Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

The pressure of parasites on a host population can be very intense. In fact, in some cases, it can be the basic element of the feedback regulation mechanism maintaining the size of the host population at a constant level. As the size of a parasite population bound to the population of a certain host is usually larger than the size of the population of a predator, the system is less susceptible to random fluctuations. Consequently, the equilibrium in the parasite-host system is usually more stable than in similar systems based on interactions of the predator-prey type. The greater host specificity of the parasite contributes to the stability of these systems. On the other hand, the frequently low food specificity of a predator allows the population of the predator not to be necessarily bound to the size of the population of a certain species of prey. As a consequence, a momentarily numerous population of a predator can completely exterminate the populations of some kinds of prey.
Similar to the predator-prey system, the parasite-host system also belongs, from the viewpoint of the host, amongst feed-back regulation systems of the turbidostat type (Flegr 1977) (see IV.4.1). This means that, in a system regulated by the action of the parasite, the host population is exposed to r selection, i.e. selection for a faster reproduction rate (and not for more effective use of nutrient sources). As a consequence, the species does not utilize the resources to the extreme of the capacity of the environment, so that a larger or smaller amount is left for other species occurring in the particular environment. Through this mechanism, the phenomenon of parasitism apparently contributes very effectively to maintenance of a high level of biodiversity in real ecosystems.
Maintenance of a high level of biodiversity through the presence of parasites is also augmented by the fact that specialized parasites add a further dimension to the multidimensional ecological environment in which organisms live. In section IV.4.1, we mentioned that two species utilizing a single resource can exist next to one another for a longtime in a single place if one of them is the subject of chemostatic and the other of turbidostatic regulation (see the caption for Fig. IV.4). Similarly, permanent coexistence is possible for two species whose populations are turbidostatically regulated through the action of two different species of parasites.
Overall, it can be stated that, in the absence of parasites, natural ecosystems would have far less biodiversity and would probably make less effective use of the available resources.