biological barriers prevent organisms from spreading to all habitats where conditions are suitable
organisms that spread by means of sweepstakes routes include species of birds called accidentals that appear in places far from their homes
biological barriers can be the result of physical features,climate,competition and behavior.
organisms can cross barriers by means of three types of natural pathways:corridors,filter routes,and sweepstakes routes
behavior barriers do not prevent the spread of species from place as effectively as physical barriers
humans may accidentally or intentionally bring some species across natural barries,and these species may have certain advantages over native species
american bison spread throughout the grasslands of North America.