Definitions of parasitism, considering its potentially opposing effects at different levels of hierarchical organization

Abstract

An annotated synthesis of textbook definitions of parasitism is presented. Most definitions declare parasitism is a long-lasting relationship between individuals of different species harming the hosts. The infection-induced costs are interpreted as diseases in the medical-veterinary literature. Alternatively, evolutionary ecologists interpret it as a reduction of host's fitness (longevity, fertility or both). Authors often assume that such effects decrease host population growth and select for antiparasitic defences, which is not necessarily true because infections may simultaneously express opposite effects at different levels of biological organization. (i) At the cellular level, infection-induced cell growth, longevity and multiplication may yield tumours maladaptive at higher levels. (ii) At the individual level, reduced host longevity, fertility or both are interpreted as disease symptoms or reduced fitness. (iii) Contrary to common sense, the growth rate of infected host lineages may increase in parallel with the individuals' reduced survival and fertility. This is because selection favours not only the production of more offspring but also their faster production. (iv) Finally, infections that reduce host individuals' or lineages' fitness may still increase infected host populations' growth rate in the context of ecological competition. Therefore, differences between parasitism and mutualism may depend on which level of organization one focuses on.

Editorial: Parasitism: the good, the bad and the ugly

“…Parasitism is a tight association between species in which one organism, the parasite, lives on or inside the host, causing it harm, and is structurally adapted to this way of life ( 1 ). Until the twenty-first century, parasitism was studied by parasitologists, rather than ecologists or evolutionary biologists.…”

“…Today, parasitism is a major element of evolutionary ecology, as nearly all free-living animals, are hosts to at least one parasite species ( 2 , 3 ). Since it is in the parasite's evolutionary interest for its host to flourish, long-term coevolution can lead to a stable relationship bordering on mutualism ( 1 , 4 ). According to Lynn Margulis, when resources are scarce, natural selection moves relationships from parasitism to mutualism, as was brilliantly illustrated in Margulis' endosymbiosis theory, where eukaryotic mitochondria and chloroplasts descended from formerly free-living prokaryotes ( 5 ).…”

Towards Resolving Three Problems in Ecological Parasitology: Parasite Aggregation, Determinants of Parasitism, and Parasite Detoxification of Contaminants

“…The explanation for this pattern that these parasites are bioaccumulating contaminants at such high relative rates as to decrease the availability of the contaminant to hosts has now been given a name: the "parasite detoxification hypothesis" (Jeantet et al, 2023). This can be thought of as a specific example of the more general concept of "conditionally helpful parasites," an extension of the idea that the definition of parasitism and its potential contrast with mutualism is context-specific (Fellous and Salvoudon, 2009;Rózsa and Garay, 2023).…”