Do diseases have a role in ecology?….To put it simply..Yes!
Populations sizes cannot grow indefinitely. In population ecology you will often hear of the term ‘carrying capacity’. This is simply the population size at which the population growth rate equates to zero. Reproduction, mortality. immigration and emigration are the four processes that strongly influence population size and these are constrained by factors such as food availability, competition, predation and disease. When a population is smaller than the carrying capacity there is a positive growth rate, so a population increase is seen. When the population is larger than the carrying capacity, you get the opposite effect. A negative growth rate and a decrease in population size. In reality, population sizes will fluctuate until the size equals the carrying capacity at which time the growth of a population will stop, representing a stable equilibrium of population size.
Density dependence is also a major population feedback mechanism that acts to regulate population size. Density dependence simply means that one or more population processes are related to population size. This often occurs because the strength/frequency of interactions between individuals is related to population size. So what would the effect of disease and density dependence be on a population size?
This would usually have a negative effect by reducing the reproduction rate and/or increasing the mortality rate as an increase in population size would increase the probability of disease transmission. This therefore regulates population size and the carrying capacity is reached at the population size at which the per capita birth rate equals the per capita death rate.