Spatial variation in parasite-induced mortality in an amphipod: shore height versus exposure history

Characterizing the causes of spatial and temporal variation in parasite-induced mortality under natural conditions is crucial to better understanding the factors driving host population dynamics. Our goal was to quantify this variation in the amphipod Paracalliope novizealandiae, a second intermediate host of the trematode, Maritrema novaezealandensis. If infection and development of trematode metacercariae are benign, we expected mature metacercariae to accumulate within amphipods inhabiting high infestation areas. In field samples, intensity levels of mature metacercariae decreased linearly when amphipods harbored >5 immature metacercariae. This finding is consistent with the hypothesis that the parasite can be detrimental at high intensities of infection. Short-term field experiments showed that host survival also declines with the intensity of new infections and drops below 80% when early stage metacercariae reach 10 amphipod(-1). However, parasite effects varied over space and time. High-shore amphipods suffered an increased risk of infection in the summer and a lower likelihood of survival: there was a 10-30% decrease in survivorship for any given infection intensity at high- versus low-shore locations. We also tested for differences in the susceptibility of naive and exposed populations using transplant experiments, and found that naive amphipods acquired greater parasite loads (on average, 4.7 vs. 2.8 metacercariae amphipod(-1)). Because survival decreases rapidly with infection intensity of both early- and late-stage metacercariae, naive populations would suffer considerably if the parasite were to increase its range. Our results indicate that trematode infections cause high mortality in amphipods during summer months under natural conditions, and emphasize that the effects of parasitism vary at local spatial scales and with exposure history.