Effect of species richness on disease risk: dilution effect and underlying mechanisms

Summary any pathogens infect multiple host species which can differ in their reservoir competence. Consequently the species richness and composition of the host community can considerably influence the dynamics of disease transmission. Recently, an increasing number of studies reported the existence of a dilution effect whereby high host species richness reduces the disease risk. However, the generality of the dilution effect and its mechanisms are still highly debated. In this thesis, I tested the existence of a dilution effect in bovine tuberculosis (BTB) and investigated the underlying mechanisms of the dilution effect. I detected a possible dilution effect in BTB, where higher mammal species richness reduced the probability of occurrence of BTB at a regional level in Africa, after correcting for cattle density (Chapter 2). This dilution effect might be caused by encounter reduction, i.e. the presence of non-competent mammal species might act as barriers to herd movement of cattle and reduce encounter rates among herds, which leads to a decreased probability of BTB outbreaks. Then I extended the study of the BTB dilution effect to the analysis of BTB persistence and recurrence (Chapter 3). The results showed that mammal species richness was also negatively correlated with the BTB persistence and recurrence. Besides, I demonstrated that the presence of African buffalo, as a maintenance host for Mycobacterium bovis (the causative agent of BTB), had a positive identity effect and increased the risk of BTB persistence and recurrence, whereas greater kudu distribution was not correlated with BTB persistence or recurrence. In addition, BTB persistence and recurrence were correlated with different sets of risk factors. In Chapter 4, I showed that interspecific variation in species’ reservoir competence could be partly explained by life-history traits in three vector borne diseases, namely Lyme disease, West Nile Encephalitis (WNE) and Eastern Equine Encephalitis (EEE). Species with larger body mass (for hosts of Lyme disease and WNE) or smaller clutch size (for hosts of EEE) had a lower reservoir competence. Given that both larger body mass and smaller clutch size are linked to higher extinction risk of local populations, the results indicate that species with a higher reservoir competence are more likely to remain in the community when biodiversity declines, and thereby potentially increase the risk of transmitting these pathogens. This might be a possible mechanism underlying the dilution effect. Combing the results about the relationships between species’ reservoir competence and life-history traits, I constructed a compartmental model to investigate the effect of connectivity on the risk of directly transmitted diseases in metapopulations (Chapter 5). I showed that different indicators of disease risk (infection prevalence and number of infected individuals) reacted differently to increasing connectivity. Higher connectivity can not only decrease disease risk due to the dilution effect by increasing species richness, but can also increase disease risk through increasing contact rates among patches (facilitation effect). The net impact of connectivity depends on the relative importance of the dilution versus facilitation effect. These results may reconcile the current debate on the dilution effect, and contributes to a better understanding of the impacts of fragmentation on disease risks and the generality of the dilution effect. M Finally, I combined these findings and reviewed the evidence and critiques on the dilution effect (Chapter 6). Latest studies (also the BTB study in this thesis) tried to test species identity effects, caused by particular species in communities, and found that the identity effect and dilution effect can operate simultaneously in the host community. I suggest that the identity effect could act as an additional mechanism explaining the effect of species richness on disease risk. A weak correlation between host reservoir competence and local extinction risk can create inconsistent effects of host species richness on disease risk. Moreover, different indicators of disease risk may react differently to the changes in species richness. This could also be one of the reasons for the controversial results from previous studies that used different indicators (e.g., prevalence or number of infection) of disease risk. In conclusion, this thesis presents both evidence and critique for the existence of the dilution effect. Since factors may simultaneously influence community compostion and the characteristics of pathogen transmission (e.g., susceptibility, survival of pathogen etc.), future studies should also consider these factors, rather than only species richness, to better understand the effect of species richness on disease risk.