Co-evolutionary dynamics of a host-parasite interaction model: obligate versus facultative social parasitism

Host-parasite co-evolution can have profound impacts on a wide range of ecological and evolutionary processes, including population dynamics, the maintenance of genetic diversity, and the evolution of recombination. To examine the co-evolution of quantitative traits in hosts and parasites, we present and study a co-evolutionary model of a social parasite-host system that incorporates (1) ecological dynamics that feed back into their co-evolutionary outcomes; (2) variation in whether the parasite is obligate or facultative; and (3) Holling Type II functional responses between host and parasite, which are particularly suitable for social parasites that face time costs for host location and its social manipulation. We perform local and global analyses for the co-evolutionary model and the corresponding ecological model. In the absence of evolution, our ecological model analyses imply that an extremely small value of the death rate for facultative social parasites, primarily due to hunting/searching for potential host species, can drive a host extinct globally under certain conditions, while an extremely large value of the death rate can drive the parasite extinct globally. The facultative parasite system can have one, two, or three interior equilibria, while the obligate parasite system can have either one or three interior equilibria. Multiple interior equilibria result in rich dynamics with multiple attractors. The ecological system, in particular, can exhibit bi-stability between the facultative-parasite-only equilibrium and the interior coexistence equilibrium when it has two interior equilibria. Our analysis on the co-evolutionary model provides important insights on how co-evolution can change the ecological and evolutionary outcomes of host-parasite interactions. Our findings suggest that: (a) The host and parasite can select different strategies that may result in local extinction of one species. These strategies can have convergence stability (CS), but may not be evolutionary stable strategies (ESS); (b) The host and its facultative (or obligate) parasite can have ESS that drive the host (or the obligate parasite) extinct locally; (c) Trait functions play an important role in the CS of both boundary and interior equilibria, as well as their ESS; and (d) A small variance in the trait difference that measures parasitism efficiency can destabilize the co-evolutionary system, and generate evolutionary arms-race dynamics with different host-parasite fluctuating patterns.