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Parasites in Food Webs: The Ultimate Missing Links

Most might not have even considered it, but adding parasites to a food web may be extremely beneficial when studying the spread of infectious diseases. Parasites are so small, compared to lions and zebras, but they are everywhere in biological systems. This article reflects on the complexity of adding parasites to food webs, and why it may or may not be a good idea. Parasites can be eaten directly or indirectly, they can live on or within a host, and they have complex life cycles that may or may not depend on transferring to hosts. This is one reason they cause a problem; parasites are a lot harder to place in a food web than a lion and a zebra. Not only are they hard to place, but placing them may also cause instability in a food web, depending on the way they are placed. However, whether they actually cause instability or cancel out the instability through their complex characteristics is highly unresolved as an issue. Seeing as stability is essential to accurate observations and studies made using information from a food web, adding parasites to food webs may do more harm than good. Food webs can be extremely complex, with connections that have weights or strengths depending on the frequency of or dependence on the interactions between nodes.

The discovery of parasites in specific nodes (animals) of a food web may be beneficial for discovering links in a food web, in terms of predator-prey relationships. Since parasites are very selective about choosing a host, it might be easy to see a new relationship that was unexpected between a predator and a prey when a parasite that is specifically found in the prey is then found in the predator, showing a new link between the two. However, it is important to distinguish “incidental predation”, like the example given of a rabbit accidentally consuming a host ant known to carry a specific parasite. Should a wolf consume the rabbit and later be found to have the parasite, it would not make sense to assume the wolf spends time consuming ants. Therefore, the introduction of parasites may increase connectivity of the network, but it also complicates which connections should be made. One important topic discussed was that parasites may effect energy flow within a food web. The connectivity of a food network relies also on the strengths of the connections to determine who eats who and how often. Parasites that disable prey defensive functions could easily increase energy flow because they are far easier for a predator to catch. However, top predators that have functions disabled that they rely on to effectively consume energy may severely affect the energy flow to that predator. It is mentioned this may also decrease the likelihood of extinction through predation. Obviously introducing parasites to a food web increases complication, but if done right, it may also answer a lot of questions about ecological relationships previously not known.

 

https://onlinelibrary.wiley.com/doi/full/10.1111/j.1461-0248.2008.01174.x

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