Conceptual Models

Developing the models in the previous chapter, imagine a population in which both reproduction and mortality are density dependent, as shown in Figure 27.1a. The place where the lines depicting these relationships intersect marks the equilibrium population level (E), to which numbers tend to return after any perturbation. If mortality during migration is also density dependent, and additive to other mortality, the equilibrium population level (E1) is reduced as shown. The same holds if either reproduction or mortality is density independent. In Figure 27.1b, for example, breeding is shown as density dependent and mortality as density independent. Whether migration mortality is density dependent or density independent, it can reduce subsequent breeding numbers only if it is additive to other mortality, and not compensated by reduction in some other form of mortality before the breeding season. Where mortality in the non-breeding season is

Figure 27.1 Model depicting the relationship between per capita reproduction and per capita mortality in a population. Where the lines cross marks the equilibrium population size (E). In (a), both reproduction and mortality are density dependent, as is additional migration mortality, which reduces the equilibrium population size (E1). In (b), reproduction is density dependent while mortality is density independent, as is additional migration mortality, which again reduces the equilibrium population size.

Plus migration mortality

Plus migration mortality

Winter mortality

Net breeding output

E1 E Population size

E1 E Population size

Winter mortality

Net breeding output

Plus migration mortality Winter mortality Net breeding output

Population size

0 0

Post a comment