Parasite populations are nested within their host, and the spatial scale of host-parasite interactions should reflect this aspect of the relationship. Several studies report that the population density of parasitic species scales with parasite body size, and the spatial scale of the interaction has been interchangeable and represented as within a host or ecosystem. Nonetheless, it remains unclear whether the patterns emerging from these analyses using disparate sampling units are repeatable, and it is also likely the processes generating the patterns at different scales are not comparable. The aim of this study was to directly compare the population density and body mass scaling across parasitic taxa, while describing parasites nested within their host and ecosystem. We sampled three, 100-m transects along two riverine ecosystems, across 4 seasons to evaluate the effects of spatial scale of parasite population allometries in fish hosts. We used linear mixed effect models to assess the relationship between parasite density represented as n/m2 stream sampled and n/gram host, and host and parasite body size.
Results/Conclusions
In the fall, winter and spring we collected 793 fish individuals representing 23 fish species, and 26,771 parasite individuals representing 37 parasite taxa. Across both spatial scales of within the host and ecosystem, there was no significant scaling relationship between parasite density and parasite body size (ecosystem scale: slope = -0.05, CI (-0.16, 0.07); host scale: slope = 0.002, CI (-0.13, 0.14)). There was a positive scaling relationship between host body size and parasite density with an ecosystem (slope = 0.20, CI (0.04, 0.35)). However, we also report a strong negative relationship between parasite density and host body size at the level within a host (slope = -0.47, CI ( -0.67, -0.27)). Our data reveals a lack of relationship between parasite body size and population density at both spatial scales, but shows scaling relationships with host body size that differ based on the definition of the boundary of a host-parasite interaction. Moving forward in these analyses, a more transparent use of parasite spatial scales is needed to evaluate how parasites fit into ecological theory.