marine waters known as zooplankton. The term plankton comes from the Greek, πλαγκτον, meaning "wanderer" or "drifter". Dispersal by dormant stages Many animal species, especially freshwater invertebrates, are able to disperse by wind or by transfer with an aid of larger animals (birds, mammals or fishes) as dormant eggs, dormant embryos or, in some cases, dormant adult stages. Tardigrades, some rotifers and some copepods are able to withstand desiccation as adult dormant stages. Many other taxa (Cladocera, Bryozoa, Hydra, Copepoda and so on) can disperse as dormant eggs or embryos. Freshwater sponges usually have special dormant propagules called gemmulae for such a dispersal. Many kinds of dispersal dormant stages are able to withstand not only desiccation and low and high temperature, but also action of digestive enzymes during their transfer through digestive tracts of birds and other animals, high concentration of salts and many kinds of toxicants. Such dormant-resistant stages made possible the long-distance dispersal from one water body to another and broad distribution ranges of many freshwater animals. Quantifying dispersal Dispersal is most commonly quantified either in terms of rate or distance. Dispersal rate (also called migration rate in the population genetics literature) or probability describes the probability that any individual leaves an area or, equivalently, the expected proportion of individual to leave an area. The dispersal distance is usually described by a dispersal kernel which gives the probability distribution of the distance traveled by any individual. A number of different functions are used for dispersal kernels in theoretical models of dispersal including the negative exponential distribution, extended negative exponential distribution, normal distribution, exponential power distribution, inverse power distribution, and the two-sided power distribution. The inverse power distribution and distributions with 'fat tails' representing long-distance dispersal events (called leptokurtic distributions) are thought to best match empirical dispersal data. Consequences of dispersal Dispersal not only has costs and benefits to the dispersing individual (as mentioned above), it also has consequences at the level of the population and species on both ecological and evolutionary timescales. Many populations have patchy spatial distributions where separate yet interacting sub-populations occupy discrete habitat patches (see metapopulations). Dispersing individuals move between different sub-populations which increases the overall connectivity of the metapopulation and can lower the risk of stochastic extinction. If a sub-population goes extinct by chance, it is more likely to be recolonized if the dispersal rate is high. Increased connectivity can also decrease the degree