272 Origin of Biomes and Succession 



Reversal of Succession 



The proposition of reversing succession is simply that of affecting 

 a community so that the area it occupies becomes more suitable 

 for a community of an earlier successional age. Factors producing 

 this condition can be classified into non-biotic and biotic factors. 



NON-BIOTIC FACTORS 



These may be divided into short-term and long-term factors. Short- 

 term factors include the effects of catastrophes and weather ( as dis- 

 tinct from climate); long-term factors include geomorphic changes. 



Short-term events. Catastrophes and weather include fire, tidal 

 waves, landslides, scouring and silting by floods, high winds, forma- 

 tion of oxbows and ponds by local shifts in stream channels, severe 

 droughts, unsual temperature drops, and all similar events which 

 disrupt the biota of a community. These are chiefly terrestrial events. 

 Others having comparable effect undoubtedly occur in marine en- 

 vironments. Volcanoes and earthquakes contribute catastrophic fac- 

 tors on land and presumably do the same on the ocean floor. 

 Because of an extreme paucity of knowledge concerning the effects 

 of these factors on marine biota, however, it is necessary to base 

 the following discussion on terrestrial examples. 



Fire may have been of greater importance in maintaining se- 

 quences of subclimax areas than is generally conceded. Studying 

 the efi^ects of fire in the Florida Everglades, Robertson (1954) 

 found that about 70 per cent of the 100 or more endemic herbs 

 and low shrubs of the Everglades region occur only in commu- 

 nities whose existence is dependent on frequent fires. Watchers in 

 the Everglades region saw fires start from lightning strikes in saw- 

 grass and in tree islands. In all, fires attributed to lightning in the 

 Everglades area numbered 12 in 1951 and 11 during the first half 

 of 1952. Some of these fires were extinguished by the rain accom- 

 panying the electrical storm, but several became widespread. Rob- 

 ertson concluded that a checkerboard of frequent fires has been 

 the principal factor maintaining the present subclimax areas in the 

 Everglades for the full geologic life of these communities. Reason- 

 ing from these observations it is possible that most if not all of the 

 extensive pine forests of the southeastern and south central United 

 States may owe their continued existence to the destruction of 

 deciduous hardwood seedlings by fire. 



Natural fires have been recorded for many other areas. Prebble 



