Disease Transmission: Consider the Community

Shaw, Kelsey; Civitello, David

"Tadpoles" by Takashi(aes256)
“Tadpoles” by Takashi(aes256)

Worms, germs, and other types of disease agents (“parasites”) are abundant and important in natural communities, and ecologists continue to struggle with predicting how infectious diseases change when the number or composition of species in an ecological community changes. Some studies have found that parasitism decreases in the presence of a diverse number of species (dilution effect), while others have found that it increases (amplification effect). Yet often these studies do not investigate the driving forces behind these patterns. In this review, we emphasize a complementary approach to understanding how parasites thrive or suffer when embedded in different ecological communities.

We specifically highlight the role of non-focal species, defined as any species that cannot be successfully infected by a parasite of interest. Our approach focuses on targeted, mechanism-based questions, such as: how would a highly abundant and efficient predator of hosts affect the transmission of one of their parasites? Overall, non-focal species can either increase or decrease parasite transmission through three broad categories: (i) changing the likelihood that a parasite encounters a susceptible host, (ii) changing the likelihood of successful infection given contact, and (iii) changing the number of new parasites released from the infected host. As an example, tadpoles will often swim vigorously to avoid trematode parasites, but in the presence of a predator they will remain still, thus rendering them more vulnerable by increasing the likelihood of encountering the parasite.

We envision that this approach is particularly useful when a single non-focal species can impact more than one category simultaneously. Taking into consideration these multiple potential effects could help explain otherwise perplexing outcomes. Understanding how non-focal species can impact disease transmission is important for learning the driving forces behind observed dilution and amplification effects, as well as for understanding how human actions can affect our own vulnerability to disease.

Read the review in full here

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