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Cestodes are parasitic flatworms, of the phylum Plathelminthes, which tipically live in the digestive tract of adult vertebrates (definite hosts) and frequently parasite other species of animals in the early stages of their developments (intermediate hosts). One possible example of this system is the interaction between different species of helminthes that parasite ungulates in North America in their early development. The ungulates are the main resource in wolfs' diet in this region and, upon predation, the juvenile parasites infect the wolfs, their definite hosts, completing the parasites' cycle.

tapeworm-taenia-hydatigena-parasite-of-dogs-picture-id90062360

However, wolfs can also predate alternative prey, like beavers, which are not intermediate hosts to many wolf parasites. A recent study (Friesen and Roth, 2016) has shown that alternative resources use is correlated with reduced incidence of parasites in wolfs. This suggest that alternative resources use by wolfs might have an important influence on the parasite prevalence on the community, what can be beneficial both for wolfs and ungulates.

Propose a simple but realistic mathematical model to describe the coupled dynamics of populations of protected and unprotected plants, herbivores and predators. Your model should take into account at least two of the explanations presented above for the decreasing of predation in more protected plants. You are welcome to build on the case study of Kaplan & Lather (2010) or choose any other one with similar features.

Here are some questions you may explore, but further well-grounded insights are also welcome.

• Are the tri-trophic systems ¹⁾ based on protected and unprotected plants viable under realistic values of the parameters of your model?
• Under which conditions protected and unprotected plants can coexist?
• What is a more effective way to control crop pests: to plant protected varieties or to increase predator densities?

• Friesen, O.C. and Roth, J.D., 2016. Alternative prey use affects helminth parasite infections in grey wolves. Journal of Animal Ecology.