Human alteration of ecosystems often includes changes to species diversity. Habitat destruction, climate change, and overharvesting can eliminate species from ecosystems, whereas invasions represent species additions. How these changes should alter host-parasite dynamics and infectious disease risk for focal hosts (including humans) remains an open question, however. Although some have suggested that biodiversity should decrease (dilute) infection risk, others have questioned the generality of this paradigm, suggesting that whether a particular species decreases (dilution effect) or increases (amplification effect) risk might depend on its characteristics. Furthermore, it has been proposed that this controversy could come down to scale dependence; different groups might be reaching different conclusions because they are using different response variables that operate at different scales. Here we present a model that investigates how a species alters disease risk for a focal host, while explicitly accounting for spatial and temporal scale.
Results/Conclusions
Our model demonstrates that whether a species increases or decreases disease risk for the focal host depends on its density, competence, and contact rate, and its ability to alter focal host density. Moreover, we find that outcomes for the focal host can depend on the spatial and temporal scales at which host-parasite dynamics are studied. Due to the balance between the functional and numerical responses, dilution effects tend to dominate at scales smaller than those over which parasites reproduce, whereas amplification effects tend to occur at larger scales, although the opposite pattern is also possible. In short, we find no evidence for a general relationship between biodiversity and disease. Therefore, if win-win outcomes for people and nature depend on dilution effects, then it will be important to consider whether these outcomes reverse at broader scales. Nevertheless, where our model identifies scale-invariant dilution effects, these might represent ecological levers for health.