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In Praise of Predator Parasites: A Chat with Biologist Ryan Hechinger

You are what you eat, and what eats you.
No parasites here. Photo: USGS

Think of the food chains you were given in middle school: You're likely to remember the big predators, a couple smaller animals that get eaten by the predators, and the plant "producers"--but absent from those rudimentary food chains, and overlooked on many more serious ones, is the role that parasites play.

That's a mistake, according to some ecologists: Parasites, which for their purposes include most bacteria, viruses, and fungi in addition to the "classic" parasites such as tapeworms, likely play a much more important role in the food web than they're given credit for. A small group of ecologists are increasingly buying into the "metabolic theory of ecology"--that energy flow is all that really matters in an ecosystem.

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While that theory may work on a macro level, it might also work from organism to organism: Ryan Hechinger, a researcher at University of California Santa Barbara, recently published a study that calculated the total energy parasites were sapping from rats, a finding that Hechinger says may eventually help humans fight disease.

Motherboard spoke with Hechinger about why parasites are overlooked by ecologists, why that's slowly starting to change, and why your grandma's method for beating a cold may have been a good one.

MOTHERBOARD: Tell me a little bit about the point of doing this study--haven't we known since forever that being sick makes you tired?

Hechinger: We deal with ecology and evolution. A lot of what we focus on is bringing consideration of parasites to mainstream ecology. We try to put parasites into the picture. We want to deal with them as an energy-level interaction. Everyone who has taken ecology, they know predators and competitors are important, but parasitism is almost always swept under the rug. As human beings, we know that parasites are important for how we perform for thousands of years - modern ecology has proven it in one case study after another parasitism is important, but they still don't show up.

So you want people to think of parasites as predators?

They are directly predators; it's just not as apparent. They consume but they're not killing their host like a predator. A parasite consumes from one individual resource throughout its whole life, while a predator kills their prey by definition.

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Why do ecologists miss parasites? It's because they are out of sight, out of mind. You don't see a tapeworm inside an antelope's gut that may have sucked energy from the antelope through its life. You don't see tapeworms that maybe made that antelope so much slower that maybe made it so the cheetah was able to eat that one.

Any excuse to revisit this footage.

Is it possible to quantify how important parasites are to a particular ecosystem?

We say that if parasites are really important, we should be able to measure their biomass--do a case study and ask how many tons of parasite flesh is there and how does it compare to predators? It turns out they have a substantial biomass. There is more parasite biomass than bird biomass in some estuaries. That's a crude proxy for energetic performance.

How about on an individual-by-individual level?

Energy is the next logical way to go. Energy--calories in, calories out-- can provide a universal currency for ecology. We're beginning to think about parasites inside of individual hosts. What I did in a recent study [published last week in The American Naturalist] is created [energy] equations and tested them by going over old data from classic ecological parasitology from the 1960s.

In that study, a parasitologist crammed a bunch of tapeworms into rats to see if there was a limit to how many it can hold. Turns out, it's not the space, it's the energy consumed that puts a cap on how many worms can be in the rat. After a few weeks, he put in thorny-headed worms, which are smaller than tapeworms. The total number of grams of worms inside the rats went down when he crammed in more worms, but because each individual worm was smaller, they were burning more calories.

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Why is that?

When you're smaller, you "burn hotter." A hummingbird burns way more calories per gram than an ostrich, for example.

So what does this mean for us?

There may be medical applications: With infectious diseases, we usually focus on the uniqueness of each individual one. We treat diseases based on their particulars. But by focusing on energy, we may be able to find universal truths or generalities about the way parasites impact their hosts.

Parasites' energy doesn't come from thin air--the host expends energy fighting parasites, the parasites sap energy directly from a host. I doubt we'll find energy unimportant in fighting disease. A focus on energy may point us towards some novel therapies to prevent or decrease pathology of a disease. You think of some of the old sayings "starve a cold or feed a flu"--that's almost certainly related to energetics. You either starve out the parasite or give yourself more energy to fight it.