Any ecological community can be characterized by its structure (number of species, interaction strength within the food web, average length of food chains, etc.), by certain quantities (especially biomass and the rate of production of biomass, which we can summarize as 'productivity') and by its temporal stability (Worm & Duffy, 2003). In the remainder of this chapter, we examine some of the interrelationships between these three.
keystone species can occur throughout the food web
Much of the very considerable recent interest in this area has been generated by the understandable concern to know what might be the consequences of the inexorable decline in biodiversity (a key aspect of structure) for the stability and productivity of biological communities.
We will be particularly concerned with the effects of food web structure (food web complexity in this section; food chain length and a number of other measures in Section 20.4) on the stability of the structure itself and the stability of community productivity. It should be emphasized at the outset, however, that progress in our understanding of food webs depends critically on the quality of data that are gathered from natural communities. Recently, several authors have called this into doubt, particularly for earlier studies, pointing out that organisms have often been grouped into taxa extremely unevenly and sometimes at the grossest of levels. For example, even in the same web, different taxa may have been grouped at the level of kingdom (plants), family (Diptera) and species (polar bear). Some of the most thoroughly described food webs have been examined for the effects of such an uneven resolution by progressively lumping web elements into coarser and coarser taxa (Martinez, 1991; Hall & Raffaelli, 1993, Thompson & Townsend, 2000). The uncomfortable conclusion is that most food web properties seem to be sensitive to the level of taxonomic resolution that is achieved. These limitations should be borne in mind as we explore the evidence for food web patterns in the following sections.
First, however, it is necessary to define 'stability', or rather to identify the various different types of stability.
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