Washington, July 24 : In a study of parasites living in three estuaries on the Pacific coast of California and Baja California in the US, researchers have determined that biomass of these parasites exceeds that of top predators, in some cases by more than 20 times.
Biomass is the amount of living matter that exists in a given habitat. It is expressed either as the weight of organisms per unit area or as the volume of organisms per unit volume of habitat.
Until now, scientists have believed that because parasites are microscopic in size they comprised a small fraction of biomass in a habitat, while free-living organisms such as fish, birds and other predators make up the vast majority.
"We quantified the biomass of free-living and parasitic species in three estuaries and discovered that parasites have substantial biomass in these ecosystems," said Armand Kuris, a zoologist at the University of California at Santa Barbara (UCSB), and a lead author of the paper.
"Parasites have as much, or even more, biomass than other important groups of animals--like birds, fish and crabs," said Ryan Hechinger, a marine scientist at UCSB and co-lead author of the paper.
"Unlike large animals, such as birds, parasites are tiny and often easy to overlook," said Sam Scheiner, NSF program director for EID (Ecology of Infectious Diseases) Program. "These results will cause ecologists and other scientists to completely reconsider their views," he added.
The researchers quantified parasites and free-living organisms in the Carpinteria Salt Marsh in California, and in the Bahia San Quintn and Estero de Punta Banda estuaries in Baja California.
Their study included 199 species of free-living animals, 15 species of free-living vascular plants, and 138 species of parasites.
"A lot of work we've done has suggested that parasites are important in ecosystems," said Kevin Lafferty, a marine ecologist with the United States Geological Survey. "But no one's actually looked at them as a group throughout an ecosystem," he added.
"Also, no one's considered parasites from the perspective of how much they weigh, because it's always been assumed they weigh almost nothing," said Lafferty. "Now we know that's not true," he added.
For example, in an estuary there are more kilograms of trematode worms - parasites - than kilograms of birds, the scientists found.
The study's results have a potential impact on the perceived role of parasites in the ecosystem.
From an ecological perspective, parasites serve both as regulators to prevent species from becoming numerically dominant, and as indicators of the health of a particular ecosystem.
According to Kuris, understanding the enormity of parasite biomass and the burden it places on available hosts could also lead to new strategies in the management of infectious diseases.