Traditionally, most ecologists understand the world from a human scale. Ecosystems are often understood as large visible units of the landscape , usually homogeneous land patches or a series of adjacent patches with intense flows of individuals, energy or biomass and nutrients. However, there is more in a landscape than meets the eye. An arguably homogeneous land patch within a landscape hosts many small ecosystems, or microhabitat patches, where many different communities of microbes dwell and interact. For example, imagine you are standing in a clearing of an open forest in a temperate region. A terrestrial ecologist studying macroscopic organisms would think he is looking at part of a single ecosystem. On the contrary, a microbial ecologist will identify a plethora of different ecosystems, including leaf litter of different degrees of humidity, the bark of each different tree and shrub species, treeholes, temporary puddles and pools, moss cushions of different life forms growing over different substrates, etc. Not to mention soil communities. In other words, a 1 ha clearing within a forest could be considered a whole landscape for many groups of microbes. A key question in microbial ecology is thus whether the patterns and organization of microbial communities differ from those of macroscopic organisms just in terms of scale or they are so radically different that the rules affecting macrobes cannot be extrapolated to microbes. The debate on this question extends to the biogeography of microorganisms. Strikingly, it has been argued that most microorganisms do not have biogeography; that is, that contrary to macroorganisms, the distributions of microorganism species are just limited by local environmental conditions (e.g. Fenchel & Finlay, 2003, 2004 and below). But, are microbes so different from their larges relatives than they follow different ecological and biogeographical rules?

I wish to thank Diego Fontaneto for the invitation to write this chapter and participate in the corresponding symposium, and to an anonymous referee for a thoughtful review of the previous version of this manuscript. I am also indebted to Noemí Guil, Sara Sánchez Moreno and Diego Fontaneto, for introducing me to the world of the very very small things, as well as for many insightful discussions throughout the last ten years. JH is funded by a Spanish CSIC JAE-Doc research grant, and obtained additional funding from a travel grant of the Azorean Biodiversity Group – CITA-A.

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