Evidence for the importance of trophically transmitted parasites and predation at the lower latitudes of species ranges

The lack of information concerning how parasitism maps onto host geographical distributions represents a striking gap in ecological knowledge. This knowledge gap limits our understanding of a wide range of phenomena, including the consequences of climate change-induced range shifts of both hosts and parasites. To help solve this problem, we created a predictive theoretical framework and quantified latitudinal variation in parasitism by animal and protozoan parasites throughout the entire contiguous geographical ranges of four estuarine fish species. To circumvent frequent limitations in data and to permit revealing of novel biogeographical patterns, we (i) quantified parasitism at the individual host level, (ii) quantified parasite species diversity and biomass load, and (iii) tracked functionally distinct parasitic consumer strategies. Parasite diversity always increased at lower latitudes, and this pattern was largely driven by parasites using trophic transmission. Furthermore, in three of four cases, the role of fish as predator- versus prey-host for trophically transmitted parasite (TTP) species increased at lower latitudes. Parasite diversity patterns followed predictions more consistently than did biomass load, indicating that increased predation at lower latitudes may decrease observed parasite biomass load. These findings suggest a particularly strong role for TTPs and predation in community structure and dynamics at lower latitudes.