Do parasites upset food web theory?

Jun 11, 2013
A network of 4,671 feeding interactions among 68 parasites (in blue) and 117 free-living taxa (green = basal taxa, red = consumers) in the food web of Estero de Punta Banda, Baja California, Mexico. The vertical axis corresponds to trophic level. Credit: Images produced by J.A. Dunne using Network3D software

Parasites comprise a large proportion of the diversity of species in every ecosystem. Despite this, they are rarely included in analyses or models of food webs. If parasites play different roles from other predators and prey, however, their inclusion could fundamentally alter our understanding of how food webs are organized. In a paper published 11 June in the open access journal PLOS Biology, Santa Fe Institute Professor Jennifer Dunne and her team test this assertion and show that including parasites does alter the structure of food webs, but that most changes occur because of an increase in diversity and complexity, rather than from unique characteristics of parasites.

"Current models and theory were developed with data for free-living species," said Dunne. "We wanted to understand whether including parasites alters in unique ways, or if observed changes are consistent with the addition of any types of species and links to a food web."

The group of researchers, which included parasitologists and food web ecologists, analyzed highly resolved for seven coastal estuary and . They compared three versions of each food web dataset: webs without parasites; webs that included parasites and all of their links to other species; and an intermediate case that included parasites but excluded the "concomitant" links between a predator and the parasites of its prey.

The team found that including parasites altered many aspects of network structure, such as the distribution of feeding links per species, the average shortest feeding chain between pairs of species, and the proportion of species that are or cannibals. But a closer look suggested that most of these changes were generic effects of increasing the overall diversity and complexity of the network, rather than unique effects attributable to the parasites' roles in food webs.

"Our analyses show that in many ways parasites are similar to other species in terms of their effects on food web organization," said Dunne. However, the team did find two cases where parasites seem to play special roles that alter aspects of food web structure. One case is when a parasite is eaten along with its host. "The physical intimacy between a parasite and its host is not found as frequently between free-living predators and prey," Dunne said. "The fact that predators incidentally feed on the parasites of their prey can alter certain patterns of interactions among species."

The other case appears to result from the complex life cycles of many of the parasites in these food webs. can shift hosts in a dramatic fashion, for example by starting out with a cricket as a host, but later requiring a fish host. This results in a more structurally complex feeding niche than is seen for most free-living predators.

"Our research extends the generality of food web theory and provides a more rigorous framework for assessing the impact of any species on trophic organization," said Dunne. "However, it also reveals limitations of current food web models when they are applied to the more diverse and highly resolved data that researchers are increasingly compiling."

Explore further: Identifying potential disease-transmission sources in animal species by calculating risk to humans

More information: Dunne JA, Lafferty KD, Dobson AP, Hechinger RF, Kuris AM, et al. (2013) Parasites Affect Food Web Structure Primarily through Increased Diversity and Complexity. PLoS Biol 11(6): e1001579. doi:10.1371/journal.pbio.1001579

Related Stories

A new model for understanding biodiversity

Nov 21, 2011

(PhysOrg.com) -- Animals like foxes and raccoons are highly adaptable. They move around and eat everything from insects to eggs. They and other "generalist feeders" like them may also be crucial to sustaining ...

Do parasites evolve to exploit gender differences in hosts?

Feb 28, 2012

Some disease-causing parasites are known to favor one sex over the other in their host species, and such differences between the sexes have generally been attributed to differences in immune responses or behavior. But in ...

Recommended for you

Male monkey filmed caring for dying mate (w/ Video)

Apr 18, 2014

(Phys.org) —The incident was captured by Dr Bruna Bezerra and colleagues in the Atlantic Forest in the Northeast of Brazil.  Dr Bezerra is a Research Associate at the University of Bristol and a Professor ...

Orchid named after UC Riverside researcher

Apr 17, 2014

One day about eight years ago, Katia Silvera, a postdoctoral scholar at the University of California, Riverside, and her father were on a field trip in a mountainous area in central Panama when they stumbled ...

In sex-reversed cave insects, females have the penises

Apr 17, 2014

Researchers reporting in the Cell Press journal Current Biology on April 17 have discovered little-known cave insects with rather novel sex lives. The Brazilian insects, which represent four distinct but re ...

Fear of the cuckoo mafia

Apr 17, 2014

If a restaurant owner fails to pay the protection money demanded of him, he can expect his premises to be trashed. Warnings like these are seldom required, however, as fear of the consequences is enough to ...

User comments : 0

More news stories

Making graphene in your kitchen

Graphene has been touted as a wonder material—the world's thinnest substance, but super-strong. Now scientists say it is so easy to make you could produce some in your kitchen.

Low tolerance for pain? The reason may be in your genes

Researchers may have identified key genes linked to why some people have a higher tolerance for pain than others, according to a study released today that will be presented at the American Academy of Neurology's 66th Annual ...

How to keep your fitness goals on track

(HealthDay)—The New Year's resolutions many made to get fit have stalled by now. And one expert thinks that's because many people set their goals too high.