Parasitism and Host Diet Quality in Natural Populations

Parasite-host relationships can be amazingly complex. Parasites can even alter host diets. But how exactly does this work? Dr Charlotte Narr, a Research Scientist at Colorado State University’s Natural Resource Ecology Laboratory, recently published an article on just this topic in the Journal of Animal Ecology. 

When you’re sick, your relationship with food changes. You might lose your appetite or have trouble digesting certain foods. Pregnancy can have an even bigger effect on what and how much you eat. Now, imagine that you are an organism that feeds constantly to support your continuous investment in reproduction. Then you get sick. And you aren’t alone, just about everyone in your population is constantly eating to produce offspring and, just like you, they’ve gotten sick.

It isn’t good for your offspring, the sickness. You inadvertently ate the parasite with your food, and now it’s stealing the nutrients you ingested with it, repurposing them for its own reproduction, and messing with your digestion. You can’t produce as many offspring, and the ones you do produce are also sick. So your offspring produce fewer offspring, and you can see that your population growth rate is starting to drop.

Daphnia female - Dieter Ebert

Daphnia magna with eggs in brood sac (Photo: Dieter Ebert)

You’re a water flea, Daphnia magna, to be precise, and your relationship with food is legendary. So much so that the shuffling of nutrients between zooplankton like you and your algal food inspired the term ‘consumer-driven nutrient recycling’. Nutrients pass back and forth between you and your food in an elegant cycle. You can’t help but to ‘eat local’, and the nutrients that you don’t absorb from your food go straight back into the ecosystem to fertilize your food. This means that when you and your offspring eat food faster than it can grow, you starve and die. Alternatively, if you don’t eat enough, the food that’s growing around you becomes depleted in nutrients, and that’s not good for you, or the other grazers in the ecosystem that depend on the same food, either. If this parasite alters the way you eat and excrete nutrients, it could affect the entire ecosystem by shifting the nutrient content of primary producers.

Experimental evidence indicates that parasites do alter your relationship with food. Experiments have shown that Daphnia eat less, reproduce less, and excrete more nutrients like nitrogen (N) and phosphorus (P) when they’re infected. But do these shifts translate into real world changes to your ecosystem? And, if so, can these changes make you even sicker or, worse yet, alter your ability to drive nutrient recycling?

According to this new study in the Journal of Animal Ecology, they might. Narr and colleagues sampled rock pools on skerry islands in the Baltic Sea near the Tvärminne Zoological Station.  They quantified the relationship between the prevalence of a microsporidian parasite and the density and nutrient content of its host (Daphnia) and the diet of its host (algae). They also set up a mesocosm experiment to see how increasing the nutrient content (reducing the N:P ratio) of algae affected infected Daphnia populations and the population of the parasite.

Rockpools

Rock Pools on Skerry islands in the Baltic Sea. Dieter Ebert has been studying 100s of populations of Daphnia and their parasites in these pools for decades. Nitrogen and phosphorus concentrations in pools vary based on proximity to the sea and inputs from biotic sources like bird poop. The consequences of these inputs are apparent in the pictures above with some pools exhibiting very high levels of primary production (Left) and others nearly oligotrophic (Right).

As it turns out, algae in rock pools with a higher prevalence of infected Daphnia are depleted in nutrients (have higher N:P ratios) relative to those with a lower prevalence of infection. It’s unclear if this relationship is driven by parasite-induced shifts in consumer-driven nutrient recycling or effects of diet quality on host (and parasite) ingestion rates, but the authors make the case that it could be both. The trends in the field are consistent with parasite-induced shifts in consumer-driven nutrient recycling observed in the lab, but the mesocosm experiment showed that P additions reduce the parasite loads of individual Daphnia.

Mesocosm experiment

Narr and colleagues set up a mesocosm experiment on one of the skerry islands near the Tvärminne Zoological Station in the Baltic Sea. They added phosphorus to infected Daphnia populations in buckets to create ambient, moderate, and high N:P ratios, and then measured the growth of the populations over 56 days and spore loads of individual adult Daphnia at the end of the experiment.

So, there you are, a sick Daphnia with diminished offspring counts. Do you need to worry that your diminished appetite is making you sicker or ruining your ecosystem? That depends on the parasite infecting you and the specific nutrients humans are adding to your ecosystem (read the paper for more details). But it might be time accept that nutrient cycles don’t always revolve around your nutritional needs. Parasites can drive nutrient recycling too.

More Info:

Narr et al. (2019) Nutrient availability affects the prevalence of a microsporidian parasite. Journal of Animal Ecology. doi: 10.1111/1365-2656.12945

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