Once the genetics of the fundamental niche have been brought to equilibrium, another set of processes can reduce the actual set of resources used. These include individual decision-making, which, though it acts within the constraints of genetics, and hence cannot extend the fundamental niche, can reduce it. In general, the solutions to the optimal decisions are to be found within the realms of behavioural ecology theory (Chapter 7). Foraging models that consider the optimal use of resources are particularly appropriate, and we will consider one more such model here, the optimal diet model, which evolved from work by MacArthur and Pianka (1966). This, like the marginal value theorem, is a rate maximization model, and assumes simply that two resources are available at different frequency in the environment,
Fig. 9.3 Optimal diet selection. An individual arrives at a low value resource (grey circle). Should it use it or move on in the hope of finding a better resource (large circles)? When high value resources are abundant, it is best to move on, because of the opportunities that are lost by staying (a), but when good resources are scarce, it loses nothing by staying (b).
which give different fitness returns (as a result of the outcome of the evolution of the fundamental niche). The resources have different handling times (times taken to exploit them). On encountering the lower quality resource, should an organism accept it or wait until it finds the next higher quality resource? The solution is simple: it should accept it as long as if when doing so it does not gain energy at a lower rate than if it searched for and used the higher quality resource. Therefore, if the better resource is rare or has a larger handling time, or is not much better than the worse resource, all these favour acceptance of the worse resource (Figure 9.3). However, if the best quality resource is far superior, common, or takes little time to handle,then selection favours rejection of the lower quality resource and hence reduction of the realized niche.
Models based on such 'time limited dispersers' have also been written for egg-laying taxa and those selecting habitats to develop in, which for some taxa, such as most herbivorous insects, is the same thing (see Jaenike 1990; Mayhew 1997). The general predictions of these and similar models are that specialization (reduction of the realized niche) is favoured by (1) abundance of the high quality resource; (2) low density of competitors; (3) low variability in resource abundance; (4) longevity; and (5) high egg loads. Unsurprisingly these predictions are very similar to those that predict a specialized fundamental niche, although the variables explored do not always overlap. How do all these predictions stand up to data?
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