UCSB Study: Parasites Outweigh Predators

Parasites have traditionally been ignored by ecological studies as a result of the presumption that they are too small to matter, but a study headed by three UCSB professors concludes that parasites actually outweigh many of what are commonly considered top predators. Dr. Armand Kuris, Dr. Ryan Hetchinger and Dr. Kevin Lafferty, along with 15 co-authors including UCSB graduate and several undergraduate students, have concluded that the out of sight, out of mind philosophy is no longer adequate for studies of ecosystems and food webs when at least half of all species on earth are parasites.

The study was published in the July 24 issue of the scientific journal Nature.

If parasites were so negligibly small, then how could they cause so much damage and require so much energy from their hosts to fight them off? Studies had previously shown that parasites are capable of causing tremendous damage to their hosts. This is evidenced in the human population by the 1.5 billion people infected with the Ascaris worm or “round worm,” which feed on human blood to lay worm eggs. In response, the human body must fight off the infection by increasing blood cell capacity. Through this process, the parasite is continually drawing energy from the human.

Einstein’s famous physics equation, e = mc², is a proportionality of weight to energy that says if something is small, then it has a small effect. Consider all of the energy required to fight AIDS or malaria, said Kuris. Either Einstein was wrong, or infectious things are bigger than presumed.

After five years of collecting more than a million lines of data from plant and animal samples in three different locations along the Pacific Coast and Baja California, the study adjusts the hierarchy of energy control in the food web. On the biomass scale, parasites weighed in to a comparable and in some cases larger to biomass as top predators such as birds, crabs and fish, which are normally perceived as controlling species. Biomass is a measure of the total weight of a specific organism within a given area. So, even though they are individually small, at each of the estuaries the biomass of trematodes was three to nine times the biomass of birds. “How can you understand the natural world without understanding one of the major parts?” asked Hetchinger.

Each animal and plant has a biomass which harnesses the energy in a certain way, so by having a considerably large biomass, parasites control a considerable amount of energy. At the ecosystem community level, there is a huge energy drain directly caused by parasites because hosts use a lot of energy to repair damage, fight and defend against parasites. One of the types highlighted in the paper were the parasitically castrated types – organisms which have been completely taken over and, instead of fulfilling its biological role of surviving and reproducing, function exclusively a parasite baby-making machine, explained Hetchinger.

But parasites provide an integral role within an ecosystem and the food web by regulating the population of their host species. For example, a particular parasite on the brain of a fish make them act more conspicuously, so they are up to 30 times more likely to be caught by a bird. Parasites are a natural component of a healthy ecosystem. “Parasites are labeled as bad and dirty, but they are just organisms tying to make their living. Their purpose is the same as any other living thing. That’s the only purpose that anything has,” Hetchinger said.

If parasites are considered on equal footing as other species, how does that change the scientific perception of the patterns of the world? Adding parasites to the picture brings us closer to being able to see the whole picture.

“Infectious disease is not the kind of thing that ecological studies focus on, but if they could visualize the parasites in the world, ecologists would always be forced to consider parasites as part of the overall problem,” Kuris said.