Distribution by Dispersal
Distribution by Dispersal
By dispersal, animals spread into new localities from their places of origin. Dispersal involves emigration from one region and immigration into another. Dispersal is a one-way, outward movement that must be distinguished from periodic movement back and forth between two localities, such as seasonal migration of many birds. Dispersing animals may move actively under their own power, or they may be passively dispersed by wind, by floating or rafting on rivers, lakes, or the sea, or by hitching rides on other animals. Animals are expected to expand their geographic distributions in this manner across all favorable habitat that is accessible to them. For example, as the last Pleistocene glaciers retreated northward, habitats favorable for many temperate species became available on formerly glaciated territory in North America, Europe, and Asia. Species that originated immediately south of the glaciated territory prior to glacial retreat then expanded northward as new habitats appeared. Because the reproductive rate of animal populations is great, there is a continuous pressure on populations to expand across all favorable habitats.
Dispersal easily explains the movement of animal populations into favorable habitats that are geographically adjacent to their places of origin. This movement produces an expanded but geographically continuous distribution. Can dispersal also explain the origins of geographically disjunct distributions? For example, flightless ratite birds (Figure 39-14) inhabit disjunct landmasses primarily of the Southern Hemisphere including Africa, Australia, Madagascar, New Guinea, New Zealand, and South America. These landmasses are separated from each other by ocean, a very strong barrier to ratite dispersal. To explain this distribution by dispersal, one must postulate a center of origin from which the group dispersed to reach all of the widely separated landmasses on which it is now found. Because ratites do not fly, a dispersalist hypothesis requires intermittent, passive rafting of individuals across the ocean. Is this hypothesis reasonable? We know from studies of the Galápagos Islands and Hawaii (Organic Evolution) that occasional, long-distance dispersal of terrestrial animals and plants across oceans does occur. This is the only way that terrestrial animals could colonize islands produced by oceanic volcanoes. For flightless birds and many other discontinuously distributed animals, however, there is an alternative to the hypothesis that disjunct distribution was produced by dispersal over unfavorable habitat. This is the hypothesis of vicariance (L. vicarius, a substitute).
By dispersal, animals spread into new localities from their places of origin. Dispersal involves emigration from one region and immigration into another. Dispersal is a one-way, outward movement that must be distinguished from periodic movement back and forth between two localities, such as seasonal migration of many birds. Dispersing animals may move actively under their own power, or they may be passively dispersed by wind, by floating or rafting on rivers, lakes, or the sea, or by hitching rides on other animals. Animals are expected to expand their geographic distributions in this manner across all favorable habitat that is accessible to them. For example, as the last Pleistocene glaciers retreated northward, habitats favorable for many temperate species became available on formerly glaciated territory in North America, Europe, and Asia. Species that originated immediately south of the glaciated territory prior to glacial retreat then expanded northward as new habitats appeared. Because the reproductive rate of animal populations is great, there is a continuous pressure on populations to expand across all favorable habitats.
Dispersal easily explains the movement of animal populations into favorable habitats that are geographically adjacent to their places of origin. This movement produces an expanded but geographically continuous distribution. Can dispersal also explain the origins of geographically disjunct distributions? For example, flightless ratite birds (Figure 39-14) inhabit disjunct landmasses primarily of the Southern Hemisphere including Africa, Australia, Madagascar, New Guinea, New Zealand, and South America. These landmasses are separated from each other by ocean, a very strong barrier to ratite dispersal. To explain this distribution by dispersal, one must postulate a center of origin from which the group dispersed to reach all of the widely separated landmasses on which it is now found. Because ratites do not fly, a dispersalist hypothesis requires intermittent, passive rafting of individuals across the ocean. Is this hypothesis reasonable? We know from studies of the Galápagos Islands and Hawaii (Organic Evolution) that occasional, long-distance dispersal of terrestrial animals and plants across oceans does occur. This is the only way that terrestrial animals could colonize islands produced by oceanic volcanoes. For flightless birds and many other discontinuously distributed animals, however, there is an alternative to the hypothesis that disjunct distribution was produced by dispersal over unfavorable habitat. This is the hypothesis of vicariance (L. vicarius, a substitute).
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