Parasites can exacerbate or buffer host response to environmental conditions, and thus hosting a parasite can affect host performance. Likewise, environmental context can alter parasite performance directly, creating a range of host-parasite interactions across environmental gradients. Extant variation in species interactions across environmental gradients could give us insight into how species will respond to climate change. Using field outplants, we examine growth of a shell-boring endolithic cyanobacteria and its mussel host, Mytilus californianus across an intertidal gradient. To isolate erosion rate from mussel growth rates, we paired dead shells with living mussels across the intertidal. We evaluated solar angle of the outplants, intertidal elevation and temperature. We fit field measured performance rates to a simple model of mussel growth and erosion, and evaluated the effects of changes in environmental conditions on predicted host and parasite extent.
Results/Conclusions
The proportion of mussels that are eroded by endolithic cyanobacteria varies across the intertidal, with high levels of erosion in the upper intertidal and low levels in the lower intertidal. We found that mussel growth rates were highest in the lower intertidal, and that erosion rates were highest in the upper intertidal. Increased erosion rates were associated with decreases in mussel growth rate across the intertidal, but also within subset intertidal heights. Interestingly, we found lower erosion in the living mussels compared to dead shells, even when corrected for the increase in mussel shell created by the growth of living mussels. Solar angle had no effect on the mussel growth rates or cyanobacterial erosion rates. Model results suggested that field measurements of cyanobacterial erosion and mussel growth, in combination, can create the variation in erosion found across the intertidal.