Sexton and Norris propose that it was the climate, and its role in determining the availability of favourable oceanic habitat, that restricted the distribution of T. truncatulinoides, rather than the presence of physical ocean barriers. In this new view, plankton are freely dispersed throughout the ocean but local conditions determine whether or not the species can ‘take hold’ and thrive. An analogy is that of coconuts, which sometimes wash up on the shore of Britain; cold temperatures prevent coconuts from germinating, but should the climate suddenly shift to a subtropical state, coconut trees might become a common sight lining Britain’s shores.
This new idea that there are few, if any, barriers to the free dispersal of plankton throughout the world’s oceans has been corroborated by genetic research showing that rates of gene flow throughout the oceans are remarkably high. Furthermore, distributions of a number of larger ocean-dwellers such as tuna and molluscs show that, despite having regions of favoured habitat, small numbers of them are regularly found outside of their ‘core range’. These observations suggest that species’ distributions are more controlled by habitat availability rather than by an inability to disperse.
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