The evolution of brood parasitism: The role of facultative parasitism

The hypothesis that facultative brood parasitism may serve as an intermediate step in the evolutionary transition from purely parental reproduction to obligate parasitism was investigated. The population dynamics of a host-parasite complex were computer-simulated in a model that incorporated different intensities of parasitism and host defense and considered a simplified semelparous bird species living in a homogeneous habitat. The individuals use two different breeding strategies: provide parental care or parasitize the nest of those providing parental care. Under obligate parasitism, the parasites appeared unsuccessful, drove the host population to extinction, or coexisted with the host in stable or oscillating proportions. The behavior of the system depended on both the effectiveness of the parasite and the defense of the host. Under facultative parasitism (making the best of a bad job), the parasites reduced host numbers but did not reduce the population size below the number of breeding sites. Thus, facultative parasitism provides a better opportunity for the development of defense in the host. The population of a host that shows a certain level of defense can be more successfully invaded by obligate parasites so that stable coexistence of hosts and parasites is possible.