What distinguishes a single resource from others?
Haigh and Maynard Smith (1972) was the first attempt to take a rigorous approach to answering this question. They noted that separation in time or space, and different parts of a single biological entity could constitute different resources.
The necessary condition for being distinct was that abundance of the resource at one point in space or time did not functionally determine the abundance at another point. That is certainly true of the spatial system discussed in the previous section. Abrams (1988b) later added a condition that the consumers must be capable of distinguishing the different categories of resource for them to qualify as different resources for the purposes of coexistence. Resources that differ only in spatial location or the time at which they are present are generally connected to each other. Time has a directional property that is not universally present with space (although it may be in the case of flowing water systems). In any event, the interaction between resource items that only differ in when or where they are present is quite different from the interaction of resources that have distinct properties when present at the same time and place.Even when resources are physically distinct, different resources interact in a number of ways. Haigh and Maynard Smith (1972) pointed out that different life stages or different parts of a particular biological resource constituted potentially distinct resources for consumers. Distinct resource species may compete for their own resources, but may also interact positively. The absence of resources in the LV model not only ignores these possibilities, but also suppresses thought about their potential role in interactions between consumers. That is probably why MacArthur never considered this possibility in his consumer-resource models.
3.5