This review article describes the potential mechanisms by which biodiversity affects disease risk. The authors explore the mechanisms at play in simple systems with only host and pathogen, as well as in more complex systems that include a vector species and/or multiple hosts. The reduction of disease risk by increased diversity is called the “dilution effect.” The opposite, termed the “amplification effect,” is when disease risk increases. “Both models and literature reviews suggest that high host diversity is more likely to decrease than increase disease risk” [Keesing 2006: 485].
The mechanisms by which diversity affects disease risk are as follows:
Encounter reduction: An additional species (such as a predator) suppresses the movement of host species or vector species, thereby reducing contact between susceptible hosts and infected hosts or vectors. (Alternatively, if the presence of a different species causes host species to clump together more among their own kind, then transmission could increase in an encounter augmentation.)
Transmission reduction: An additional species in a system (such as a prey) reduces host stress, boosting immune system response and lowering pathogen load. An added species could also modify host behavior in a way that reduces the duration of their encounters and thus limits transmission.
Vector or susceptible host [population] regulation: The addition of any species that reduces birth rates or increases death rates, limiting overall population, among hosts susceptible to the pathogen or among pathogen vectors. Transmission rates may be reduced, for example, with the addition of host species predators or with the addition of species that attract vectors (ticks, for instance), but then groom themselves in a way that kills many vector individuals.
Infected host mortality: An added species outcompetes infected hosts for resources or targets infected hosts for predation.
Recovery augmentation: The addition of a prey species as an added resource for host species could, for example, increase full recovery rates of host species, creating a dilution effect, or, by contrast, increase the longevity of sick hosts in an amplification effect.
When there are many hosts for a particular pathogen, some species transmit the disease more readily than others. Often, the species that most effectively spread the disease (the most competent reservoirs) are present in species-poor, degraded ecosystems, meaning that any additional host species is likely to dilute the presences of the more contagious species.
One key question in multi-host disease systems is whether the most competent reservoir is present in species-poor communities. If so, species added to these communities have, by deﬁnition, lower (if any) reservoir competence and thus have the potential to decrease disease risk. If the most competent reservoir is not present in species-poor communities, by contrast, then an increase in diversity could include the addition of the most competent reservoir itself, which is likely to result in an ampliﬁcation of disease risk. Ostfeld & Keesing (2000b) considered evidence that the most competent reservoir for a variety of vector-borne zoonoses was typically present in species-poor communities [Keesing 2006: 495].
Keesing, F., R.D. Holt & R.S. Ostfeld, 2006, Effects of species diversity on disease risk, Ecology Letters 9, https://onlinelibrary.wiley.com/doi/full/10.1111/j.1461-0248.2006.00885.x.