All of the interactions we have discussed thus far have been between species of approximately the same size. Yet, as ecologists have recently begun to recognize, diseases, pathogens, and parasites may play an extraordinarily important role in regulating populations. One example is shown in Figure 10.1, where both the numbers of larch bud moths and the fraction of moths infected with a particular virus are shown. Once again, we will address the fundamental question of population ecology — can diseases act as the agent preventing runaway exponential population growth? We are also interested in determining what kinds of dynamics are likely to result from the interaction between a species and its pathogens and parasites. In this chapter, we will focus primarily on pathogens (typically bacteria or viruses) where we say that an individual either has a disease or does not. The other possibility is to look at macroparasites, such as tapeworms, where any host individual will carry only a small number of parasites, and differing numbers of parasites have different effects on the state of the host individual.
KeywordsTransition Rate Population Growth Rate Host Individual Total Population Size Reproductive Number
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