Parasitism may be the most common lifestyle on the planet, but ecologists often ignore bloodsuckers and body invaders. That means parasites don't show up in food webs, the maplike visualizations of who-eats-who that students often learn about in high school biology. To amend this oversight, an international team of parasite and food web experts drew on data from seven different coastal and marine ecosystems, including California's Carpinteria Salt Marsh, the Baltic Sea's Flensburg Fjord, and New Zealand's Otago Harbour. Most of the parasites were animals—as opposed to bacteria or viruses—and hosts included both vertebrates and invertebrates. Tapeworms heavily infesting clams and rays represented a typical example. The team created three visualizations of each ecosystem: one without parasites; one with parasites, their hosts, and their hosts' predators (parasites in blue, free-living species in red in image above); and one with parasites and their hosts, but no predators. Somewhat predictably, including parasites significantly increased the food webs' complexity and diversity, the researchers report today in PLOS Biology. There was one surprise, however: unique triangular loops that show the normally unseen intimacy between parasites, their hosts, and their hosts' predators. Rather than getting a straightforward meal, predators often consume a significant dose of parasites, too—and sometimes that is what the parasite wants. Some of the triangular loops included double lines between parasites and predators, indicating that the parasite went on to infest the predator as its new host after being eaten. Rather than just occurring here and there, these loops can dominate patterns in the food web, revealing just how prevalent these often overlooked players can be.
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