American Society of Naturalists

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Parasitism risk and infection alter host dispersal

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Celina Beatrice Baines, Salma Diab, and Shannon J. McCauley (August 2020)

Organisms disperse to avoid parasites, but infected individuals are poor dispersers and get stuck in infected habitats.

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Healthy hosts disperse away from parasites, but infected hosts get trapped in parasite-infested habitats

One potentially effective way of avoiding parasites that are clumped in space (i.e., common in some areas, not present in others), is to move away and go to a new habitat with no, or fewer, parasites. The process of moving between habitats is called “dispersal”. In this study, the authors found that organisms often fail to use this strategy, and that the fault lies with the parasites: organisms that are infected by, or close to, parasites become poor dispersers and can get trapped in their heavily parasite-infested habitats.

While working in an array of artificial ponds at the University of Toronto’s Koffler Scientific Reserve, the authors of this study observed that many specimens they collected of an insect species called backswimmers were infected with mites. Not only do these infections look gruesome – mites are bright red and cluster in masses on the backs of these insects – but previous experiments had shown that mites reduce backswimmer fitness, meaning backswimmers should have an incentive to disperse away from mites. In a series of experiments in mesocosms (tanks of water that simulate natural ponds), the authors found that healthy backswimmers can sense mites in their environment and disperse away from them. But they found that backswimmers that are infected with mites are poor dispersers, because mites damage backswimmer wings. Finally, the authors tested these findings in a natural setting: they individually marked backswimmers in the artificial pond array, and tracked them to see if they stayed in their home pond or moved (dispersed) to new ponds. They found that backswimmers in ponds with mites are seemingly “trapped” in ponds with high mite abundance. These findings shed new light on the interactions between parasites and their hosts, helping us understand things like how parasites spread across space, and whether parasites will drive their hosts extinct.


Dispersal determines the spatial dynamics of host-parasite assemblages, particularly during invasions and disease epidemics. The risk of parasitism may create an incentive for dispersal, but infection is expected to reduce dispersal ability, which may alter the host’s dispersal response to biotic stressors including population density. We measured the dispersal of the semi-aquatic insect, Notonecta undulata, in aquatic mesocosms in which we manipulated the presence of ectoparasitic Hydrachnidia mites and of infected conspecifics. We found that parasitism risk increases host dispersal propensity. Using a flight assay, we determined that parasite infection reduces host dispersal ability. Finally, we used a mark-release-recapture study to investigate the joint effects of both parasitism risk and parasite infection on host dispersal in a natural, spatially structured population. We found that parasitism risk reduced dispersal probability, eliminated positive density-dependent dispersal, and increased dispersal distance. Infection had no effect on dispersal in the natural population. Our results show that parasites can both increase and decrease the movement rates of their hosts, depending on the ecological context, and can alter the host’s dispersal response to other biotic stressors. Future studies should consider the consequences of this double-headed impact of parasites for landscape connectivity, population persistence, and host-parasite coevolution.