Migratory animals can affect parasite dynamics in resident species in at least two important ways. First, migratory hosts can shed parasites within the home ranges of resident hosts, creating areas of high parasite density (i.e., transport effects). Second, migratory hosts may exert trophic effects that promote or reduce parasite exposure risk for resident hosts. We proposed that for environmentally-transmitted parasites in resident host populations, the magnitude of transport and trophic effects depends on how many animals (intensity) are involved in migration events and how long they remain in the range of resident hosts (duration), which can vary substantially among, and even within, movement systems. To test this, we took advantage of a vertebrate system that combines a highly mobile migratory host, a diverse assemblage of resident species, and well-studied plant-herbivore dynamics: the annual migration of ~1.4 million Serengeti wildebeest. We used data from this system to set parameters for model simulations to test the role of migration intensity and duration in parasite infection rates within resident hosts. We included the effects of resource limitations to migrant intensity and duration and explored how parasite life history strategies (particularly how incubation periods that couple or uncouple trophic and transport effects) affected responses.
Results/Conclusions
Migrations can drive net reductions in resident-host parasite burdens. When trophic and transport effects of migratory-hosts are uncoupled and duration is held constant, then increasing migration intensity does not drive consistent responses in resident-host parasite loads. Rather, a threshold migratory-host parasite burden is required to increase resident-host parasite burdens, below which, increasing migration intensity results in decreasing resident-host parasite burdens. Moreover, across all simulated parameter combinations when duration was held constant (n =12320), coupled transport and trophic simulations resulted in 42% of simulations having a positive or neutral effect on resident-host parasite burdens compared to 66% for simulations with uncoupled effects. Longer duration migration events with constant migration intensity resulted in lower resident-host parasite burdens in 99% of simulations when transport and trophic effects were uncoupled. This effect was driven by the long-term trophic effects that migratory-hosts exerted on the system. Our findings suggest that the directionality (positive or negative) in the response of parasite burdens within resident-hosts during interactions with migratory-hosts will be largely dependent on the migratory-host parasite burden, but the intensity and duration of the migration event will determine the magnitude of the response in either direction.
