But, despite the work of Darwin and others (e.g., Carlquist, 1966, 1967 Sorensen, 1986), long-distance seed dispersal has often been ignored or addressed by studies that are anecdotal in nature. He supplemented such observations with logical arguments to show that while seeming to move little, plants possessed the means to establish new colonies at great distances from their birthplace. His interest in the spread of plant populations led him to make meticulous observations on how seeds could survive immersion in salty water, be transported in mud on the feet of vertebrates, or pass unharmed through the digestive system of birds. For example, with his characteristic ability to identify the essence of a problem, Darwin (1859) argued that both adaptations for seed dispersal and accidental means of long-distance transport were likely to be of importance to plants. We do not mean to imply that biologists have not been aware of the existence and potential importance of long-distance seed dispersal. Despite its fundamental role in natural populations, relatively little attention has been paid to long-distance seed dispersal, and, in some instances, its significance has been strongly downplayed ( Howe and Smallwood, 1982). Because most seeds move short distances (zero to a few tens of metres see Howe and Smallwood and citations therein), it follows that unusual events that move seeds long distances are of critical importance. For such species, seed dispersal represents the only way that populations can exchange individuals or colonize empty, but suitable habitat. The patchy nature of the landscape ensures that the populations of many plant species are spatially isolated from each other, often by hundreds of metres or more. The checkered landscapes visible from the air vividly demonstrate that the world is a patchy place. We close the paper by discussing important but as yet largely unexplored areas for future research. At present, several promising genetic approaches for estimating long-distance seed dispersal are under active development, including assignment methods, likelihood methods, genealogical methods, and genealogical/demographic methods. Second, we argue that genetic methods provide a broadly applicable way to monitor long-distance seed dispersal to place this argument in context, we review genetic estimates of plant migration rates. To make the case for the importance of long-distance seed dispersal, we review existing data and models of long-distance seed dispersal, focusing on situations in which seeds that travel long distances have a critical impact (colonization of islands, Holocene migrations, response to global change, metapopulation biology). First, we argue that long-distance seed dispersal is of critical importance and, hence, that we must collect better data from the tails of seed dispersal curves. This paper is structured around two lines of argument. However, because long-distance seed dispersal is inherently hard to measure, there are few data sets that characterize the tails of seed dispersal curves. Long-distance seed dispersal influences many key aspects of the biology of plants, including spread of invasive species, metapopulation dynamics, and diversity and dynamics in plant communities.
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