ABSTRACT
Dispersal undoubtedly occurs at present but it is normally difficult to observe. The chief problem in detecting dispersal in action is that detailed species distributions are scarce. Most instances of organisms moving to new areas probably pass unnoticed-is a new sighting an individual that has moved in from elsewhere, or is it an individual that was born in the area but has not been seen before? In addition, it is sometimes difficult to account for a range change. Species ranges alter through dispersal and local extinctions. Acting in tandem, dispersal and extinction may lead to range expansion (through all or any of the dispersal processes), to range contraction (from local extinction), or to range ‘creep’ (through a mixture of spread and local extinction). It is far from easy to establish the processes involved in actual cases.
