarce, despite the fact that densities of other invertebrates were similar to those reported in the temperate zone (Yule 1996, Dobson et al. 2002, Rincon et al. 2005, Wantzen and Wagner 2006). In these studies, shredders represented <7% of total macroinvertebrate abundance, while in European rivers this value ranged from 10% to 43% (Dobson et al. 2002, Hieber and Gessner 2002). Nonetheless, leaf breakdown rates in streams in Costa Rica (Rosemond et al. 1998) and in Colombia (Mathuriau and Chauvet 2002) were rapid, suggesting a greater role of microorganisms in tropical streams. Possible explanations for these findings include higher temperatures in the tropics, lower quality

TABLE 7.2 Estimated half-lives (days) based on exponential decay in ash-free dry mass of four leaf species in a stream treated with an insecticide, compared to a reference stream. (From Wallace et al. 1982a, b.)

Reference Treatment Change in

Dogwood (Cornus florida) Red maple (Acer rubrum) White oak (Quercus alba) Rhododendron (Rhododendron maxima)

Reference Treatment Change in

stream

stream

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