Trematodes don't get the respect they deserve. Long overlooked by ecologists, these parasitic worms and other moochers constitute more than 1% of the total biomass in some environments, a new study shows. That may not seem like much, but it dwarfs the contribution of many birds and other top predators. The findings could dramatically change our view of how food webs work.
The beefed-up reputation of trematodes comes thanks to Armand Kuris, a parasite ecologist at the University of California, Santa Barbara. For 5 years, Kuris and his colleagues waded through the mud and sand flats of three salt marshes, one in southern California and two in Baja California. They surveyed 215 free-living plant and animal species--including fish, birds, crabs, worms, and snails--and dissected more than 17,000 specimens to catalog the parasites. Although parasites are ubiquitous and often weaken their hosts or curtail their reproduction, their overall role in an ecosystem--in particular how they factor into the energy flow through the food web--is a mystery.
Plants accounted for the most biomass--the amount of living material per hectare--weighing in at more than 95% of the total, the team reports in the 24 July issue of Nature. Clams and other bivalves, crabs, and snails made up the bulk of the animal biomass, which amounted to less than 5% of the total. As for parasites, the team dug out 138 species, which accounted for 0.2% to 1.2% of the overall biomass. The biomass of trematode worms, also called flukes, was three to nine times that of the marshes' birds--and about on par with that of the fish and burrowing shrimp.
The wiliness of the trematodes helps explain how they came to represent such a big chunk of the animal biomass, say the researchers. As part of their complex life cycles, the worms "castrate" snails, commandeering the host's gonads and diverting energy meant for snail reproduction into trematode reproduction. The overall mass of infected snails sometimes surpassed that of uninfected snails by as much as 30%. The snails seem to survive just fine, but when they feed, they're really fueling parasite reproduction and not their own, the researchers report. As a result, the trematodes become much better breeders than the snails themselves.
The efficiency of a food web depends on how well plants convert energy from the sun into food and how much of that energy is passed on to meat-eaters. Given these results, parasites "clearly have a major influence on how energy flows through ecosystems," says Peter Hudson, a population biologist at Pennsylvania State University in State College, who wasn’t involved in the research. "This is a great, if not a heroic, paper [that] shows us just how significant parasites are to ecosystem functioning."
Parasites can help stabilize a food web by keeping the size of their hosts' populations in check. Thus they warrant conservation consideration, says Mark Huxham, an ecologist at Napier University in Edinburgh, U.K. These results "may help people to think about conservation of these whole systems, including parasites, rather than just the iconic species like birds."