30 ANIMAL ECOLOGY 



sort of way upon plant succession on land. Herbivorous 

 animals are often the prime controlling factor in the particular 

 kind of succession which takes place on an area. Farrow ^* 

 says : "It is thus seen that variation in the intensity of the 

 rabbit attack alone is sufficient to change the dominant type 

 of vegetation in Breckland from pine woodland to dwarf 

 grass-heath vegetation through the phases of Calluna heath 

 and Carex arenaria^ and that for each given intensity of rabbit 

 attack there is a certain associated vegetation." He showed 

 that there were definite zones of vegetation round each rabbit 

 colony and even each hole, since the distance from the colony 

 resulted in different intensities of attack. The curious fact 

 also appears that Carex arenaria will dominate Calluna when 

 both are eaten down intensely. It is the relative intensity 

 of attack that matters. The example quoted here is only one 

 of many which could be given, although the situation has not 

 been worked out so ingeniously or fully for any other animals 

 or place. 



17. Hofman's work ^^ has made it probable that the in- 

 fluence of rodents in burying seeds of conifers may sometimes 

 determine the type of succession which starts after a forest 

 fire. The Douglas Fir {Pseudotsuga taxifolia) is a dominant 

 tree over large parts of the Cascade and Coast region of 

 Washington and Oregon. When the seed crops are light 

 they are largely destroyed by an insect {Megastigmus spermo- 

 trophns) and by rodents. But when there is an unusually 

 heavy crop large quantities of surplus seed are collected and 

 stored in caches by the rodents. Since in many cases the 

 animals do not return to the caches the seeds remain there for 

 a good many years, and if there is a fire in the forest or the 

 trees are cut down, large quantities of the stored seed germinate. 

 More of the Douglas fir seed than of other species of trees 

 such as hemlock and cedar is cached by rodents, so that the 

 Douglas has accordingly an advantage in the early stages of 

 succession. The same thing holds good for the white pine ; 

 its seeds are much eaten by rodents, which gather them and 

 store them in the ground. But here other factors such as 

 germination come in and affect the succession. 



