Species and Species Change 125 



fall are predictable in general, but the exact quantitative values of 

 these fluctuations fall into more random oscillations. Unusual quan- 

 titative values such as lack of rainfall or late freezes may kill entire 

 local populations of a species, causing peripheral contraction or 

 internal lacunae in the range. Again because of ecological fluctua- 

 tions, by the next season or even only a few hours later these same 

 depleted areas would again be perfectly habitable for the species 

 if it reached the area. 



Local extinction by unusual ecological fluctuations would even- 

 tually exterminate every species were it not for the inherent prop- 

 erty of every species to disperse. By dispersal, depleted areas may 

 be repopulated and the range reconstituted. Areas which are only 

 temporarily suitable ecologically may also be populated periodi- 

 cally, thus affecting another variable in range periphery. 



Because dispersal is at least partly random, many individuals of 

 a species will spread into areas outside the habitable range and 

 perish. However, over the years this is not really a loss to the 

 species because, by this mechanism, the species maintains what 

 might be called a constant expansion pressure at the edge of its 

 range. 



How far species habitually disperse beyond the border of their 

 ranges is not completely known, but it is undoubtedly different 

 from year to year and species to species. Many kinds of flies are 

 known to disperse readily for distances of five to 30 miles ( Schoof , 

 1952). In the case of the migratory high plains grasshopper Dis- 

 sosteira longipennis, Wakeland ( 1958 ) reported dispersal with some 

 regularity in an arc of several hundred miles radius beyond the 

 area where breeding occurs ( Fig. 49 ) . Wind-borne seeds may have 

 an even greater radius of regular dispersal. More sedentary or- 

 ganisms have correspondingly lower dispersal potentials. For snakes 

 and terrestrial amphibians some writers have estimated the net 

 annual dispersal distance of an individual at only a mile or two, 

 representing the total of its day-to-day travels. Plants with heavy 

 seeds such as oaks and hickories normally have a dispersal distance 

 limited to the daily activities of squirrels, but a tornado or flood 

 might carry an occasional nut many miles. For rare species such 

 as animals confined to caves or local streams, valleys or peaks, httle 

 is known concerning dispersal, although it surely occurs. 



The exact oscillations in ranges are poorly known, but several 

 examples in the insect order Orthoptera give some idea of the po- 

 tentials involved in the interplay between ecological fluctuations 

 and dispersal. The range of the North American high plains grass- 



