By Gail Gallessich

Parasites play surprisingly important roles in food webs, scientists reported in a recently published version of the Proceedings of the National Academy of Sciences. Scientists with UC Santa Barbara, the U.S. Geological Survey, and Princeton University contributed to the research.

The study was performed in Santa Barbara County at the Carpinteria Salt Marsh. The Carpinteria Salt Marsh Reserve is one of several natural reserves UCSB supervises as part of a University of California system of natural areas for research and teaching.

Food webs trace the flow of energy through an ecosystem. They extend the concept of food chains––those who-eats-whom sequences–to biological communities. Food webs rarely include parasites because of the difficulty in quantifying them by standard ecological methods.

“Food web theory is the framework for modern ecology,” said lead author Kevin Lafferty, a UCSB research scientist and USGS marine ecologist. “Parasites have been missing from this framework and, as a result, we know relatively little about the role of parasites in ecosystems. It’s like driving with a highway map, but with no knowledge of the smaller road network. To reach most destinations, you need a map with both.”

Using data from four relatively comprehensive food webs that contain parasites, Lafferty and his co-authors found that parasites dominated the food web links among species: On average, a food web contained more links between parasites and their hosts than between predators and their prey.

“Parasites may well be the thread that holds the structure of ecological communities together,” said study co-author Andrew Dobson of Princeton University.

Additionally, the researchers’ analyses revealed new patterns. It’s well known that vulnerability to predators decreases at the top trophic level. In this study, the scientists found that vulnerability of hosts to parasites also increased with trophic level. Consequently, animals at mid-trophic levels are the most vulnerable, being subjected to diverse parasites and many predators.

“When you add parasites to food webs, the pyramid contains a second inverted pyramid of parasites that are as abundant as all the other species,” said Dobson.

When they analyzed the Carpinteria Salt Marsh food web, the researchers calculated that parasites were involved in 78 percent of the links among species. Due to the diversity of parasites in prey species, the Carpinteria web had over twice as many predator-parasite as predator-prey links––1021 links compared with 505.

Parasitic trematode worms’ life cycles require sequential infection of multiple hosts. A common snail is the first intermediate host to at least 19 different kinds of trematodes at Carpinteria, involving many different kinds of birds as final hosts.

The authors found that without the snail and its trematodes, 977 links would disappear from the food web. In related work, Lafferty and co-author Armand Kuris, UCSB professor of biology, are counting trematodes in snails to assess salt marsh health.

“Few food webs have been able to consider parasites, and it will take a lot of work to include them, but the message is that you can’t fully understand food webs without parasites,” said Lafferty.