Browsing by Author "Jalo, Mikko"

As biodiversity is being lost worldwide at an accelerating rate due to anthropogenic activities, the frequency and severity of many infectious diseases has been observed to increase. Together these patterns have brought forth an urgent need to understand the possible linkages between biodiversity and disease risk. Two contradicting hypotheses have been proposed to explain the diversity-disease relationship. The dilution effect hypothesis suggests that increasing host community species diversity ‘dilutes’ disease risk, whereas the amplification effect hypothesis predicts disease risk to increase with increasing diversity. Even though most of the studies support the dilution effect, there remains an intensive debate regarding the generality of this effect. As most of the research efforts to understand the relationship between diversity and disease have focused on animals and crop plants or have been carried out experimentally, one of the research gaps is how relevant the dilution effect is in wild plant communities. In nature, plants and their diseases are affected simultaneously by multiple abiotic and biotic environmental factors that might confound or supersede the effects of diversity. It is also poorly understood, whether we might expect dilution effects to occur not only on diversity gradients driven by anthropogenic diversity loss, but also on natural diversity gradients. To study the possible association between host community species diversity and disease risk in the wild and to test whether this association could be detected after accounting for the effects of abiotic factors, I surveyed grassland vascular plant communities for their species diversity and foliar disease symptoms along a natural diversity gradient driven by elevation. I also recorded data on the mean soil surface temperature in the surveyed plant communities and used structural equation modelling to differentiate and compare the effects of biotic and abiotic variables on disease risk. The data were collected on Mount Calanda in the Swiss Alps during summer 2019. In this thesis I show that host community species diversity and disease risk are negatively associated with each other along a natural diversity gradient driven by elevation. Furthermore, this negative effect can be detected even after accounting for the effects of elevation and mean soil surface temperature on disease. Together the results support the occurrence and the ecological relevance of the dilution effect in wild plant communities along natural diversity gradients and suggest that diversity might protect wild plant communities from increased disease risk. Future studies should aim to identify the exact mechanisms of the association to help us better understand when and where we might expect dilution effects to occur in the wild. This knowledge can be used to predict how epidemics, that affect the well-being of ecosystems, humans and wildlife, are born in the changing world.