Mapping parasitism onto host geographic ranges: Do host populations differ in their levels or types of parasites between range edges and range centers?

Anai Novoa, Scripps Institution of Oceanography, UC San Diego, La Jolla, CA and Ryan F. Hechinger, Marine Biology Research Division, Scripps Institution of Oceanography, La Jolla, CA

Background/Question/Methods
The rise in the number of species shifting their range boundaries in response to global warming has led to concerns about whether range-expanding species will bring their parasites with them, thereby exposing their new communities to novel parasites. Hindering efforts to assess this possibility is our near complete lack of knowledge about how parasitism maps onto host geographic ranges. Do host populations have different levels or types of parasites at range edges compared to range centers? Are there differences in parasitism at northern versus southern range boundaries?
Here, we aim to address this issue using a hypothesis-driven, observational ecology framework. We formulated a priori hypotheses originating from basic principles from biogeography, ecological parasitology, and epidemiology. We then quantified parasitism by animal and protozoan parasites of four estuarine fish species sampled throughout their entire geographic ranges. We quantified parasite load using different currencies and diversity metrics. We also recognize distinct types of parasitic consumer strategies, which differ in fundamental ecological attributes (including how parasites impact host fitness). For each fish species, spatial patterns of parasitism was fitted to five models representing abundant center distributions and distributions where parasitism is highest near one or both host range edges.
Results/Conclusions
Mean parasite richness per host individual followed a consistent pattern for all fish species, exhibiting a ramped south distribution, revealing that their parasite assemblages were more diverse near southern range limits. Parasite biomass load (g parasite / g host) throughout host species ranges showed two dominant patterns where biomass load was highest at either northern or southern edges. However, these patterns were only significant for two fish species, suggesting that the factor hypothesized as driving the strong parasite richness pattern may counter a similar increase in parasite biomass load at southern range limits. Specifically, the increased biodiversity at lower latitudes may be associated with smaller or fragmented host populations, which would reduce host-parasite encounter rates, thereby leading to lighter parasite loads. A specific functional group of parasites, trophically transmitted parasites, followed ramped south distribution for three fish species, consistent with the hypothesis that predation is more intense near southern limits. Our analyses show how recognizing these elements of parasite-load and diversity can improve our understanding of how parasitism interacts with host biogeography, including influencing geographic ranges and predicting impacts arising from global warming induced range shifts.