In our original definition of a population (Chapter 2) we set the spatial limits of the system in a rather arbitrary manner. This was done for practical reasons and, in particular, because it is often difficult or impossible to identify the real geographic boundaries of a population. Our population models (Figures 2.13 and 3.9) dealt with the problem of organisms moving into and out of the system by incorporating net migration (immigration-emigration) into the density-dependent process of population regulation. That is, we assumed that net migration changed in response to the density of the population. If we continue with this line of reasoning, we can draw a diagram for the interaction between two populations of the same species that occupy two spatially distinct environments as shown in Figure 5.1. In this model individuals displaced by competitive interactions from population A enter population B through the porous boundary we have set up between them – in reality this boundary is nonexistent. This kind of reasoning is convenient because it allows us to utilize the theories that we developed in the previous chapters to evaluate the dynamics of populations over broad geographic regions. However, as we shall see later, there may be better ways to define the spatial boundaries of population systems.
KeywordsBark Beetle Population System Beetle Population Epidemic Threshold Winged Morph
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