Parasites are nature's freeloaders, living off their hosts while giving nothing in return. But scientists have come to appreciate that even the greediest parasites can indirectly benefit other species by manipulating ecosystems—providing food, assisting predators and even building habitats.

Indeed, some parasites are now counted among the ranks of "ecosystem engineers"— organisms that create, modify, maintain or destroy habitats. Beavers are the most familiar example. By building dams, they can radically alter landscapes, channeling streams into meadows and ponds. Likewise, woodpeckers move to different trees every season and create new nests, leaving behind vacant cavities throughout the forest that many other animal species can inhabit.

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But how could organisms as small and selfish as parasites have a similarly significant impact on their surrounding environments? Frequently, they do so through the manipulation of other, larger species. Scientists have identified a number of examples:

Fish and Worms

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Japanese biologists are trying to save an endangered species of trout, the Kirikuchi charr, by learning more about its diet. In doing so, they made a remarkable discovery.

The charr live in streams inhabited by Gordionus chinensis—a type of nematomorph, otherwise known as a horsehair worm. Their offspring are larval worms that infect aquatic insects such as mayflies.

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Upon reaching adulthood, mayflies leave their streams and live on land. When they die, they fall onto the forest floor, with the larval nematomorophs still alive inside their carcasses.

Crickets scavenging for food eat the mayfly carcasses and become infected with the larvae. As the nematomorph grows within the cricket, it wants to return to its adult habitat, the water. But, crickets are land-based insects. So, what to do?

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Researchers have learned that nematomorphs can alter the behavior of crickets, compelling them to seek water. The crickets jump into the stream, and the adult nematomorph explodes through the cricket's anus to go searching for a mate, leaving its twitching former host to die on the water's surface.

You'd think things couldn't get any worse for the cricket by that point, but once it's in the water, it will likely get eaten by a charr. Biologists have found that infected crickets account for 60% of the fish's caloric intake. The parasite delivers food to the charr, assisting in their survival.

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Ecologists Kevin D. Lafferty and Armand M. Kuris say that, beyond helping to sustain the charr, the nematomorphs have a broader, indirect effect on their entire ecosystem:

The nematomorph moves a substantial amount of energy from the forest to the stream. In return, satiated by crickets, charr consume fewer aquatic insects. This shifts energy from the stream back to the forest because many surviving aquatic insects, such as mayflies and damselflies, metamorphose into flying adults that move back to the forest and become predators and prey for terrestrial animals.

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Shell Games

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The New Zealand Cockle is a type of clam that burrows beneath mudflats and sand flats. Sea anemones sometimes live on cockles, anchoring themselves to their hard shells.

But, there's a parasite that preys upon cockles, called Curtuteria australis. The parasite infests the foot of the cockle, where it forms a hard cyst. As more parasites accumulate, the cockle loses its ability to bury itself in the sediment. As a result, the cockle is stranded on the surface, where it is vulnerable to being eaten by shorebirds, which is the next destination of the parasite in its lifecycle.

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It's a bad deal for the cockle, but not all of them are eaten. In fact, as their shells become exposed, other creatures, such as limpets, are able to live on their surface. On measure, this increases biodiversity, by reducing the competition for a suitable habitat between limpets and anemones.

Christmas Spirit

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You know how people kiss under the mistletoe during the Christmas season? Yeah, sorry to break the mood, but you're smooching beneath a parasite. The plant's seeds drill through tree bark with a thread-like probe and then grow by sucking water and nutrients from their hosts.

David Watson, an Australian ecologist who studies mistletoe, conducted an experiment: with the help of some volunteers, he removed all the mistletoe that were living in one part of a forest. After a few years, they discovered that the bird population had decreased by one-third. They simply left.

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As NPR reports, most of the birds that left were those that fed on insects:

What do insects have to do with mistletoe? "It's a byproduct of how parasitic plants do their parasitizing," explains Watson. Parasitic plants are packed with nutrients that they gobble up from their hosts. They suck up all these salts and minerals to create a water gradient between them and their host so they can draw water out of their hosts.

"Parasitic plants the world over have 15 [to] 20 times more concentrated nutrients than their hosts," Watson says.

And because they're moochers, they don't really care about conserving their resources — they can just suck out more. Not so with regular trees, which pull out the good stuff from their leaves before allowing them to fall. But mistletoes just drop their leaves with all the vitamins inside.

"So there is this rain of enriched litter — a bit like mulch, a fertile mulch," as Watson puts it, that falls onto the forest floor under infected trees.

More goodies on the soil, more bugs, more birds that eat the bugs — it might mean more lizards and more mammals too.

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Standoff

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Isle Royale National Park in Lake Superior is a unique habitat where the struggle between predator and prey involves just two species: moose and wolves.

Originally, neither animal lived on the fir- and spruce-covered island. The moose first arrived about a century ago, either by swimming or walking on ice. Then, 50 years later, the gray wolves showed up, after a cold winter allowed them to cross 15 miles of ice from the Canadian shore.

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In the decades since, the island has become an outdoor laboratory, where biologists have been conducting the world's longest-running study of predator-prey dynamics.

One key area of focus is ecological balance. If the moose population were to become too large, they would overgraze and begin to die off—which would also deprive the wolves of their food supply.

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Both species have seen their populations fluctuate due to disease and weather. But, one theory is that a precarious standoff is maintained by tapeworms that infect the moose as a temporary host and the wolves as a final host. While the parasite has little discernable effect on the wolves, it can be debilitating for the moose, creating large cysts in the animals' lungs, braincase and liver.

The moose, in its weakened state, makes things easier for the wolves. Although wolf packs are impressive hunters, an adult moose is formidable prey. "An old forest with lots of downed trees also allows a moose to 'comb' the wolves from its backside by running or twirling around and violently throwing them against trees," explains biologist Rolf Peterson. "This behavior helps explain the broken ribs often found in the skeletons of old wolves on Isle Royale."

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The tapeworm, therefore, helps keep the moose population down, which prevents overgrazing and potential mass starvation for both species.

Not Always Helpful

Of course, like other ecosystem engineers, parasites are also capable of damaging entire habitats. Often, they accomplish this by infecting and interfering with the positive work being done by other engineer species.

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In the Pacific Northwest, for instance, blue mud shrimp play a significant role in the local environment. For starters, they're a significant food source for birds, fish and grey whales. But, mud shrimp populations also filter up to 60% of phytoplankton standing stock within estuaries and enhance carbon and nutrient cycling. However, an invasive species of parasite, Orthione griffenis, is damaging the ability of the shrimp to reproduce and is shrinking the population. Biologists worry that this will have far-reaching implications for other ocean species living in the region.

It's only in recent years that scientists have begun to understand and appreciate the potential for a single parasite species to alter an entire ecosystem. To paraphrase a famous Jedi master, judge them not by their size.

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