Disentangling transport and trophic effects of animal movement on environmental parasite abundance.

Migratory wildlife plays an outsized role in disease transmission. Transmission risk is often assumed to be scaled with migratory host density through parasite transport effects, but in environmentally transmitted parasites, migratory hosts can also influence parasite availability via trophic effects. Trophic effects can either amplify or dampen transport effects, making the net impact of migratory hosts on resident hosts difficult to predict. We propose that the net effect is shaped by two attributes of migrant movement: intensity of use (i.e., number of migrants) and duration of use (i.e., length of stay). Using gastrointestinal nematodes (GIN) as a model, we experimentally varied transport and trophic effects of a migratory grazer wildebeest (Connochaetes taurinus) by manipulating the intensity and duration of dung addition and grazing across five treatment combinations in replicated plots, and measuring their effects on the density of infective third-stage GIN larvae in pasture. We found that: (1) higher dung addition increased GIN larvae density, (2) simulated grazing reduced the density of GIN, particularly in treatments with high dung addition, and (3) longer duration and lower intensities of use reduced GIN density for the subsequent hosts compared to treatments with single bouts of dung addition and grazing. Our results indicate that migratory hosts directly facilitate parasite spread via transport effects, while infection risk tends to decline with increasing intensity and duration of trophic interactions. Our results highlight the underappreciated role of transport and trophic interactions in shaping parasite spread in migrant-resident systems.