The unavoidable costs and unexpected benefits of parasitism: Population and metapopulation models of parasite-mediated competition

Abstract

When faced with limited resources, organisms have to determine how to allocate their resources to maximize fitness. In the presence of parasites, hosts may be selected for their ability to balance between the two competing needs of reproduction and immunity. These decisions can have consequences not only for host fitness, but also for the ability of parasites to persist within the population, and for the competitive dynamics between different host species. We develop two mathematical models to investigate how resource allocation strategies evolve at both population and metapopulation levels. The evolutionarily stable strategy (ESS) at the population level is a balanced investment between reproduction and immunity that maintains parasites, even though the host has the capacity to eliminate parasites. The host exhibiting the ESS can always invade other host populations through parasite-mediated competition, effectively using the parasites as biological weapons. At the metapopulation level, the dominant strategy is sometimes different from the population-level ESS, and depends on the ratio of local extinction rate to host colonization rate. This study may help to explain why parasites are as common as they are, and can serve as a modeling framework for investigating parasite-mediated ecological invasions. Furthermore, this work highlights the possibility that the 'introduction of enemies' process may facilitate species invasion.

Biological invasion and parasitism: invaders do not suffer from physiological alterations of the acanthocephalanPomphorhynchus laevis

“…This might be explained by the fact that P. laevis has a wide geographical distribution (Kennedy, 2006) and that the amphipod had already experienced infection with this parasite species in its area of origin or during the invasion process (Moret et al 2007). Although infected by P. laevis , G. roeseli suffers lower levels of damage caused by parasitism (immune depression, lower bacterial resistance, energetic budget modification) relative to the local species G. pulex , and can be maintained in the population and act as a reservoir (Kuo et al 2008). Hence, it is not surprising that infection is as prevalent in G. roeseli as in G. pulex .…”

“…Because immune defences are costly (Bonneaud et al 2003; but see Schmid-Hempel, 2003 for a review), it has been suggested that introduced populations could evolve lower investment in resistance and could down-regulate their level of immunity as a plastic response to the absence of enemy. This would minimize the cost of immunity, and could allow the shifting of resources from defences towards growth and/or reproduction, providing further benefits to the invading populations (Kuo et al 2008). However, such an evolutionary down-regulation of immune defences among invaders might nevertheless be limited because native and invasive populations sharing the same habitat suffer from similar risks of infection, and non-indigenous species often acquire parasites from the novel environment (Colautti et al 2004).…”

The Ecological Importance of Amphipod–Parasite Associations for Aquatic Ecosystems

“…The immune depression of G. pulex constitutes a parasite-mediated disadvantage towards G. roeseli. Moreover, G. roeseli, being less damaged by and more tolerant to P. laevis, contributes to maintain the parasite population acting as a reservoir [173]. Additionally, Galipaud et al [150] suggest a contribution of parasitism to the invasion success of G. roeseli that, unlike the native gammarids (G. fossarum/G.…”

Pulse Exposure in Ecotoxicology

“…Host-parasite interactions are ubiquitous in nature and are important in shaping the life history strategies of both hosts and their parasites (Kuo et al 2008). Based on life history traits such as size and mode of replication, parasites are separated into two categories.…”