Soil biomass in the below-ground subsystem can be structured through food chains that originate either from the primary production of plant roots (grazing food chains), or from labile or recalcitrant litter and debris (the decomposer food chains). We review ecology theory on how such structures are inevitably interlinked, which theoretically may lead to nematode control by natural mechanisms; we also give practical examples of the functional involvement of nematodes.
According to the Green World Hypothesis (GWH), plants are abundant because herbivores are top-down controlled by their predators and parasites, whereas plants themselves are bottom-up controlled by resource availability. In regulating the herbivore populations, predator and parasite populations are also resource-limited (Hairston et al. 1960). Should the GWH apply to plant-parasitic nematodes in three-level food chains, a given plant parasite would not only control its host plant (by reducing its primary production), but also be controlled by a natural enemy, e.g. a fungal or bacterial parasite. An interesting analogy of the GWH is the Brown Ground Hypothesis (BGH), in which essentially the same regulatory processes are applied to decomposition in ecosystems, or 'why there is so much carbon in soil' (Allison 2006). Organic matter accumulates in soil because there is a large amount of input of dead material (notably of plant origin) and the microbial organisms involved in its decomposition are top-down controlled.
A sometimes large proportion of roots in the rhizosphere can be inactive. In grassland ecosystems, for example, a layer of dead roots frequently accumulates in the most superficial layers of soil (Watt et al. 2006). Yet organic matter in soil in the form of dead organisms, leaf litter and root deposition is not a blind alley for energy and biomass in the soil ecosystem. Although soil is rich in carbon compounds, nitrogen is generally a limiting factor (Ingham et al. 1985). Therefore, dead material is a major input of organic matter into the system, a food source for decomposers that eventually mineralize these nutrients and make them again available to the plants. Exudates and leachates from living plant roots, for example, also support communities of decomposer microorganisms, which are thought to be selected by their interaction with the plants: they specialise in the decomposition of the plant exudates and leachates and promptly make nutrients available back to the plant (Grayston et al. 1998).
By bringing together concepts taken from the GWH and the BGH, two parallel energy chains can be identified in soil food webs, both culminating at the top-level consumers: one starting from biomass originating from plant primary production, and one starting from dead organic matter (Fig. 2.1). Predators and parasites in soil acquire energy from both the grazing and the decomposer chains and therefore predation/parasitism on the primary consumers can be driven by the decomposer chain (Moore et al. 2004). Energy channels are defined as a group of species consuming biomass that originates from the same primary energy source (Moore and Hunt 1988). In soil food-webs, the dead biomass based energy chain is developed along two lines, one starting from bacteria and one from fungi. Therefore, there are three parallel energy channels in the soil food-web: the root channel, a primary production channel based on the plant and following on to its herbivores and their predators and parasites; the bacterial channel, a decomposition channel based on
Fig. 2.1 The involvement of plants, bacterial-feeding, fungal-feeding, plant-parasitic nematodes and their microbial enemies in the three parallel energy channels in the soil food-web: A - the bacterial energy channel, originating on labile organic matter, B - the fungal energy channel, based on the decomposition of recalcitrant organic matter and C - the grazing channel, using plant primary production as an energy source. All three channels are joined together at the top consumer level (Moore and Hunt 1988), the microbial enemies of nematodes and therefore a generalist microbial enemy could obtain energy from the three channels
Fig. 2.1 The involvement of plants, bacterial-feeding, fungal-feeding, plant-parasitic nematodes and their microbial enemies in the three parallel energy channels in the soil food-web: A - the bacterial energy channel, originating on labile organic matter, B - the fungal energy channel, based on the decomposition of recalcitrant organic matter and C - the grazing channel, using plant primary production as an energy source. All three channels are joined together at the top consumer level (Moore and Hunt 1988), the microbial enemies of nematodes and therefore a generalist microbial enemy could obtain energy from the three channels high quality, N-rich debris; and the fungal channel, based on low-quality, C-rich compounds (Fig. 2.1).
In analyses of soil decomposer food-webs in grassland ecosystems, bacterial-feeding nematodes have only been associated with the bacterial-based energy channel and fungal-feeding nematodes with the fungal-based energy channel. Plant-parasitic nematodes have exclusively been allocated to the root channel, as they depend solely on primary production as their food source. However, all three channels are not separate: predatory nematodes were associated not only with the bacterial-based energy channel (89.9%), but also with the fungal-based energy channel (10.6%), and (weakly) with the root channel (0.4%). The association of predatory nematodes with the primary production channel seems to imply that there may be a weak trophic link to plant-parasitic nematodes (De Ruiter et al. 1995).
Although conceptually useful, in nature, organisms are not simply organised in food chains, but rather in food-webs, with complex and indirect interactions between them, regardless of their trophic level. The sum of indirect effects that result from food-webs can easily overshadow the biomass/energy transfer of the three-level food chain, which could be demoted from the designation 'trophic cascade' to the more modest 'trophic trickle' (Strong et al. 1999). Indeed, three-level food chains are thought to be unstable and tend to chaotic dynamics (Hastings and Powell 1991), which certainly is not expected to express the natural functioning of the soil ecosystem. To understand the ecology of soil, we need to consider how food-webs, and not simply food chains, work.
The Santa Rosalia theory proposes that ecological interactions such as competition have a major role in the maintenance of biodiversity (Hutchinson 1959). This theory aims to explain 'why there are so many species of organisms' and was put forward after the observation of several species of plankton inhabiting a small pond by the Santa Rosalia caves. The number of species in a community and also their functional differences increase food-web complexity, which seems to promote coexistence. Long food chain loops with weak links that form in complex multi-trophic interaction webs may also be responsible for the stability of ecosystems (Neutel et al. 2002). If there is a high degree of functional differences between species, then inter-specific facilitation as opposed to competition can occur; this mechanism is thought to be involved in driving decomposition processes in soil (Heemsbergen et al. 2004) and has also been shown to support the coexistence of different plant species in coastal dune systems (Stubbs and Wilson 2004).
Plant identity, as substantiated before in this chapter, is thought to be a driver of soil food-webs, leading to changes in the soil community both among and within trophic groups (Wardle et al. 2003). In a biodiversity field experiment, the soil food-webs of plant individuals were most similar within the same plant community. Individual plant soil food-webs varied between plant communities and between plant species; this variation could be detected even between plant individuals (Bezemer et al. 2010). Soil communities are also known to feed back to their host plants (Bever 1994). Soil biota, therefore, also determine the abundance of plant species, as the most abundant species have strong positive feedbacks with their own soil and rare species have a negative feedback effect (Klironomos 2002). Soil community feedbacks can also maintain the coexistence of competitor plants, where otherwise one would exclude the other (Bever 2003).
Negative feedbacks caused by soil-borne disease complexes composed of fungal pathogens and plant-parasitic nematodes have been correlated to both degeneration and successional replacement of marram grass Ammophila arenaria in coastal sand dunes (Van der Putten and Peters 1997; Van der Putten et al. 1993). However, if plants are released, even if only partially or temporally from their own natural enemies, they will have an increased competitive advantage and may outcompete other plants if they remain constrained by their natural enemy community (DeWalt et al. 2004). Also above-ground studies suggest that plant coexistence can be maintained by such indirect effects when parasites disproportionately repress the population density of the dominant host plant species (Yorozuya 2006).
Indirect effects encompass a wide range of interactions and can be defined as occurring when the impact of one species on another requires the presence of a third (Strauss 1991; Wooton 1994). Tritrophic interactions in which plants can communicate with the enemies of their enemies, giving indirect control, have been the object of much study, and are a good example of indirect effects (Price et al. 1980). The four most studied types of indirect effects are apparent competition (the sizes of two different populations being mediated by a shared predator), indirect facilitation (a population benefiting from the predation of another), exploitative competition (two different populations being limited by the same resource) and the above-mentioned trophic cascades (van Veen et al. 2006; White et al. 2006). Indirect effects comprise not only density-mediated effects, but also trait-mediated ones, including life-history traits and plastic or evolutionary adaptations of populations (Luttbeg et al. 2003).
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