members of the human microbiome. Dispersal Dispersal is a vital component of an organism's life-history, and the potential for dispersal determines the distribution, abundance, and thus, the community dynamics of species at different sites. A new habitat must first be reached before filters such as organismal abilities and adaptations, the quality of a habitat, and the established biological community determine the colonization efficiency of a species. While larger animals can cover distances on their own and actively seek suitable habitats, small (<2 mm) organisms are often passively dispersed, resulting in their more ubiquitous occurrence. While active dispersal accounts for rather predictable distribution patterns, passive dispersal leads to a more randomized immigration of organisms. Mechanisms for passive dispersal are the transport on (epizoochory) or in (endozoochory) larger animals (e.g., flying insects, birds, or mammals) and the erosion by wind. A propagule is any material that functions in propagating an organism to the next stage in its life cycle, such as by dispersal. The propagule is usually distinct in form from the parent organism. Propagules are produced by plants (in the form of seeds or spores), fungi (in the form of spores), and bacteria (for example endospores or microbial cysts). Often cited as an important requirement for effective wind dispersal is the presence of propagules (e.g., resting eggs, cysts, ephippia, juvenile and adult resting stages), which also enables organisms to survive unfavorable environmental conditions until they enter a suitable habitat. These dispersal units can be blown from surfaces such as soil, moss, and the desiccated sediments of temporary or intermittent waters. The passively dispersed organisms are typically pioneer colonizers. However, wind-drifted species vary in their vagility (probability to be transported with the wind), with the weight and form of the propagules, and therefore, the wind speed required for their transport, determining the dispersal distance. For example, in nematodes, resting eggs are less effectively transported by wind than other life stages, while organisms in anhydrobiosis are lighter and thus more readily transported than hydrated forms. Because different organisms are, for the most part, not dispersed over the same distances, source habitats are also important, with the number of organisms contained in air declining with increasing distance from the source system. The distances covered by small animals range from a few meters, to hundreds, to thousands of meters. While the wind dispersal of aquatic organisms is possible even during the wet phase of a