Figure 3.2 Depiction of systems in which the number of producer and consumer species remain constant but the number of pathways of materials or energy flow change. Systems with low diversity of flow pathways (a) should be less reliable (less sustainable) than systems with a high diversity of flow pathways (c).

the companion chain. Because Cu can switch to the second producer when abundance of the first producer becomes rare, there will be a more steady (reliable) flow of energy and material up the chain to the primary and secondary consumer than in the less interconnected system. By extension (Figure 3.2c), the stability (reliability) of production and materials fl ow will be increased as the two parallel chains become fully interconnected. That is, the degree of interconnectedness determines the level of diversity in pathways of flow from producers to secondary consumer. The most interconnected systems (i.e., most diverse systems) should be the ones with the highest reliability in function. This intuitive argument is known as the diversity-stability hypothesis (McCann 2000).

The idea was, however, refined when formal mathematical analysis revealed that system stability derives not simply from the degree of network connections but also from pattern in the distribution of the strengths of consumer-producer interactions (McCann 2000). That is, reliability depends also on the distribution of rates at which energy and materials flow through the various pathways. Recent theory has shown that the most stable kinds of systems should be those with many weak and a few strong consumer-producer interactions (McCann 2000). Such skewed distributions of interaction strengths have indeed been discovered empirically for many ecosystems. The explanation for this pattern is that the many weak interactions ought to counterbalance and therefore limit energy flow through pathways involving very strong consumer-producer relationships, thereby inhibiting destabilizing "runaway" consumption (McCann 2000).

In addition, complexity, if interpreted in terms of both numbers of species performing similar function roles and diversity in functional roles, can increase the opportunities for emergence of stable chains of interacting species relative to simple linear chains. This is because in complex systems the chances are

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