Food web structure and its influence on competition

The early theory based on the LV model and MacArthur's interpretation of his consumer-resource model both suggest that the nature of competition is relative­ly independent of the presence and abundance of species other than the competing consumers.

These multi-level effects of adaptive foraging behaviour were explored using food chain models in the 1980s and 1990s (Abrams 1982,1984b, 1992c, 1995). Higher-level species usually suppress foraging by their prey, and therefore have a positive effect two levels below. Increased resource abundance can increase or decrease foraging; these respectively are likely to have positive and negative effects two levels above. A system with two adjacent levels of a food chain both exhibiting adaptive foraging influenced by predation risk has a variety of potential outcomes (Abrams 1992c). These have not been explored in the context of competition, but there are very likely a wide variety of complex effects that may occur.

The results presented in Chapters 8 and 9 show the importance of the time lags in consumer responses to resources in determining the dynamics of temporally vari­able consumer-resource systems. Time lags are likely to play an even larger role in the dynamics of competitors that are parts of larger food webs, which is to say most competitors in natural systems. The potential role of these lags for interspecific inter­actions was something that was emphasized by Schoener (1989,1993). Schoener also noted how difficult they would be to study in real-world food webs.

The argument for a food web approach (i.e., one that includes resources and species on trophic levels above and/or below the ones that define the interaction) is accept­ed for other indirect interactions. For example, Wise and Farfan (2021, p. 1) recently wrote that, ‘Ecologists have long debated how simple mathematical models should be and how much to simplify food webs by lumping taxa into functional groups. For both modellers and empiricists, the food chain is the ultimate simplification’. The lumping of all species on a trophic level into one group is something that would never be con­sidered if the existence or diversity of species on a single level was the topic of interest. However, it is clear that these authors consider lumping species within a level to be more easily justified than making an entire level implicit. Explicit modelling of inter­mediate species/entities is needed just to be consistent with the approaches used in studying other systems. It is required both because: (1) any single intermediate entity may have a wide range of dynamics, depending on the functional forms of the various components of its growth rate; and (2) there are likely to be a number of dynamically distinct components in the intermediate entity.

12.6