Abstract

Parasites are often distributed heterogeneously across host populations, but the controls of this heterogeneity across regional scales often remain unclear. Here, we test the relative importance of natural and anthropogenic environmental factors and biological attributes of host populations on the large-scale variability in infection probability and parasite abundance. We quantified a larval acanthocephalan parasite (Profilicollis altmani), as well as the trematode Maritrema sp. and the nematode Proleptus sp., infection of mole crabs Emerita analoga using a hierarchical design at 8 sites spanning 500 km of the south-central shoreline of Chile. At each site, we measured the variables of beach size, morphodynamics, distance to nearest port, distance to tributaries, mole crab density, seagull density, and host body size. Using mixed-effects models, we analysed the associations of acanthocephalan infection probability and parasite abundance in mole crabs as functions of body size and the environmental site-level factors. Models accounted for 20 and 61 % of variation in acanthocephalan infection probability and abundance, respectively, with mole crab body size exclusively accounting for nearly all of the model fits. Also, acanthocephalan abundance decreased with increasing mole crab density, suggesting a possible encounter-dilution effect. Co-infection was strong, with the 2 other larval parasite species correlated positively with acanthocephalan parasites. The low influence of environmental variables on acanthocephalan infection could be because spatial variability in these factors is relatively low or their influences are quick to saturate. Therefore, in this system, parasite infection is apparently more strongly related to the duration of host exposure than spatially variable environmental factors, even across large spatial scales.