**5. Effect of the edaphic biota on decomposition — Top-down/bottom-up control**

The edaphic organisms organize in an intricate trophic web, and the diversity of the litter mixture would alter the interactions among these organisms and therefore the effects of this interaction on litter decomposition. The knowledge regarding trophic webs related to the decomposition process is very weak, especially in tropical biomes. This lack of knowledge is caused primarily by the weakness of taxonomic and biological knowledge [64, 65], both

passage of litter along the digestive tract of detritivores facilitates the degradation by the enzymes of decomposer organisms [66, 67]. Thus, the action of animals may increase the abundance and/or species richness of microorganisms, with a consequent increase in decom‐

Biodiversity and Ecosystem Functioning: a Conceptual Model of Leaf Litter Decomposition

http://dx.doi.org/10.5772/57396

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**Figure 3.** Simplified flowchart of the edaphic trophic web, depicting possible pathways of the effect of diversity of

On the other hand, the higher abundance and/or diversity of decomposers may lead to an increase of abundance and/or species richness of fungivores and bacteriovores (called here microbivores), due to an increase of resources available to these animals. An increase of microbivores may augment a predation pressure on decomposers, decreasing their abundance and activity. As a consequence, there would be a decrease of litter decomposition (continuous line in Figure 3). This control pathway of the decomposers by the microbivores could represent

Moreover, predators, not depicted in the trophic web of Figure 3, may control top-down both detritivores and microbivores, making this trophic web more complex and difficult to interpret. The study of edaphic trophic webs, its influence on litter decomposition process and the effects of litter mixture diversity on this web must thus be studied through well designed experiments. Correlational studies may give weak evidences regarding the relative role of each group of organisms within this trophic web, since several of the involved factors in these webs tend to covariate and confound data interpretation [65]. The exclusion of some groups or trophic levels may represent, hence, an option to study these trophic webs, provided that the techniques involved in the exclusion process do not interfere in the decomposition process or in other organisms. A critique of the use of fumigation as an arthropod exclusion method is

According to the above discussions, it is possible to notice that a single response relating the effects of litter diversity on decomposition does not exist. The interactions among the factors

given by [65], due to the nutrient addition to litter caused by the technique.

litter mixture on litter decomposition. See text for further information.

a top-down control in the decomposer trophic level.

**6. Conclusions and perspectives**

position rates.

**Figure 2.** Factors and processes that associate plant diversity in litter mixture to litter decomposition.

regarding the decomposers themselves and their consumers, from different trophic levels, associated to the trophic web. The study of these trophic webs necessarily involves two components that may influence the litter decomposition process: (1) the diversity of litter mixture composing the litter; and (2) the diversity and the trophic links among decomposers and detritivores. Once the trophic web related to litter decomposition is, at least partially, donor-controlled – in the case, the leaf litter – it is expected these webs to be controlled predominantly by bottom-up effects, since consumers do not control resource abundance (and, in our example, nor the diversity).

In Figure 3 we represent a simplified trophic web of detritivores and decomposers, recognizing three different control pathways of the represented trophic levels. The more direct pathway, symbolized by the dotted line, represents a direct effect of the diversity of litter mixture accounting for higher resource heterogeneity to decomposers (but see Figure 2 and discussion above). This pathway may promote the increase of decomposer diversity and activity, directly augmenting the decomposition process, without the interference of organisms from other trophic levels. However, such effects would only be possible if the effects promoted by the increase of litter diversity on the edaphic biota are positive (see Figure 2 and discussion above).

The activities of the animals that inhabit litter may interfere in the above described patterns in different ways. Following the dashed line in Figure 3, the activity of litter breaking by arthropods may increase the resource availability to the decomposer community, since the passage of litter along the digestive tract of detritivores facilitates the degradation by the enzymes of decomposer organisms [66, 67]. Thus, the action of animals may increase the abundance and/or species richness of microorganisms, with a consequent increase in decom‐ position rates.

**Figure 3.** Simplified flowchart of the edaphic trophic web, depicting possible pathways of the effect of diversity of litter mixture on litter decomposition. See text for further information.

On the other hand, the higher abundance and/or diversity of decomposers may lead to an increase of abundance and/or species richness of fungivores and bacteriovores (called here microbivores), due to an increase of resources available to these animals. An increase of microbivores may augment a predation pressure on decomposers, decreasing their abundance and activity. As a consequence, there would be a decrease of litter decomposition (continuous line in Figure 3). This control pathway of the decomposers by the microbivores could represent a top-down control in the decomposer trophic level.

Moreover, predators, not depicted in the trophic web of Figure 3, may control top-down both detritivores and microbivores, making this trophic web more complex and difficult to interpret. The study of edaphic trophic webs, its influence on litter decomposition process and the effects of litter mixture diversity on this web must thus be studied through well designed experiments. Correlational studies may give weak evidences regarding the relative role of each group of organisms within this trophic web, since several of the involved factors in these webs tend to covariate and confound data interpretation [65]. The exclusion of some groups or trophic levels may represent, hence, an option to study these trophic webs, provided that the techniques involved in the exclusion process do not interfere in the decomposition process or in other organisms. A critique of the use of fumigation as an arthropod exclusion method is given by [65], due to the nutrient addition to litter caused by the technique.
