Pathogens and parasites are the hidden players of many of nature’s most bizarre and beautiful patterns and processes. For example, the extraordinary levels of plant animal biodiversity we find in the tropics is thought to be due, at least in part to the high levels of disease and natural enemies we find in those environments. Similarly, the medical community and casual observers are often fascinated by the boom and bust cycles seen in disease epidemics. How does disease spread so quickly through some populations but not others? What about certain habitats keeps the rate of new infections low? Well, PhD student Maja Šljivar (Figure 1) and her advisor, Dr. Spencer Hall (see Lab website), are interested in these types of questions, but maybe from an angle you haven’t thought of before.
Maja’s research focuses on how food web structures affect the rates, processes, and outcomes of disease. We often think of food webs in some kind of stepwise order: example, phytoplankton get their energy from the sun and are eaten by zooplankton, who are eaten by tiny fish, who are then eaten by bigger fish, etc. However, have you ever stopped to wonder what happens when a pathogen or parasite attacks the zooplankton? It’s not really consuming or eating the tiny marine creatures, but it still changes the rules of the game. The Hall Lab tackles these questions by studying the (surprisingly) adorable water flea, Daphnia dentifera (Figure 2).
Every fall, in freshwater lakes throughout the Midwest, a terrible fungal-plague sweeps through Daphnia populations, filling up these tiny translucent creatures with deadly fungal spores until the host bursts, releases the spores back into the water, and dies. How does this terrible plague happen you ask? You see Daphnia normally consume tiny algae found in freshwater lakes. As the Daphnia float along sucking up algae in order to grow to adulthood and then make new baby Daphnia, they also unintentionally eat fungal spores dispersed in the water, thus becoming infected hosts. However, the Daphnia and algae are not the only players in this food web. Chaoborus is a midge (type of small fly) and predator of Daphnia. Conventional wisdom specifies that predators tend to eat the weakest or sickest prey – think lions eating sick gazelles, or wolves eating deer. Under the conventional wisdom scenario, predators preferentially consume infected hosts, thus reducing the net prevalence of disease and the size of epidemics. Yet this is not what Maja has found to be the case in the algae-Daphnia–Chaoborus food web. Critically, her research shows that when you have high predator densities of Chaoborus, which does not preferentially eat sick hosts (i.e., Chaoborus is a non-selective predator), the size of epidemics is much higher.
The reason for this is directly related to classic food web theory: more predators, equals fewer prey, which leads to an increase in the resources (i.e., algae) the prey consume. And, because there are fewer Daphnia dining at the algae buffet, they wind up consuming more algae as well as more fungal spores. The net outcome leads to higher loads of fungal spores reproducing in the tiny Daphnia bodies and a higher dispersion back into the lake. Larger epidemics can be the result of higher predator densities, when that predator is a non-selective consumer of the host-prey.
Tiny Daphnia floating around in freshwater lakes and food web theory may seem only tangentially related to our daily lives, but believe it or not, these questions can also connect to human health. How predators influence disease and epidemics depends on the other members of their habitat and food web system. As a final example, Maja pointed me towards a recent scientific study on schistosomiasis, which is considered a neglected tropical disease infecting more than 200 million people. The parasitic worm that causes the disease first infects freshwater snails (intermediate host) before infecting humans (the secondary host) through the skin. In the mid-1980’s, the construction of a dam on the Senegal River removed the predators (prawns) of the intermediate snail hosts, which ultimately caused massive increases in both the snail population and rates of infection in villages along the river. This food web has multiple interacting players: worms, snails, predators, humans. Fortunately, researchers have recently shown that reintroducing prawn predators back into the system, combined with preventative drug treatments in susceptible humans, could eliminate schistosomiasis from these sites (Sokolow et al. 2015, 2017).
So the next time you think about birds eating worms, seals eating fish, or Chaoborus eating Daphnia, take a second to ponder the hidden pathogens and parasites of our world. These tiny organisms have the power to bring both the consumer and the consumed to their knees.