Can infection tolerance be a co-opted as a competitive strategy? Theoretical and empirical studies have demonstrated that parasites can facilitate coexistence and even cause reversals in competitive dominance among competing species susceptible to a common parasite. However, it is unclear how associations with parasites that can enhance a particular host’s competitive ability evolve. Elucidating the evolution and the characteristics of these host-parasite associations will increase understanding of how diseases influence community structure and improve predictions of competition and invasion outcomes.
Parasites can alter competitive outcomes when one host species experiences reduced fitness costs from infection compared to its competitors. Hence, the potential for parasite tolerance to be co-opted as a competitive strategy depends on the coevolution of host tolerance and parasite virulence.
We analyzed models of host-parasite dynamics to investigate when host tolerance could be a competitive strategy. Within-host resources were characterized using the Y-model of life-history resource allocation such that parasites could steal resources from the pre-allocation resource budget, reproductive resource budget, or the maintenance/growth budget. Using the adaptive dynamics framework, we predicted outcomes of tolerance-virulence coevolution, and identified which outcomes could potentially result in the co-opting of host tolerance as a competitive strategy.
Results/Conclusions
We found that infection tolerance could, under certain conditions, be co-opted as a competitive strategy, such that tolerant hosts that coevolved with the parasite were able to coexist or exclude species that would otherwise be competitively dominant when in parasite-free environments. When parasites exploited pre-allocation resources or resources allocated to reproduction, host tolerance could facilitate coexistence or dominance reversals. However, when parasites exploited resources allocated to maintenance/growth, host tolerance could not be co-opted as a competitive strategy. Thus, the potential for tolerance to be a competitive strategy is constrained by the mode of parasite exploitation.
These results suggest that the potential for tolerance to be a competitive strategy is more likely for host species carrying certain types of parasites. This accords with empirical examples of competition or invasion mediated by parasites. More generally, our findings provide further evidence of how ecological interactions may shift between beneficial and harmful: In the absence of competition, the parasite negatively impacted host fitness; however, in the presence of competition, the parasite functions as a mutualist, positively impacting host fitness.