Why you should care about parasites

Parasites do cause suffering, and even veterinarians and public health researchers are apt to view them as scourges to be eradicated. But Kevin Lafferty, a California scientist who’s spent three decades studying the parasites that infect sea otters, estuary snails, fish and other marine creatures, views them with an ecologist’s lens: They’re an integral part of marine ecosystems, that must be factored in to understand how those natural systems work. Even ecologists haven’t traditionally included them in the food webs — an issue that Lafferty worked hard to change.

For more information, check out our article “In praise of parasites.”

Video transcript:

Knowable: “Parasites don’t make the cut on most of our favorite creature lists, but maybe that’s because we simply don’t know the crucial and surprising roles they play in the ecosystems all around us. These endangered Japanese trout rely on an unlikely ally to survive: tiny parasitic worms that need to make their way from land to water in order to mate. To get there, the worm infects crickets and prompts them to fling themselves into streams. The sacrifice delivers the worm to water, and the unlucky crickets provide up to 60 percent of the endangered fishes’ food supply. Parasites like this one may be critical to the functioning of many ecosystems, but they have historically been left out of the maps scientists create to help understand the flow of energy through a habitat.”

Kevin Lafferty (USGS parasitologist): “Ecologists have built hundreds of food webs and they haven’t put parasites in them — and what we’ve lost from that is the ability to even think about parasites and their roles in ecology.”

Knowable: “Take the African Savannah where — almost any child will tell you — lions are king.”

Kevin Lafferty: “Normally we think the lions are untouchable, but in fact they have all these ticks, fleas, protozoans, 31 species of tapeworms and nematodes and trematodes and on and on. And, in fact, there’s more things eating lions than things that lions eat.”

Knowable: “Though we rarely see them, parasites may make up as many as half of all known species. The roles they play are not always what you’d expect. In the salt marshes of Southern California, for example, a trematode worm moves from one host to another as it develops, living in snails as larvae, and reproducing only in the gut of shorebirds. It makes the transition through an unusual route: The brain of a fish. Fish with parasites on the brain are more likely to flash their silvery bellies toward the sky — a beacon to hungry birds overhead. The result: Infected fish are up to 30 times more likely to get caught, delivering the parasite to its final avian home.

“Parasites may regulate population numbers by feeding on their host species and competing with others for food. In the few cases where parasites have been added to food webs, the enhanced complexity is obvious.”

Kevin Lafferty: “This is a food web that we constructed for Estero de Punta Banda, which is an estuary in Baja California, Mexico.”

Knowable: “Here, each ball represents a species and each gray line connects a consumer to the species that it eats.”

Kevin Lafferty: “The species down at the bottom are the plants, and they’re in green, and then all the free living animals are in red. We can also see that there are trophic levels so we can go from plants up to herbivores and then to different levels of predation as we move from the bottom to the top.

“The next step, then, is to add parasites and see what changes. You can see that food web is a lot taller now with the parasites, because of the additional trophic levels that parasites provide.”

Knowable: “As scientists work to understand how parasites fit into ecology, some have even begun to talk about parasite conservation, arguing that losing certain parasites might have ramifications we are just beginning to understand. Some parasites even benefit humans directly.”

Kevin Lafferty: “If you’re into eating organic foods, you can thank lots of parasites that take out some of the insect pests that reduce crop yields, and we use those instead of pesticides, arguably to our better health.”

Knowable: “Parasites may also be worth preserving because they’re just really cool. We may never love the nine-meter worm that infects sperm whales as much as we love its hosts, but you can’t deny that it’s an impressive specimen. And it’s hard not to be left speechless by the marine louse that gobbles up a fish’s tongue, only to replace the organ with its own body.”

Kevin Lafferty: “A lot of them are just fascinating examples of evolutionary strategies, some of which are sort of, you know, disgusting perhaps, but certainly sinister and fascinating all the same.”