Causes and consequences of soil bacterial rarity
In the highly diverse community of soil bacteria most species occur at extremely low numbers and are consequently called the “rare biosphere”. In this thesis I studied why these species are rare whereas others are common. I found that many rare bacterial species can grow rapidly and are doing well in competition, but that rare species could be more vulnerable to predation. This might account for their rarity in soil. Still growth, competitiveness and predation resistance differed greatly between rare species. In addition, I tested if rare soil bacterial species can protect plants from insect attack by stimulating resistance mechanisms in the plant. However, there was no effect of rare bacteria on plant resistance. Rather plant response varied with different bacterial communities regardless of whether rare species were present or not. Overall, my thesis demonstrates that there are likely no general causes of bacterial rarity or general consequences for plant health.