CAUSES 571 



able that a similar relationship exists with respect to the status of grouse abundance through- 

 out any range area. The data also suggest that conditions of general scarcity which occur 

 from time to time over wider areas may be primarily a consequence of some combination of 

 factors causing a loss of productivity over a preponderance of the component coverts, but 

 that the actual populations of many of these coverts may often fall little lower than they do 

 in other years. 



Synchronism, with respect to the status of grouse abundance over any range area, thus 

 seems to have been greatest during recurrent major troughs, but to have occurred a year or so 

 later than many parts of the area initially reached a low level. Considerable lack of syn- 

 chronism has been characteristic of the early stages of most declines. The term "spotty" has 

 been applied to conditions of this kind when grouse are still plentiful in many localities but 

 noticeably scarce in others. Lack of synchronism has also been frequent during times of 

 generally high abundance. 



CAUSES 



The reproductive potential of a species constantly tends to increase its numbers. The 

 potential productivity of the ruffed grouse is greatly in excess of that which can actually be 

 realized because the range will accommodate only about so many. For this reason low popu- 

 lations tend to rise, but those already at a peak have little o|)|)<)rlunity to increase and tend 

 to fall back. A wide variety of influences operate as limiting factors. Most of these vary 

 from year to year, and even from season to season, causing grouse populations to fluctuate 

 continually in numbers. Moreover, radical changes in the environment may cause local 

 scarcity at any time. 



The observations of this Investigation as to the manner in which these influences have 

 affected grouse productivity on the areas studied have been discussed in the preceding chap- 

 ter. These data have shown that differences in the degree of survival during one or more life 

 periods rather than changes in the reproductive potential of the birds are the chief reasons 

 for the population changes recorded. It is probable that this is typical of the behavior of 

 such populations. 



It has also been shown that, in addition to the constant but irregular fluctuation of local 

 populations, there have been recurrent periods of general abundance and of general scarcity 

 over greater areas. It has been shown, too, that these periods, particularly those of scarcity, 

 have recurred at intervals of eight to ten years in many parts of the species' north-central 

 and northeastern range during the time since 1900. Furthermore, data have been presented 

 which suggest that such longer-term fluctuations in the average abundance of grouse over 

 extensive areas may be primarily a result of the recurrent synchronization of a preponde- 

 rance of the local populations at low average levels, although at such times many of the com- 

 ponent units exhibit densities little, if any, lower than are often reached in other years. 



In other words it seems evident that the immediate causes of these major fluctuations in 

 general abundance lie among the same agencies which limit survival on local areas regard- 

 less of to what extent they may be governed by more deep-seated influences. 



Relative Significance of Losses During Different Life Periods 



The data increasingly point to the success or failure of the annual increment of young as the 

 key to the problem. As already discussed under Net Productivity*, this has been the most 

 frequent cause of low fall populations on the Investigation's study areas although just when 



* See Chapter XH, p. 539. 



