Where Are The Parasites In Food Webs

Published Date: 02 Nov 2017

Why parasites are "ultimately missing links" in food webs?

Food web is a network of food chains in an ecosystem. It traces the flow of energy through an

ecosystem. There are five trophic levels in a food chain: quaternary consumer, tertiary consumer,

secondary consumer, primary consumer, and primary producer. Each trophic level absorbs energy by

consuming trophic level below respectively. Besides predation, parasitism is the most general

consumer lifestyle. However, parasites are rarely included in the food webs.

Parasites are consumers that feed on a living host. They spend a period of time in host

organisms and cause harm to the hosts, by deriving nourishment and protection, without immediately

killing them. Parasites can be found everywhere in our environment, and some may cause infections

and illnesses. It has been reported that in 1993 waterborne parasite Cryptosporidium caused a

massive outbreak in Milwaukee, Wisconsin, affecting an estimated of 403 000 people. (Morris et al., 1995)

Where are the parasites in food webs?

Parasites are rarely included in food web because it is difficult to measure and observe

parasites using ecological techniques. Some parasites are so small that they are invisible to the naked

eye, and are hidden inside their hosts. Many typical parasites often have complicated life cycles with

multiple hosts and feeding preferences. As a result, they can occupy several trophic levels in a food

web. Nowadays there is more evidence that proves parasites can drastically affect food-web structure.

The inclusion and exclusion of parasite-host interactions clearly indicate that parasites influence the

community dynamic. (Sukhdeo, 2012)

Why parasites should be included in food webs?

Parasites life cycle depends on transmission from intermediate host to final host. Asexual

reproduction or development takes place inside intermediate host. Parasites reach definitive host

through predation in order to complete their life cycles. They sexually reproduce and mature inside

definitive host. In order to transmit from intermediate host to definitive host, parasites modified host

behaviour by targeting the central nervous system. They influence the hormone secretions or disrupt

the nervous system, which contributes to the unusual physical behaviours such as abnormal

responses to environmental stimuli, disorientation and increase in exposure to predators. (Lafferty et al., 1996)

Parasite manipulation of intermediate host behaviour, and transmission to final host have

been widely studied. Lafferty and his colleagues did research on Euhaplorchis californiensis, larval

trematode that lives in southern California salt marshes. It has a three-host life cycle. This parasitic

worm matures sexually in shorebird. Eggs produced by parasite are released in the faeces of bird,

which are spread into the salt marshes. Horn snail, the intermediate host, is found typically in marshy

areas. It ingests the faeces which contained eggs and becomes sterile as trematodes castrate the snail.

Parasites divert snail host's energy and undergo development stage. They develop into cercariae,

disk-shaped larva with a tail-like appendage, when they are ready to leave the snail. Cercariae swim

out into the marsh, and infect second intermediate host, the killifish. (Zimmer, 2003)

The cercariae attach onto the gills of the killifish and migrate along a nerve into the brain as

metacercaraie. They form a thin carpet-like layer on the brain surface. According to Lafferty, the

parasitized killifish perform 4 times conspicuous swimming than unparasitized fish. They do all risky

movements, such as twitching, exposing its belly while swimming on one side, or darting to water

surface. These altered behaviours increase the chances of infected fish to be caught and consumed by

predator. Shorebird is the definitive host of trematode life cycle. Once it ingests the infected killifish,

parasite resides in the bird’s gut and produces eggs that are released in the stool back into the salt

marshes. Eventually, parasites achieve transmission and complete their life cycle. (Zimmer, 2003)

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Figure : Life cycle of Euhaplorchis californiensis [Adapted from Friend et al., 1999]

Lafferty’s approach proved that parasites increase susceptibility of infected individuals to

predation by behaviour manipulation to promote transmission. Parasites can selectively strengthen

links between predator and prey by reducing the competitive ability of parasitized hosts prey against

an uninfected predator. Parasite-mediated competition has been used to describe the effect of

parasites in the competition ability between two species. Recent studies have revealed the complexity

in parasite-mediated interactions (Hatcher et al., 2006). Parasites can prevent competitive exclusion,

where two species having the same feeding preference in the same area. This allows the coexistence

of a competitively inferior species with a dominant species. For example, two species of Anolis

lizards coexist in Caribbean islands. Anolis gingivinus is a stronger competitor which

outcompetes Anolis wattsi throughout the island. Malarial parasite, Plasmodium azurophilum, uses

both lizards as host. Nevertheless, only A. gingivinus is heavily infected. This suggests that the

competitively inferior lizard is able to coexist because of the decrease in competitive ability of the

dominant lizard by Malarial parasite (Schall, 1992).

Parasites have substantial effects on host development, reproduction, survival, and

competitive ability. They divert host energy to reduce host’s reproductive success, and increase

vulnerability to predation and physiological stress. The effect on the host depends on the parasite’s

life history. As mentioned previously, trematodes act as parasitic castrators in snails. At least 19

trematode species use the common snail as their first intermediate host. If the snail host and flatworm

parasites are absent from the food web, there will be a corresponding 977 links disappearing from the

web. (Lafferty et al., 2006)

With the intention of observing the productivity of snails, Lafferty placed the healthy snails

and infected snails in different cages at salt marsh. Algae were grown on the bottom of the cages.

The results suggest that the growth of uninfected snails is faster than infected snails. They produced

more eggs, and were able to thrive in crowded environments. In consequence, the productivity of

healthy snails in nature is not quick enough to take full advantage of the marsh as parasites were

competing intensely. Moreover, snail population would be doubled with the absence of parasites in

the marsh. There will be a decline in algae production due to the overpopulation of snails, therefore

benefiting predators to thrive. (Lafferty, 1993)

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Figure : Parasites affects predator's feeding preference and host's feeding intensity.

[Adapted from: http://www.sciencedirect.com/science/article/pii/S1385110112000834]

Alternatively, parasites achieve transmission via the food chain. Thus they contribute to the

flow of energy through food webs. Energy is passed from hosts to parasites. Parasites alter prey

capture rates so that they are protected from many predators while living inside their hosts. Another

field experiment was carried out to measure the rate of energy flow after being altered by parasites

through a forest-stream ecosystem. Gordionus spp. is a nematomorph parasite that is commonly

found in damp areas such as streams and puddles. It infects Orthoptera such as crickets and

grasshopper, and causes them to crawl into a stream where parasite develops into a free-living adult.

Due to their abundance, these parasitized orthopterans are consumed by predator trout. The trout

obtained almost 60% of its energy from this interaction. Benthic invertebrates in stream are less

likely to be consumed by trout, leading to the potential for cascading. These results show that flow of

energy through ecosystems can be altered by parasites. (Sato, 2011)

Parasites affect food web structure

Parasites have dominated the food web links. There are more predator–prey links in a food

web compared with parasite–host links. It was reported that after the inclusion of parasite links there

are approximately 80% of parasites involved in the food web links of the Carpinteria Salt Marsh,

Recent studies have found that parasites affect chain length, link density, and asymmetry of

interactions of a food web. Despite of the least susceptibility of top trophic levels to predator are

shown in most food web studies, the inclusion of parasites revealed that mid-trophic levels will more

likely to have the highest susceptibility to predators, instead of the lower trophic levels. This proves

that the properties and data of the food web is insufficient without the presence of parasite. (Lafferty et al., 2006).

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Figure : Susceptibility to pradators peaks at mid-trophic levels in the Carpinteria Salt Marsh food webs.

[Adapted from Lafferty et al., 2006]

There is far more information about the potential role parasites in food web that has not been

investigated. Nonetheless, ecologists have just stated to make progress in examining the parasites’

roles in community ecology. Hence, a lot of work and efforts are required in order to include

parasites into the food web. Overall, we now understand parasites play important roles in food webs,

competitive interactions, flow of energy, as well as the development and productivity of animals.

This clearly indicates that parasites have a major contribution in the structure of ecological

communities.