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FIGURE 8.8. Diagram of soil food-web structure in the four different grassland communities, showing the relative abundance within a given functional group. The numbers of icons on a given line are relative to each other; thus twice as many icons indicates that organisms are twice as abundant. B = Bromus, H = Hilaria, S = Stipa (from Belnap and Phillips, 2001).

FIGURE 8.8. Diagram of soil food-web structure in the four different grassland communities, showing the relative abundance within a given functional group. The numbers of icons on a given line are relative to each other; thus twice as many icons indicates that organisms are twice as abundant. B = Bromus, H = Hilaria, S = Stipa (from Belnap and Phillips, 2001).

Western Europe, where it preys upon earthworms. It was established in Great Britain by unknown means, but probably on soil associated with nursery stock. Boag and Yeates (2001) suggest that the avenue of introduction was rather circuitous: initially the flatworms spread from botanic gardens to horticultural wholesalers, then to domestic gardens, and finally invaded agricultural lands. They note that it is one of twelve alien terrestrial planarians in Britain considered to be a pest, and hence seems to obey the "rule of tens" (sensu Williamson, 1996), in which only one in ten invasive species assumes an outbreak or pest status. The indigenous flatworms, including Arthurdendyus, are not a problem in New Zealand because New Zealand has a drier and warmer climate than in the west of Scotland, where the outbreaks are the most severe. Interestingly, under minimum tillage practice in New Zealand, where crop residues provide refuges (Yeates et al., 1999), flatworms have the potential to reduce lumbricid earthworm populations. Invasions by exotic earthworm species are also becoming a problem in many temper ate and tropical areas, as discussed further in Chapter 4 (Hendrix and Bohlen, 2002).

In general, numerous theoretical studies have usually supported Elton's (1958) biotic resistance hypothesis, in which more diverse communities better resist invading species (see Byers and Noonburg, 2003, and references cited therein). In a mathematical overview of more general aspects of biotic resistance to invasive species, Byers and Noonburg (2003) demonstrate that invasibility is influenced not only by the number of native species present, but also by the number of resources present in a given ecosystem. Building on a Lotka-Volterra competition model, Byers and Noonburg's model predicts that increasing invasi-bility with native diversity across large scales is the result of decreasing mean interaction strength as resources increase. The strength of the positive relationship between native and exotic species diversity and relative contribution of factors extrinsic to the community depend on whether niche breadth increases with the number of available resources. Interestingly, the same mechanism—the sum of interspecific competitive effects (Sa^)—drives the opposite pattern of decreasing invasibility with native richness at small scales because resource numbers are held constant. As a consequence, Byers and Noonburg (2003) conclude that Elton's biotic resistance hypothesis, interpreted as a small-scale phenomenon, is consistent with large-scale patterns in exotic species diversity.

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Worm Farming

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