Impacts of two virulent, sterilizing pathogens on Daphnia individuals, populations, and communities
Parasites that sterilize their hosts without strongly impacting host lifespan are predicted by theory to have especially large impacts on host populations. In cases where sterilizing parasites can affect multiple host species, infections may spillover from one host to another, impacting community dynamics. If those hosts are a dominant member of the food web, sterilizing pathogens have the potential to have ecosystem-level effects.
We studied two widespread, sterilizing parasites of the important freshwater crustacean Daphnia: the oomycete brood parasite Blastulidium paedophthora, which attacks developing embryos, and the bacterial obligate killer Pasteuria ramosa, which manipulates host energetics to sterilize the host. First, we used life table studies to quantify the virulent effects of the brood parasite. Second, we tested for cross-species transmission in the lab. Finally, we analyzed the dynamics of both of these parasites in 15 lake populations, tracking infection and host dynamics in all Daphnia (D. dentifera, D. dubia, D. parvula, D. pulicaria, and D. retrocurva) and Ceriodaphnia dubia. We did this in two consecutive years (2013 & 2014) from July through November. We analyzed both the frequency of infections and their relationships with host dynamics.
We found that, like the bacterium Pasteuria, the brood parasite strongly impacts fecundity (p < 0.05 for 4/5 life tables) but not lifespan (p > 0.16 for all 5 life tables). Interestingly, most infected hosts continued to produce clutches of embryos, which were then attacked by the parasite. For both parasites, infected Daphnia were able to infect Daphnia of other species, as well as Ceriodaphnia, showing that both are multihost parasites.
Infections of both parasites were observed in all six host species and all 15 lakes. However, there was substantial variation between lakes in the prevalence of infection, with infections rare in some lakes but common in others. In 2014, maximum infection prevalences of the brood parasite reached 4.9-8.7% of the entire population and 9.1-20% of the asexual adult female population. Maximum prevalences of the bacterium ranged from 0.2-54.5% of the population. Interestingly, for both parasites and in all host species, there is a striking pattern where there are no sampling dates that have high host density and high parasite prevalence, suggesting population-level impacts of both parasites. Ongoing analyses are continuing to describe these effects and determine the impacts of community composition on host-parasite dynamics.