Food webs trace the flow of energy through an ecosystem. They extend the concept of food chainsthose "who-eats-whom" sequencesto biological communities. Food-web studies rarely include parasites because of the difficulty in quantifying parasites by standard ecological methods. Parasites seem small and invisible, hidden inside their hosts. However, parasites strongly affect food-web structure, and parasite links are necessary for measuring ecosystem stability, according to a new study by scientists at the U.S. Geological Survey (USGS), Princeton University, and the University of California, Santa Barbara (UCSB). The study was published in the online Early Edition of the Proceedings of the National Academy of Sciences, week of July 10-14, 2006.
"Food-web theory is the framework for modern ecology," said Kevin Lafferty, a USGS scientist at UCSB and lead author of the study. "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-web studies that include parasites, Lafferty and his coauthors examined whether and how parasites affected the food webs. They found that parasites dominated the food-web links between 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 coauthor Andrew Dobson of Princeton University.
Additionally, the researchers' analyses revealed new patterns. They found that vulnerability to predators decreased with the maximum trophic level, or top predators, but that vulnerability of hosts to parasites increased with trophic level. Animals at mid-trophic levels were the most vulnerable to natural enemies, being subjected to both diverse parasites and many predators.
"The work illustrates that 'the pyramid of life' we learn about in kindergarten is wrong!" said Dobson. "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."
When they analyzed the Carpinteria salt-marsh food web, which had the most complete set of parasites in the four food-web studies, the researchers calculated that parasites were involved in 78 percent of the links between species. Owing to the diversity of parasites in prey species, the Carpinteria web had more than twice as many predator-parasite as predator-prey links (1,021 versus 505). Parasites also increased food-web "connectance" by 93 percent and "nestedness" by 439 percent, two characteristics of food-web structure that make the food web more robust to the threat of extinction.
Parasitic trematode worms with complex life cycles involving sequential infection of multiple hosts were involved in numerous links in the Carpinteria salt-marsh food web. A common snail is the first intermediate host to at least 19 different types of trematode at Carpinteria, involving many different types of bird as final hosts. The authors found that without the snail and its trematodes, a corresponding 977 links would disappear from the corresponding food web. In related work, Lafferty and the third author of the study, Armand Kuris of UCSB, are currently using techniques that count trematodes in snails to assess salt-marsh health (see Sound Waves article, "Biologists Count Parasites to Assess Health of Marsh").
"Few food-web studies 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. Kuris added that this, along with other recent studies, indicates that most ecological investigations should evaluate the role of parasites because infectious diseases can be such important players.
The Proceedings of the National Academy of Sciences study opens the door, said Dobson. "Once we understand food-web structure, we will have a much better understanding of how the loss of biodiversity will affect the quality of life for the surviving species."
The print edition of the paper appeared in the July 25, 2006, issue. Here is the full citation:
Lafferty, K.D., Dobson, A.P. and Kuris, A.M., 2006, Parasites dominate food web links: National Academy of Sciences, Proceedings, v. 103, no. 30, p. 11,211-11,216 [URL http://www.pnas.org/cgi/content/abstract/0604755103v1].
in this issue:
Parasites, the Thread of Food Webs?