UC San Diego

Despite the fundamental importance of parasites, relatively few studies have adequately mapped basic patterns of parasitism onto host geographic ranges. This knowledge gap hinders our ability to evaluate and understand species biogeographical distributions, including their determinants and consequences. In the context of a predictive theoretical framework, I quantified how parasitism by animal and protozoan parasites varies among latitudes within the geographical ranges of individual species. Chapter 1 shows how quantifying parasitism (1) at the single host (parasite infracommunity) level, (2) considering distinct parasitic consumer strategies, and (3) by using parasite diversity and biomass load as currencies permitted me to document novel biogeographical distributional patterns of parasitism of four estuarine fish species. Parasite diversity increased at lower latitudes and was largely driven by parasites using trophic transmission. These findings indicate a particularly strong role for trophically transmitted parasites and predation in community structure and dynamics at lower latitudes. In Chapter 2, I documented the latitudinal variation in parasitism throughout the contiguous geographical range of the striped shore crab, Pachygrapsus crassipes. This crab commonly uses two distinct habitats throughout its range—living on mud in low-energy estuarine habitats and on rock in the high-energy rocky intertidal zone—providing a striking opportunity to examine latitudinal variation in parasite diversity and load while simultaneously examining the influence of habitat on parasitism. I showed that P. crassipes appears to be affected by parasites (1) that are trophically transmitted, overall; (2) in estuaries than in the rocky intertidal zone; and (3) down south in each habitat. In Chapter 3, I quantified the latitudinal variation in the probability of being infected by parasites, parasite diversity, and biomass load for 13 species sampled throughout their entire range, four northern species sampled at their center and southern edge, and two southern species sampled at their northern edge. I documented three strong patterns of parasitism—peaking at mid, low, and high latitudes—were repeatable among different host species and explicable from the theoretical framework. These trends have overlapping and different implications for how parasitism may play a role in the ecology of host range shifts and setting species range limits.